=head1 NAME

Exegesis_07 - Formats

=head1 AUTHOR

Damian Conway <damian@conway.org>

=head1 VERSION

  Maintainer: Allison Randal <allison@perl.org>
  Date: 26 Feb 2004
  Last Modified: 26 Feb 2004
  Number: 7
  Version: 1

=over

I<What a piece of work is Perl 6!>

I<How noble in reason!>

I<How infinite in faculty!>

I<In C<form> how express and admirable!>

=over 

=item E<ndash> W. Shakespeare, "Hamlet" (Perl 6 revision)

=back

=back

Formats are Perl 5's mechanism for creating text templates with
fixed-width fields. Those fields are then filled in using values from
prespecified package variables. They're a useful tool for
generating many types of plaintext reports E<ndash> the I<r> in I<Perl>,
if you will.

Unlike Perl 5, Perl 6 doesn't have a C<format> keyword. Or the
associated built-in formatting mechanism. Instead it has a Form.pm
module. And a C<form> function.

Like a Perl 5 C<format> statement, the C<form> function takes a series
of format (or "picture") strings, each of which is immediately
followed by a suitable set of replacement values. It interpolates
those values into the placeholders specified within each picture string,
and returns the result.

The general idea is the same as for Perl's two other built-in string
formatting functions: C<sprintf> and C<pack>. The first argument
represents a template with I<N> placeholders to be filled in, and the
next I<N> arguments are the data that is to be formatted and
interpolated into those placeholders:

    $text = sprintf $format_s, $datum1, $datum2, $datum3;
    $text =    pack $format_p, $datum1, $datum2, $datum3;
    $text =    form $format_f, $datum1, $datum2, $datum3;

Of course, these three functions use quite different mini-languages to specify
the templates they fill in, and all three fill in those templates in quite
distinct ways.

Apart from those differences in semantics, C<form> has a syntactic
difference too. With C<form>, after the first I<N> data arguments we're
allowed to put a second format string and its corresponding data, then a
third format and data, and so on:

    $text = form $format_f1, $datum1, $datum2, $format_f2, $datum4, $format_f3, $datum5;

And if we prettify that function call a little, it becomes obvious that it has
the same basic structure as a Perl 5 C<format>:

    form
         $format_f1,
             $datum1, $datum2, $datum3,
         $format_f2,
             $datum4,
         $format_f3,
             $datum5;

But the Perl 6 version is implemented as a vanilla Perl 6 subroutine,
rather than hard-coded into the language with a special keyword and
declaration syntax. In this respect it's rather like Perl 5's
little-known C<formline> function E<ndash> only much, much better.

So, whereas in Perl 5 we might write:

    # Perl 5 code...

    our ($name, $age, $ID, $comments); 

    format STDOUT
     =================================== 
    | NAME     |    AGE     | ID NUMBER |
    |----------+------------+-----------|
    | @<<<<<<< | @||||||||| | @>>>>>>>> |
      $name,     $age,        $ID,
    |===================================|
    | COMMENTS                          |
    |-----------------------------------|
    | ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< |~~
      $comments,
     ===================================
    .

    write STDOUT;


in Perl 6 we could write:

    print form
        " =================================== ",
        "| NAME     |    AGE     | ID NUMBER |",
        "|----------+------------+-----------|",
        "| {<<<<<<} | {||||||||} | {>>>>>>>} |",
           $name,     $age,        $ID,
        "|===================================|",
        "| COMMENTS                          |",
        "|-----------------------------------|",
        "| {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[} |",
           $comments,
        " =================================== ";

And both of them would print something like:

     ===================================
    | NAME     |    AGE     | ID NUMBER |
    |----------+------------+-----------|
    | Richard  |     33     |    000003 |
    |===================================|
    | COMMENTS                          |
    |-----------------------------------|
    | Talks to self. Seems to be        |
    | overcompensating for inferiority  |
    | complex rooted in post-natal      |
    | materal rejection due to physical |
    | handicap (congenital or perhaps   |
    | the result of premature birth).   |
    | Shows numerous indications of     |
    | psychotic (esp. nepocidal)        |
    | tendencies. Naturally, subject    |
    | gravitated to career in politics. |
     ===================================


At first glance the Perl 6 version may seem like something of a
backwards step E<ndash> all those extra quotation marks and commas that
the Perl 5 format didn't require. But the new formatting interface
does have several distinct advantages:

=over

=item *

it uses the standard Perl 6 subroutine call syntax, so we can use the
full power of Perl data structures and control flow when setting up 
formats;

=item *

it delimits every field specification by braces, which allows for a much
wider range of field types;

=item *

it removes the special meanings of C<'@'>, C<'^'>, C<'~'>, and C<'.'> in
formats, leaving only C<'{'> as special;

=item *

it provides an extension mechanism for creating new field types;

=item *

it greatly simplifies the common task of formatting data into a string
(rather than requiring the format data to be written to an output stream);

=item *

it doesn't destroy the contents of data variables when
formatting them across multiple lines;

=item *

it's easy to create new formats on-the-fly, rather than being
forced to statically declare them at compile-time (or in a run-time
string C<eval>);

=item *

it allows calls to C<form> to be nested;

=item *

it supports dynamically computed page headers and footers, which
may themselves make use of nested calls to C<form>;

=item *

it doesn't rely on package variables, typeglobs, or a global accumulator;

=item *

it doesn't require a (frequently cryptic)  call to the
mysterious C<write> function E<ndash> and hence frees up C<write> to be
used as the true opposite of C<read>, should Larry so desire.

=back

Of course, this is Perl, not Puritanism. So those folks who happen to
I<like> package variables, global accumulators, and mysterious writes,
can still have them. And, if they're particularly nostalgic, they can
also get rid of all the quotation marks and commas, and even retain the
dot as a format terminator. For example:

    sub myster_rite {                           
        our ($name, $age, $ID, $comments);     
        print form :interleave, <<'.'               
             =================================== 
            | NAME     |    AGE     | ID NUMBER |
            |----------+------------+-----------|
            | {<<<<<<} | {||||||||} | {>>>>>>>} |
            |===================================|
            | COMMENTS                          |
            |-----------------------------------|
            | {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[} |
             =================================== 
            .
              $name,     $age,        $ID,
              $comments;
    }

    # and elsewhere in the same package...

    ($name, $age, $ID, $comments) = get_data();
    myster_rite();

    ($name, $age, $ID, $comments) = get_more_data();
    myster_rite();

Let's take a look...


=head1 What's in a name?

But before we do, here's a quick run-down of some of the highly arcane
technical jargon we'll be using as we talk about formatting:

=over 

=item Format

A string that is used as a template for the creation of I<text>. It will
contain zero or more I<fields>, usually with some literal characters and
whitespace between them.

=item Text

A string that is created by replacing the fields of a format with specific
I<data> values.  For example, the string that a call to C<form> returns.

=item Field      

A fixed-width slot within a format string, into which I<data> will be formatted.

=item Data      

A string or numeric value (or an array of such values) that is
interpolated into a format, in order to fill in a particular field.

=item Single-line field

A field that interpolates only as much of its corresponding data value as
will fit inside it within a single line of text.

=item Block field

A field that interpolates all of its corresponding data value, over a
series of text lines E<ndash> as many as necessary E<ndash> producing a
I<text block>.

=item Text block

The column of newline-separated text lines. A text block is produced
when data is formatted into a block field that is too small to contain
the data in a single line

=item Column

The amount of space on an output device required to display one single-width
character. One character will occupy one column in most cases, the most
obvious exceptions being CJK double-width characters.

=back


=head1 Why, how now, ho! From whence ariseth this?

Unlike C<sprintf> and C<pack>, the C<form> subroutine isn't built into Perl 6.
It's just a regular subroutine, defined in the Form.pm module:

    module Form
    {
        type FormArgs ::= Str|Array|Pair;

        sub form (FormArgs *@args is context(Scalar)) returns Str
            is exported
        {
            ...
        }

        ...
    }

That means that if we want to use C<form> we need to be sure we:

    use Form;

first. 

Note that the above definition of C<form> specifies that the subroutine takes
a list of arguments (C<*@args>), each of which must be a string, array
or pair (C<type FormArgs ::= Str|Array|Pair>). And the
C<is context(Scalar)> trait specifies that each of those arguments will be
evaluated in a scalar context.

That last bit is important, because normally a "slurpy" array parameter like
C<*@args> would impose a list context on the corresponding arguments. We don't
want that here, mainly because we're going to want to be able to L<pass
arrays to C<form>| "Therefore, put you in your best array..."> without
having them flattened.


=head1 How called you...?

Like all Perl subroutines, C<form> can be called in a variety of contexts.

When called in a scalar or list context, C<form> returns a string
containing the complete formatted text:

    my $formatted_text = form $format, *@data;

    @texts = ( form($format, *@data1), form($format, *@data2) );  # 2 elems

When called in a void context, C<form> waxes lyrical about human
frailty, betrayal of trust, and the pointlessness of calling out
when nobody's there to heed the reply, before dying in a highly
theatrical manner.


=head1 He doth fill fields...

The format strings passed to C<form> determine what the resulting
formatted text looks like. Each format consists of a series
of field specifiers, which are usually separated by literal characters.

C<form> understands a far larger number of field specifiers than C<format> did,
but they're easy to remember because they obey a small number of conventions:

=over

=item *

Each field is enclosed in a pair of braces.

=item *

Within the braces, left or right angle brackets (C<< < >> or C<< > >>), bars
(C<|>), and single-quotes (C<'>) indicate various types of single-line fields. 

=item *

Left or right square brackets (C<[> or C<]>), I's (C<I>), and double-
quotes (C<">) indicate block fields of various types.

=item *

The direction of the brackets within a field indicates the direction
towards which text will be justified in that field. For example:

    {<<<<<<<<<<<}   Justify the text to the left
    {>>>>>>>>>>>}                  Justify the text to the right
    {>>>>>><<<<<}                 Centre the text
    {<<<<<<>>>>>}   Fully  justify  the  text  to  both  margins

This is even true for numeric fields, which look like:
C<<<<<<< {>>>>>.<<} >>>>>>>. The whole digits are right-justified before
the dot and the decimals are left-justified after it.

=item *

An C<=> at either end of a field (or both ends) indicates the data
interpolated into the field is to be vertically "middled" within the
resulting block. That is, the text is to be centred vertically on the
middle of all the lines produced by the complete format.

=item *

An C<_> at the start and/or end of a field indicates the interpolated data 
is to be vertically "bottomed" within the resulting block. That is, the
text is to be pushed to the bottom of the lines produced by the format.

=back


=head1 The fields are fragrant...

That may still seem like quite a lot to remember, but the rules have
been chosen so that the resulting fields are visually mnemonic. In other
words, they're supposed to look like what they do. The intention is that
we simply draw a (stylized) picture of how we want the finished text to
look, using fields that look something like the finished product
E<ndash> left or right brackets brackets showing horizontal alignments,
a middlish C<=> or bottomed-out C<_> indicate middled or bottom vertical
alignment, etc., etc. Then C<form> fits our data into the fields so it
looks right.

The typical field specifications used in a C<form> format look like this:

                                      Field specifier
    Field type                 One-line             Block
    ==========                ==========          ==========

    left justified            {<<<<<<<<}          {[[[[[[[[}
    right justified           {>>>>>>>>}          {]]]]]]]]}
    centred                   {>>>><<<<}          {]]]][[[[}
    centred (alternative)     {||||||||}          {IIIIIIII}
    fully justified           {<<<<>>>>}          {[[[[]]]]}
    verbatim                  {''''''''}          {""""""""}

    numeric                   {>>>>>.<<}          {]]]]].[[}
    euronumeric               {>>>>>,<<}          {]]]]],[[}
    comma'd                   {>,>>>,>>>.<<}      {],]]],]]].[[}
    space'd                   {> >>> >>>.<<}      {] ]]] ]]].[[}
    eurocomma'd               {>.>>>.>>>,<<}      {].]]].]]],[[}
    Swiss Army comma'd        {>'>>>'>>>,<<}      {]']]]']]],[[}
    subcontinental            {>>,>>,>>>.<<}      {]],]],]]].[[}

    signed numeric            {->>>.<<<}          {-]]].[[[}
    post-signed numeric       {>>>>.<<-}          {]]]].[[-}
    paren-signed numeric      {(>>>.<<)}          {(]]].[[)}

    prefix currency           {$>>>.<<<}          {$]]].[[[}
    postfix currency          {>>>.<<<DM}         {]]].[[[DM}
    infix currency            {>>>$<< Esc}        {]]]$[[ Esc}

    left/middled              {=<<<<<<=}          {=[[[[[[=}
    right/middled             {=>>>>>>=}          {=]]]]]]=}
    infix currency/middled    {=>>$<< Esc}        {=]]$[[ Esc}
    eurocomma'd/middled       {>.>>>.>>>,<<=}     {].]]].]]],[[=}
    etc.

    left/bottomed             {_<<<<<<_}          {_[[[[[[_}
    right/bottomed            {_>>>>>>_}          {_]]]]]]_}
    etc.


=head1 What a block art thou...

When data is interpolated into a line field, the field grabs as much of the
data as will fit on a single line, formats that data appropriately, and
interpolates it into the format.

That means that if we use a one-line field, it only shows as much of the data
as will fit on one line. For example:

    my $data1 = 'By the pricking of my thumbs, something wicked this way comes';
    my $data2 = 'A horse! A horse! My kingdom for a horse!';

    print form
        "...{<<<<<<<<<<<<<<<<<}...{>>>>>>>}...",
            $data1,               $data2;

prints:

    ...By the pricking of ... A horse!...

On the other hand, if our format string used block fields instead, the
fields would extract one line of data at a time, repeating that process as
many times as necessary to display all the available data. So:

    print form
        "...{[[[[[[[[[[[[[[[[[}...{]]]]]]]}...",
            $data1,               $data2;

would produce:

    ...By the pricking of ... A horse!...
    ...my thumbs,         ... A horse!...
    ...something wicked   ...       My...
    ...this way comes     ...  kingdom...
    ...                   ...    for a...
    ...                   ...   horse!...


We can mix line fields and block fields in the same format and C<form> will
extract and interpolate only as much data as each field requires. For example:

    print form
        "...{<<<<<<<<<<<<<<<<<}...{]]]]]]]}...",
            $data1,               $data2;

which produces:

    ...By the pricking of ... A horse!...
    ...                   ... A horse!...
    ...                   ...       My...
    ...                   ...  kingdom...
    ...                   ...    for a...
    ...                   ...   horse!...

Notice that, after the first line, the single-line
C<<<<<<< {<<<<<<} >>>>>>> field is simply replaced by
the appropriate number of space
characters, to keep the columns correctly aligned.

The usual reason for mixing line and block fields in this way is to
allow numbered or bulleted points:

    print "I couldn't do my English Lit homework because...\n\n";

    for @reasons.kv -> $index, $reason {
        my $n = @reasons - $index ~ '.';
        print form "   {>}  {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[}",
                       $n,  $reason,
                   "";
    }

which might produce:

    I couldn't do my English Lit homework because...

         10. Three witches told me I was going to be    
             king.                                      

          9. I was busy explaining wherefore am I Romeo.

          8. I was busy scrubbing the blood off my      
             hands.                                     

          7. Some dear friends had to charge once more  
             unto the breach.                           

          6. My so-called best friend tricked me into   
             killing my wife.                           

          5. My so-called best friend tricked me into   
             killing Caesar.                            

          4. My so-called best friend tricked me into   
             taming a shrew.                            

          3. My uncle killed my father and married my   
             mother.                                    

          2. I fell in love with my manservant, who was
             actually the disguised twin sister of the
             man that my former love secretly married,
             having mistaken him for my manservant who
             was wooing her on my behalf whilst secretly
             in love with me.

          1. I was abducted by fairies.                 


=head1 And mark what way I make...

Obviously, as a call to C<form> builds up each line of its output
E<ndash> extracting data from one or more data arguments and
formatting it into the corresponding fields E<ndash> it needs to keep
track of where it's up to in each datum. It does this by progressively
updating the C<.pos> of each datum, in exactly the same way as a
pattern match does.

And as with a pattern match, by default that updated C<.pos> is only
used internally and B<not> preserved after the call to C<form> is
finished. So passing a string to C<form> doesn't interfere with any
other pattern matching or text formatting that we might 
subsequently do with that data.

However, sometimes we I<do> want to know how much of our data a call to C<form>
managed to extract and format. Or we may want to split a formatting task
into several stages, with separate calls to C<form> for each stage.
So we need a way of telling C<form> to preserve the C<.pos> information
in our data.

But, if we want to apply a series of C<form> calls to the same data we also
need to be able to tell C<form> to I<respect> the C<.pos> information
of that data E<ndash> to start extracting from the previously preserved
C<.pos> position, rather than from the start of the string.

To achieve both those goals, we use a I<follow-on field>. That is we use
an ordinary field but mark it as C<.pos>-sensitive with a special
notation: Unicode ellipses or ASCII colons at either end. So instead of
C<<<<< {<<<<>>>>} >>>>>, we'd write
C<{E<hellip>>C<<<< <<<>>> >>>>C<E<hellip>}>
or C<<<<< {:<<<>>>:} >>>>>.

Note that each ellipsis is a single, one-column wide Unicode HORIZONTAL
ELLIPSIS character (C<\c[2026]>), I<not> three separate dots. The
connotation of the ellipses is I<"...then keep on formatting from where
you previously left off, remembering there's probably still more to
come...">. And the colons are the ASCII symbol most like a single
character ellipsis (try tilting your head and squinting).

Follow-on fields are most useful when we want to split a formatting task
into distinct stages E<ndash> or iterations E<ndash> but still allow the
contents of the follow-on field to flow uninterrupted from line to line.
For example:

    print "The best Shakespearean roles are:\n\n";

    for @roles -> $role {
        print form "   * {<<<<<<<<<<<<<<<<<<<<<<<<<<<<}   *{E<hellip><<<<<<<>>>>>>>E<hellip>}*",
                         $role,                            $disclaimer;
    }

which produces:

    The best Shakespearean roles are:

       * Macbeth                          *WARNING:          *
       * King Lear                        *This list of roles*
       * Juliet                           *constitutes      a*
       * Othello                          *personal   opinion*
       * Hippolyta                        *only and is in  no*
       * Don John                         *way  endorsed   by*
       * Katerina                         *Shakespeare'R'Us. *
       * Richard                          *It   may   contain*
       * Malvolio                         *nuts.             *
       * Bottom                           *                  *

The multiple calls to C<form> manage to produce a coherent disclaimer
because the ellipses in the second field tell each call to start
extracting data from C<$disclaimer> at the offset indicated by
C<$disclaimer.pos>, and then to update C<$disclaimer.pos> with
the final position at which the field extracted data. So the next time
C<form> is called, the follow-on field starts extracting from
where it left off in the previous call.

Follow-on fields are similar to C<<<<<< ^<<<<< >>>>>> fields in a Perl 5 format,
except they don't destroy the contents of a data source; they merely change that
data source's C<.pos> marker.


=head1 Therefore, put you in your best array...

Data, especially numeric data, is often stored in arrays.
So C<form> also accepts arrays as data arguments. It can
do so because its parameter list is defined as:

        sub form (Str|Array|Pair *@args is context(Scalar)) {...}

which means that although its arguments may include one or more arrays,
each such array argument is nevertheless evaluated in a scalar
context. Which, in Perl 6, produces an array reference.

In other words, array arguments don't get flattened automatically, so
C<form> doesn't losing track of where in
the argument list one array finishes and the next begins.

Once inside C<form>, each array that was specified as the data source
for a field is internally converted to a single string by joining it
together with a newline between each element.

The upshot is that, instead of:

    print "The best Shakespearean roles are:\n\n";

    for @roles -> $role {
        print form "   * {<<<<<<<<<<<<<<<<<<<<<<<<<<<<}   *{E<hellip><<<<<<<>>>>>>>E<hellip>}*",
                         $role,                            $disclaimer;
    }

we could just write:

    print "The best Shakespearean roles are:\n\n";

    print form "   * {[[[[[[[[[[[[[[[[[[[[[[[[[[[[}   *{[[[[[[[[]]]]]]]]}*",
                     @roles,                           $disclaimer;

And the array of roles would be internally converted to a single string, with
one role per line. Note that we also changed the disclaimer field to a regular
block field, so that the entire disclaimer would be formatted. And there was
no longer any need for the disclaimer field to be a follow-on field, since the
block field would extract and format the entire disclaimer anyway.

Note, however, that this block-based approach wouldn't work so well if
one of the elements of C<@roles> was too big to fit on a single line. In
that case we might end up with something like the following:

   The best Shakespearean roles are:

      * Either of the 'two foolish             *WARNING:          *
      * officers': Dogberry and Verges         *This list of roles*
      * That dour Scot, the Laird              *constitutes      a*
      * Macbeth                                *personal   opinion*
      * The tragic Moor of Venice,             *only and is in  no*
      * Othello                                *way  endorsed   by*
      * Rosencrantz's good buddy               *Shakespeare'R'Us. *
      * Guildenstern                           *It   may   contain*
      * The hideous and malevolent             *nuts.             *
      * Richard III                            *                  *

rather than:

   The best Shakespearean roles are:

      * Either of the 'two foolish             *WARNING:          *
        officers': Dogberry and Verges         *This list of roles*
      * That dour Scot, the Laird              *constitutes      a*
        Macbeth                                *personal   opinion*
      * The tragic Moor of Venice,             *only and is in  no*
        Othello                                *way  endorsed   by*
      * Rosencrantz's good buddy               *Shakespeare'R'Us. *
        Guildenstern                           *It   may   contain*
      * The hideous and malevolent             *nuts.             *
        Richard III                            *                  *

That's because the C<"*"> that's being used as a bullet for the first
column is a literal (i.e. mere decoration),
and so it will be repeated on I<every> line that
is formatted, regardless of whether that line is the start of a new
element of C<@roles> or merely the broken-and-wrapped remains of the
previous element. Happily, as we shall see later, this particular
problem has L<a simple solution|"These paper bullets of the brain...">.

Despite these minor complications, array data sources are particularly
useful when formatting, especially if the data is known to fit within
the specified width. For example:

    print form
        '-------------------------------------------',   
        'Name             Score   Time  | Normalized',   
        '-------------------------------------------',   
        '{[[[[[[[[[[[[}   {III}   {II}  |  {]]].[[} ',
         @name,           @score, @time,   [@score E<raquo>/E<laquo> @time];

is a very easy way to produce the table:

    -------------------------------------------
    Name             Score   Time  | Normalized
    -------------------------------------------
    Thomas Mowbray    88      15   |     5.867
    Richard Scroop    54      13   |     4.154
    Harry Percy       99      18   |     5.5  

Note the use of the Perl6-ish listwise division (C<< E<raquo>/E<laquo> >>)
to produce the array of data for the "Normalized" column.



=head1 More particulars must justify my knowledge...

The most commonly used fields are those that justify their contents: to
the left, to the right, to the left I<and> right, or towards the centre.

Left-justified and right-justified fields extract from their data source
the largest substring that will fit inside them, push that string to the
left or right as appropriate, and then pad the string out to the
required field width with spaces (or the L<nominated fill character|
"He doth fill fields with harness...">).

Centred fields (C<<<<< {>>>><<<<} >>>>> and C<{]]]][[[[}>) likewise
extract as much data as possible, and then pad both sides of it with
(near) equal numbers of spaces. If the amount of padding required is not
evenly divisible by 2, the one extra space is added I<after> the data.

There is a second syntax for centred fields E<ndash> a tip-o'-the-hat to
Perl 5 formats: C<{|||||||||}> and C<{IIIIIIII}>. This variant also
makes it easier to specify centering fields that are only three columns
wide: C<{|}> and C<{I}>.

Note, however, that the behaviour of centering fields specified this
way is exactly the same in every respect as the bracket-based versions, so
we're free to use whichever we prefer. 

Fully justified fields (C<<<<< {<<<<>>>>} >>>>> and C<{[[[[]]]]}>)
extract a maximal substring and then distribute any padding as evenly as
possible into the existing whitespace gaps in that data. For example:

    print form '({<<<<<<<<<>>>>>>>>>>>})',
               "A fellow of infinite jest, of most excellent fancy";

would print:

    (A fellow  of  infinite)

A fully-justified block field (C<{[[[[]]]]}>) does the same across
multiple lines, except that the very last line is always left-justified.
Hence, this:

    print form '({[[[[[[[[]]]]]]]})',
               "All the world's a stage, And all the men and women merely players."

would print:

    (All the world's a)
    (stage,  And   all)
    (the men and women)
    (merely players.  )

By the way, with both centred fields (C<<<<< {>>>><<<} >>>>>) and fully
justified fields (C<<<<< {<<<>>>>} >>>>>), the actual number of
left vs right arrows is irrelevant, so long as there is at least
one of each.


=head1 What, is't too short?

One special case we need to consider is an empty set of field delimiters:

    form 'ID number: {}'

This specification is treated as a two-column-wide, left-justified
block field (since that seems to be the type of two-column-wide
field most often required).

Other kinds of two-column (and single-column) fields can also
be created using L<imperative field widths|
"What you will command me will I do..."> and
and L<user-defined fields|"Define, define, well-educated infant.">.


=head1 Command our present numbers be muster'd...

A field specifier of the form C<<<<< {>>>>.<<} >>>>> or C<{]]]].[[}>
represents a decimal-aligned numeric field. The decimal marker always
appears in exactly the position indicated and the rest of the number is
aligned around it. The decimal places are rounded to the specific number
of places indicated, but only "significant" digits are shown. For example:

    @nums = (1, 1.2, 1.23, 11.234, 111.235, 1.0001);

    print form "Thy score be: {]]]].[[}",
                              @nums; 

prints:

    Thy score be:     1.0
    Thy score be:     1.2
    Thy score be:     1.23
    Thy score be:    11.234
    Thy score be:   111.235
    Thy score be:     1.000

The points are all aligned, the minimal number of decimal places are
shown, and the decimals are rounded (using the same rounding protocol that
C<printf> employs). Note in particular that, even though both C<1> and
C<1.0001> would normally convert to the same 3-decimal-place value
(C<1.000>), a C<form> call only shows all three zeros in the second case
since only in the second case are they "significant".

In other words, unless we L<tell it otherwise|"But say thou nought...">,
C<form> tries to avoid displaying a
number with more accuracy than it actually possesses (within the 
constraint that it must always show at least one decimal place).


=head2 Here are only numbers ratified.

You're probably wondering what happens if we try to format a number that's too
large for the available places (as C<123456.78> would be in the above format).
Whereas C<sprintf> would extend a numeric field to accommodate the number,
C<form> insists on preserving the specified layout; in particular, the
position of the decimal point. But it obviously can't just cut off the
extra high-order digits; that would change the value:

    Thy score be: 23456.78 

So, instead, it indicates that the number doesn't fit by filling the
field with octothorpes (the way many spreadsheets do):

    Thy score be: #####.###

Note, however, that it I<is> possible to 
L<change this behaviour|"That we our largest bounty may extend...">
should we need to.

It's also possible that someone (not you, of course!) might attempt to
pass a numeric field some data that isn't numeric at all:

    my @mixed_data = (1, 2, "three", {4=>5}, "6", "7-Up");

    print form 'Thy score be: {]]]].[[}', 
                              @mixed_data;


Unlike Perl itself, C<form> doesn't autoconvert non-numeric values.
Instead it marks them with another special string, by filling the field with
question-marks:

    Thy score be:     1.0  
    Thy score be:     2.0  
    Thy score be: ?????.???
    Thy score be: ?????.???
    Thy score be:     6.0  
    Thy score be: ?????.???

Note that strings per se aren't a problem E<ndash> C<form> will happily
convert strings that contain valid numbers, such as C<"6"> in the above
example. But it does reject strings that contain anything else besides
a number (even when Perl itself would successfully convert the number
E<ndash> as it would for C<"7-Up"> above).

Those who'd prefer Perl's usual, more laissez-faire attitude to
numerical conversion can just pre-numerify the values
themselves using the unary numerification operator (shown here in its
list form E<ndash> C<+E<laquo>> E<ndash> since we have an array of
values to be numerified):

    print form 'Thy score be: {]]]].[[}',
                              +E<laquo> @mixed_data;

This version would print:

    Thy score be:     1.0  
    Thy score be:     2.0  
    Thy score be:     0.0  
    Thy score be:     1.0  
    Thy score be:     6.0  
    Thy score be:     7.0  

(The C<1.0> on the fourth line appears because Perl 6 hashes numerify to the
number of entries they contain).


=head2 See how the giddy multitude do point...

Of course, not everyone uses a dot for their decimal point. The other main
contender is the comma, and naturally C<form> supports that as well. If
we specify a numeric field with a comma between the brackets:

    @les_nums = (1, 1.2, 1.23, 11.234, 111.235, 1.0001);

    print form 'Votre score est: {]]]],[[}',
                                 @les_nums; 

the call prints:

    Votre score est:     1,0
    Votre score est:     1,2
    Votre score est:     1,23
    Votre score est:    11,234
    Votre score est:   111,235
    Votre score est:     1,000

In fact, C<form> is extremely flexible about the characters
we're allowed to use as
a decimal marker: anything except an angle- or square bracket or
L<a plus sign|"That we our largest bounty may extend..."> is acceptable. 

As a bonus, C<form> allows us to use the specified decimal marker in
the I<data> as well as in the format. So this works too:

    @les_nums = ("1", "1,2", "1,23", "11,234", "111,235", "1,0001");

    print form 'Vos score est: {]]]],[[}',
                               @les_nums; 


=head2 Or else be impudently negative...

Negative numbers work as expected, with the minus sign taking
up one column of the field's allotted span:

    @nums = ( 1, -1.2,  1.23, -11.234,  111.235, -12345.67);

    print form 'Thy score be: {]]]].[[}',
                              @nums;

This would print:

    Thy score be:     1.0  
    Thy score be:    -1.2  
    Thy score be:     1.23 
    Thy score be:   -11.234
    Thy score be:   111.235
    Thy score be: #####.###

However, C<form> can also format numbers so that the minus sign I<trails> the
number. To do that we simple put an explicit minus sign inside the field
specification, at the end:

    print form 'Thy score be: {]]]].[[-}',
                              @nums;

which would then print:

    Thy score be:     1.0   
    Thy score be:     1.2-  
    Thy score be:     1.23  
    Thy score be:    11.234-
    Thy score be:   111.235 
    Thy score be: 12345.67- 

C<form> also understands the common financial usage where negative
numbers are represented as positive numbers in parentheses. Once again,
we draw an abstract picture of what we want (by putting parens at either
end of the field specification):

    print form 'Thy dividend be: {(]]]].[[)}',
                                 @nums;

and C<form> obliges:

    Thy dividend be:      1.0   
    Thy dividend be:     (1.2)  
    Thy dividend be:      1.23  
    Thy dividend be:    (11.234)
    Thy dividend be:    111.235 
    Thy dividend be: (12345.67) 

Note that the parens have to go I<inside> the field's braces. Otherwise,
they're just literal parts of the format string:

    print form 'Thy dividend be: ({]]]].[[})',
                                  @nums;

and we'd get:

    Thy dividend be: (    1.0  ) 
    Thy dividend be: (   -1.2  )  
    Thy dividend be: (    1.23 )
    Thy dividend be: (  -11.234)
    Thy dividend be: (  111.235)
    Thy dividend be: (#####.###) 


=head2 And stand a comma 'tween their amities...

If we add so-called "thousands separators" inside a numeric field at the
usual places, C<form> includes them appropriately in its output. It can
handle the five major formatting conventions:

    my @nums = (0, 1, 1.1, 1.23, 4567.89, 34567.89, 234567.89, 1234567.89);

    print form
        "Brittannic      Continental     Subcontinental   Tyrolean        Asiatic",
        "_____________   _____________   ______________   _____________   _____________",
        "{],]]],]]].[}   {].]]].]]],[}    {]],]],]]].[}   {]']]]']]],[}   {]]]],]]]].[}",
         @nums,          @nums,          @nums,           @nums,          @nums;

to produce:

    Brittannic      Continental     Subcontinental   Tyrolean        Asiatic
    _____________   _____________   ______________   _____________   _____________
             0.0             0,0              0.0             0,0             0.0 
             1.0             1,0              1.0             1,0             1.0 
             1.1             1,1              1.1             1,1             1.1 
             1.23            1,23             1.23            1,23            1.23
         4,567.89        4.567,89         4,567.89        4'567,89         4567.89
        34,567.89       34.567,89        34,567.89       34'567,89       3,4567.89
       234,567.89      234.567,89      2,34,567.89      234'567,89      23,4567.89
     1,234,567.89    1.234.567,89     12,34,567.89    1'234'567,89     123,4567.89

It also accepts a space character as a "thousands separator" (with, of
course, any decimal marker we might like):

    print form
        "Hyperspatial",
        "_____________",
        "{] ]]] ]]]:[}",
         @nums;

to produce:

    Hyperspatial
    _____________
             0:0 
             1:0 
             1:1 
             1:23
         4 567:89
        34 567:89
       234 567:89
     1 234 567:89




=head1 And gives to airy nothing a local habitation and a name

Of course, sometimes we don't know ahead of time just where in the world our
formatted numbers will be displayed. Locales were invented to address that
very problem, and C<form> supports them.

If we use the C<:locale> option, C<form> detects the current locale and
converts any numerical formats it finds to the appropriate layout. For
example, if we wrote:

    @nums = ( 1, -1.2,  1.23, -11.234,  111.235, -12345.67);

    print form 
            "{],]]],]]].[[}",
            @nums; 

then we'd get:

          1.0
         -1.2
          1.23
        -11.234
        111.235
    -12,345.67

wherever the program was run. But if we had written:

    print form
            :locale,
            "{],]]],]]].[[}",
            @nums; 

then we'd get:

          1.0
         -1.2
          1.23
        -11.234
        111.235
    -12,345.67

or:

          1,0
          1,2-
          1,23
         11,23-
        111,235
     12.345,67-

or:

          1,0
         (1,2)
          1,23
        (11,23)
        111,235
    (12'345,67)

or whatever else the current locale indicated was the correct local layout
for numbers.

That is, when the C<:locale> option is specified, C<form> ignores the actual
decimal point, thousands separator, and negation sign we specified in the call,
and instead uses the values for these markers that are returned by the
POSIX C<localeconv> function. That means that we can specify our numerical
formatting in a style that seems natural to us, and at the same time
allow the numbers to be formatted in a style that seems natural to the user.


=head1 Thou shalt have my best gown to make thee a pair...

Wait a minute...

Where exactly did we conjure that C<:locale> syntax from?
And what, exactly, did it create? What I<is> an "option"?

Well, we're passing C<:locale> as an argument to C<form>, and C<form>'s
L<signature|"Why, how now, ho! From whence ariseth this?"> guarantees us
that it can only accept a C<Str>, or an C<Array>, or a C<Pair> as an
argument. So an "option" must be one of those three types, and that
funky C<:identifier> syntax must be a constructor for the equivalent data
structure.

And indeed, that's the case. An "option" is just a pair, and the 
funky C<:identifier> syntax is just another way of writing a pair
constructor.

The standard "option" syntax is:

    :key( "value" )

which is identical in effect to:

    key => "value"

Both specify an autoquoted key; both associate that key with a value; both
evaluate to a pair object that contains the key and value.
So why have a second syntax for pairs?

Because it allows us to optimize the pair constructor syntax in two 
different ways. The now-familiar "fat arrow" pair constructor takes a
key and a value, each of which can be of any type. In contrast, the
key of an "option" pair constructor can only be an identifier, which 
is always autoquoted...at compile-time. So, if we use the "option" syntax
we're guaranteed that the key of the resulting pair is a string,
that the string that contains a valid identifier, and that the compiler
can check that validity before the program starts.

Moreover, whereas the "fat arrow" has only one syntax, "options" have several
highly useful syntactic variations. For example, "fat arrow" pairs can be
especially annoying when we want to use them to pass named boolean arguments
to a subroutine. For example:

    duel( $person1, $person2, to_death=>1, no_quarter=>1, left_handed=>1, bonetti=>1, capoferro=>1 );

In contrast, "options" have a special default behaviour. If we leave off their
parenthesized value entirely, the implied value is C<1>. So we could rewrite
the preceding function call as:

    duel( $person1, $person2, :to_death, :no_quarter, :left_handed, :bonetti, :capoferro );

Better still, when we have a series of options, we don't have to put commas
between them:

    duel( $person1, $person2, :to_death :no_quarter :left_handed :bonetti :capoferro );

That makes them even more concise and uncluttered, especially in
C<use> statements:

    use POSIX :errno_h :fcntl_h :time_h;

There are other handy "option" variants as well, all of which simply substitute
the parentheses following their key for some other kind of bracket (and hence
some other kind of value). The full list of "option"...err...options is:

      Option syntax              Is equivalent to
    ==================     =============================

    :key("some value")     key => "some value"

    :key                   key => 1

    :key{ a=>1, b=>2 }     key => { a=>1, b=>2 }

    :key{ $^arg * 2; }     key => { $^arg * 2; }

    :key[ 1, 2, 3, 4 ]     key => [ 1, 2, 3, 4 ]

    :keyE<laquo>eat at Joe'sE<raquo>     key => ["eat", "at", "Joe's"]

Despite the deliberate differences in conciseness and flexibility, we can use
"options" and "fat arrows" interchangeably in almost every situation
where we need to construct a pair (except, of course, where the key needs to be
something other than an identifier string, in which case the "fat arrow" is
the only alternative). To illustrate that interchangeability, we'll use
the "option" syntax throughout most of the rest of this discussion,
except where using a "fat arrow" is clearly preferable for code readability. 

Meanwhile, back in the fields...


=head1 Some tender money to me...

Formatting numbers gets even trickier when those numbers represent money.
But C<form> simply lets us specify how the local currency looks E<ndash>
including leading, trailing, or infix currency markers; leading, trailing, or
circumfix negation markers; thousands separators; etc. E<ndash> and then it
formats it that way. For example:

    my @amounts = (0, 1, 1.2345, 1234.56, -1234.56, 1234567.89);

    my %format = (
        "Canadian (English)"    => q/   {-$],]]],]]].[}/,
        "Canadian (French)"     => q/    {-] ]]] ]]],[ $}/,
        "Dutch"                 => q/     {],]]],]]].[-EUR}/,
        "German (pre-euro)"     => q/    {-].]]].]]],[DM}/,
        "Indian"                => q/    {-]],]],]]].[ Rs}/,
        "Norwegian"             => q/ {kr -].]]].]]],[}/,
        "Portuguese (pre-euro)" => q/    {-].]]].]]]$[ Esc}/,
        "Swiss"                 => q/{Sfr -]']]]']]].[}/,
    );

    for %format.kv -> $nationality, $layout {
        print form "$nationality:",
                   "    $layout",
                        @amounts,
                   "\n";
    }

produces:

    Swiss:
                  Sfr 0.0 
                  Sfr 1.0 
                  Sfr 1.23
              Sfr 1'234.56
             Sfr -1'234.56
          Sfr 1'234'567.89

    Canadian (French):
                      0,0 $ 
                      1,0 $ 
                      1,23 $
                  1 234,56 $
                 -1 234,56 $
              1 234 567,89 $

    Dutch:
                      0.0EUR  
                      1.0EUR  
                      1.23EUR 
                  1,234.56EUR 
                  1,234.56-EUR
              1,234,567.89EUR 

    Norwegian:
                   kr 0,0 
                   kr 1,0 
                   kr 1,23
               kr 1.234,56
              kr -1.234,56
           kr 1.234.567,89

    German (pre-euro):
                      0,0DM 
                      1,0DM 
                      1,23DM
                  1.234,56DM
                 -1.234,56DM
              1.234.567,89DM

    Indian:
                      0.0 Rs 
                      1.0 Rs 
                      1.23 Rs
                  1,234.56 Rs
                 -1,234.56 Rs
              12,34,567.89 Rs

    Portuguese (pre-euro):
                      0$0 Esc 
                      1$0 Esc 
                      1$23 Esc
                  1.234$56 Esc
                 -1.234$56 Esc
              1.234.567$89 Esc

    Canadian (English):
                     $0.0 
                     $1.0 
                     $1.23
                 $1,234.56
                -$1,234.56
             $1,234,567.89

Nice, eh?


=head1 Able verbatim to rehearse...

But sometimes I<too> nice. Sometimes all we want is an existing block
of data laid out into columns E<ndash> without any fancy reformatting
or rejustification. For example, suppose we have an interesting string
like this:

    $diagram = <<EODNA;
       G==C
         A==T
           T=A
           A=T
         T==A
       G===C
      T==A
     C=G
    TA
    AT
     A=T
      T==A
        G===C
          T==A
    EODNA

and we'd like to put beside some other text. Because it's already carefully
formatted, we really don't want to interpolate it into a left-justified field:

    print form
        '{[[[[[[[[[[[[[[[[[[[]]]]]]]]]]]]]]]]]]]]]]}       {[[[[[[[[[[[[[[[}',
         $diatribe,                                        $diagram;

Because that would squash our lovely helix:

    Men at  some  time  are  masters  of  their       G==C             
    fates: / the fault, dear Brutus, is not  in       A==T             
    our genes, / but in ourselves, that we  are       T=A              
    underlings.  /  Brutus  and  Caesar:   what       A=T              
    should be in that 'Caesar'?  /  Why  should       T==A             
    that DNA be sequenced more  than  yours?  /       G===C            
    Extract them together, yours is as  fair  a       T==A             
    genome; / transcribe them, it  doth  become       C=G              
    mRNA as well; / recombine them,  it  is  as       TA               
    long; clone with 'em, / Brutus will start a       AT               
    twin as soon as Caesar. / Now, in the names       A=T              
    of all  the  gods  at  once,  /  upon  what       T==A             
    proteins doth our Caesar feed, / that he is       G===C            
    grown so great?                                   T==A             


Nor would right-, full-, centre- or numeric- justification help in this
instance. What we really need is "leave-it-the-hell-alone"
justification E<ndash> a field specifier that lays out the data exactly as it
is, leading whitespace included.

And that's the purpose of a I<verbatim field>. A verbatim single-line field
(C<{'''''''''}>) grabs the next line of data it's offered and inserts as
much of it as will fit in the field's width, preserving whitespace "as
is". Likewise a verbatim block field (C<{"""""""""}>) grabs every line
of the data it's offered and interpolates it into the text without any
reformatting or justification.

And that's precisely what we needed for our diagram:

    print form
        '{[[[[[[[[[[[[[[[[[[[]]]]]]]]]]]]]]]]]]]]]]}       {"""""""""""""""}',
         $diatribe,                                        $diagram;

to produce:

    Men at  some  time  are  masters  of  their          G==C          
    fates: / the fault, dear Brutus, is not  in            A==T        
    our genes, / but in ourselves, that we  are              T=A       
    underlings.  /  Brutus  and  Caesar:   what              A=T       
    should be in that 'Caesar'?  /  Why  should            T==A        
    that DNA be sequenced more  than  yours?  /          G===C         
    Extract them together, yours is as  fair  a         T==A           
    genome; / transcribe them, it  doth  become        C=G             
    mRNA as well; / recombine them,  it  is  as       TA               
    long; clone with 'em, / Brutus will start a       AT               
    twin as soon as Caesar. / Now, in the names        A=T             
    of all  the  gods  at  once,  /  upon  what         T==A           
    proteins doth our Caesar feed, / that he is           G===C        
    grown so great?                                         T==A       

Note that, unlike other types of fields, verbatim fields don't
L<break and wrap their data|"A man may break a word with you, sir...">
if that data doesn't fit on a single line. Instead, they truncate each line to
the appropriate field width. So a too-short verbatim field:

    print form
        '{[[[[[[[[[[[[[[[[[[[]]]]]]]]]]]]]]]]]]]]]]}       {""""""}',
         $diatribe,                                        $diagram;

results in gene slicing:

    Men at  some  time  are  masters  of  their          G==C 
    fates: / the fault, dear Brutus, is not  in            A==
    our genes, / but in ourselves, that we  are              T
    underlings.  /  Brutus  and  Caesar:   what              A
    should be in that 'Caesar'?  /  Why  should            T==
    that DNA be sequenced more  than  yours?  /          G===C
    Extract them together, yours is as  fair  a         T==A  
    genome; / transcribe them, it  doth  become        C=G    
    mRNA as well; / recombine them,  it  is  as       TA      
    long; clone with 'em, / Brutus will start a       AT      
    twin as soon as Caesar. / Now, in the names        A=T    
    of all  the  gods  at  once,  /  upon  what         T==A  
    proteins doth our Caesar feed, / that he is           G===
    grown so great?                                         T=

rather than teratogenesis:

    Men at  some  time  are  masters  of  their          G==C 
    fates: / the fault, dear Brutus, is not  in            A=-
    our genes, / but in ourselves, that we  are       =T      
    underlings.  /  Brutus  and  Caesar:   what              -
    should be in that 'Caesar'?  /  Why  should       T=A     
    that DNA be sequenced more  than  yours?  /              -
    Extract them together, yours is as  fair  a       A=T     
    genome; / transcribe them, it  doth  become            T=-
    mRNA as well; / recombine them,  it  is  as       =A      
    long; clone with 'em, / Brutus will start a          G===C
    twin as soon as Caesar. / Now, in the names         T==A  
    of all  the  gods  at  once,  /  upon  what        C=G    
    proteins doth our Caesar feed, / that he is       TA      
    grown so great?                                  AT      
                                                   A=T    
                                                    T==A  
                                                      G==-
                                                  =C      
                                                        T-
                                                  ==A     


=head1 And now at length they overflow their banks.

It's not uncommon for a report to need a series of data fields in one
column and then a second column with only single field, perhaps
containing a summary or discussion of the other data. For example,
we might want to produce recipes of the form:

    =================[  Hecate's Broth of Ambition  ]=================

      Preparation time:             Method:                           
         66.6 minutes                  Remove the legs from the       
                                       lizard, the wings from the     
      Serves:                          owlet, and the tongue of the   
         2 doomed souls                adder. Set them aside.         
                                       Refrigerate the remains (they  
      Ingredients:                     can be used to make a lovely   
         2 snakes (1 fenny, 1          white-meat stock). Drain the   
         adder)                        newts' eyes if using pickled.  
         2 lizards (1 legless,         Wrap the toad toes in the      
         1 regular)                    bat's wool and immerse in half 
         3 eyes of newt (fresh         a pint of vegan stock in       
         or pickled)                   bottom of a preheated          
         2 toad toes (canned           cauldron. (If you can't get a  
         are fine)                     fresh vegan for the stock, a   
         2 cups of bat's wool          cup of boiling water poured    
         1 dog tongue                  over a vegetarian holding a    
         1 common or spotted           sprouted onion will do). Toss  
         owlet                         in the fenny snake, then the   
                                       legless lizard. Puree the      
                                       tongues together and fold      
                                       gradually into the mixture,    
                                       stirring widdershins at all   
                                       times.  Allow to bubble for 45 
                                       minutes then decant into two   
                                       tarnished copper chalices.         
                                       Garnish each with an owlet     
                                       wing, and serve immediately.   


There are several ways to achieve that effect. The most obvious is to
format each column separately and then lay them out side-by-side
with a pair of verbatim fields:

    my $prep = form 'Preparation time:        ',
                    '   {<<<<<<<<<<<<<<<<<<<<}', $prep_time,
                    '                         ',
                    'Serves:                  ',
                    '   {<<<<<<<<<<<<<<<<<<<<}', $serves,
                    '                         ',
                    'Ingredients:             ',
                    '   {[[[[[[[[[[[[[[[[[[[[}', $ingredients;

    my $make = form 'Method:                          ',
                    '   {[[[[[[[[[[[[[[[[[[[[[[[[[[[[}',
                        $method;

    print form 
        '=================[ {||||||||||||||||||||||||||} ]=================',
                                      $recipe,
        '                                                                  ',
        '  {"""""""""""""""""""""""}     {"""""""""""""""""""""""""""""""} ',
           $prep,                        $make;


We could even chain the calls to C<form> to eliminate the interim variables:

    print form 
        '=================[ {||||||||||||||||||||||||||} ]=================',
                                      $recipe,
        '                                                                  ',
        '  {"""""""""""""""""""""""}     {"""""""""""""""""""""""""""""""} ',
           form('Preparation time:        ',
                '   {<<<<<<<<<<<<<<<<<<<<}', $prep_time,
                '                         ',
                'Serves:                  ',
                '   {<<<<<<<<<<<<<<<<<<<<}', $serves
                '                         ',
                'Ingredients:             ',
                '   {[[[[[[[[[[[[[[[[[[[[}', $ingredients,
               ),
           form('Method:                          ',
                '   {[[[[[[[[[[[[[[[[[[[[[[[[[[[[}',
                    $method,
               );

While it's impressive to be able to do that kind of nested formatting
(and highly useful in extreme formatting scenarios), it's also far too
ungainly for regular use. A cleaner, more maintainable solution is
use a single format and just build the method column up
piecemeal, like so:

    print form 
        '=================[ {||||||||||||||||||||||||||} ]=================',
                                      $recipe,
        '                                                                  ',
        'Preparation time:               Method:                           ',
        '   {<<<<<<<<<<<<<<<<<<<<}          {<<<<<<<<<<<<<<<<<<<<<<<<<<<E<hellip>} ',
            $prep_time,                     $method,
        '                                   {E<hellip><<<<<<<<<<<<<<<<<<<<<<<<<<E<hellip>} ',
                                            $method,
        'Serves:                            {E<hellip><<<<<<<<<<<<<<<<<<<<<<<<<<E<hellip>} ',
                                            $method,
        '   {<<<<<<<<<<<<<<<<<<<<}          {E<hellip><<<<<<<<<<<<<<<<<<<<<<<<<<E<hellip>} ',
            $serves,                        $method,
        '                                   {E<hellip><<<<<<<<<<<<<<<<<<<<<<<<<<E<hellip>} ',
                                            $method,
        'Ingredients:                       {E<hellip><<<<<<<<<<<<<<<<<<<<<<<<<<E<hellip>} ',
                                            $method,
        '   {[[[[[[[[[[[[[[[[[[[[}          {E<hellip>[[[[[[[[[[[[[[[[[[[[[[[[[[[} ',
            $ingredients,                   $method;


That produces exactly the same result as the previous versions, because
each follow-on C<<<< {E<hellip><<<<<<<E<hellip>} >>>> field in the
"Method" column grabs one extra line from C<$method>, and then the final
follow-on C<{E<hellip>[[[[[[}> field grabs as many more as are required
to lay out the rest of the contents of the variable. The only down-side is
that the resulting code is still downright ugly. With all those tedious
repetitions of the same variable, there's far too much C<$method>
in our madness.

Having a series of follow-on fields like this E<ndash> vertically
continuing a single column across subsequent format lines E<ndash> is so
common that C<form> provides a special shortcut: the C<{VVVVVVVVV}>
I<overflow field>.

An overflow field automagically duplicates the field specification
immediately above it. The important point being that, because that
duplication includes copying the preceding field's data source, overflow
fields don't require a separate data source of their own.

Using overflow fields, we could rewrite our quotation generator
like this:

    print form 
        '=================[ {||||||||||||||||||||||||||} ]=================',
                                      $recipe,
        '                                                                  ',
        'Preparation time:               Method:                           ',
        '   {<<<<<<<<<<<<<<<<<<<<}          {<<<<<<<<<<<<<<<<<<<<<<<<<<<<} ',
            $prep_time,                     $method,
        '                                   {VVVVVVVVVVVVVVVVVVVVVVVVVVVV} ',
        'Serves:                            {VVVVVVVVVVVVVVVVVVVVVVVVVVVV} ',
        '   {<<<<<<<<<<<<<<<<<<<<}          {VVVVVVVVVVVVVVVVVVVVVVVVVVVV} ',
            $serves,                        
        '                                   {VVVVVVVVVVVVVVVVVVVVVVVVVVVV} ',
        'Ingredients:                       {VVVVVVVVVVVVVVVVVVVVVVVVVVVV} ',
        '   {[[[[[[[[[[[[[[[[[[[[}          {VVVVVVVVVVVVVVVVVVVVVVVVVVVV} ',
            $ingredients,
        '                                   {VVVVVVVVVVVVVVVVVVVVVVVVVVVV} ';

Which would once again produce the recipe shown earlier.

Note that the overflow fields interact equally well in formats with
single-line and block fields. That's because block overflow fields have
one other special feature: they're non-greedy. Unless we L<specify
otherwise|"I have got strength of limit">, all types of block
fields will consume their entire data source. For example, if we wrote:

    print form :layoutE<laquo>acrossE<raquo>,
         '{<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>E<hellip>}',
                                  $speech,
         '{E<hellip><<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>E<hellip>}',
                                  $speech,
         '{E<hellip>[[[[[]]]]]E<hellip>}   {="""""""""""""""""""=}   {E<hellip>[[[[[]]]]]]E<hellip>}',
             $speech,             $advert,              $speech,
         '{E<hellip>[[[[[[[[[[[[[[[[[[[[[[[[[]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]}',
                                  $speech;

we'd get:

    Now is the winter of our discontent / Made glorious summer
    by this sun of York; / And all the clouds that lour'd upon
    our house / In                             the deep  bosom
    of  the  ocean                             buried.  /  Now
    are our  brows                             bound      with
    victorious                                 wreaths; /  Our
    bruised   arms                             hung   up   for
    monuments;   /                             Our       stern
    alarums          +---------------------+   changed      to
    merry            |                     |   meetings, / Our
    dreadful         | Eat at Mrs Miggins! |   marches      to
    delightful       |                     |   measures. Grim-
    visaged    war   +---------------------+   hath   smooth'd
    his   wrinkled                             front;  /   And
    now,   instead                             of     mounting
    barded  steeds                             / To fright the
    souls       of                             fearful        
    adversaries, /                             He       capers
    nimbly  in   a                             lady's chamber.

That's because the two C<{E<hellip>[[[[[]]]]]E<hellip>}> block fields
on either side of the verbatim advertisement field will eat all the
data in C<$speech>, leaving nothing for the final format. Then
the advertisement will be centred on the two resulting columns of text.

But, block overflow fields are different.
They only take as many lines as are required to 
fill the lines generated by the non-overflow fields in their format.
So, if we changed our code to use overflows:

    print form :layoutE<laquo>acrossE<raquo>
         '{<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>}', $speech,
         '{VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}',
         '{VVVVVVVVVVVV}   {="""""""""""""""""""=}   {VVVVVVVVVVVVV}', $advert,
         '{VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}';

we get both a cleaner specification and a more elegant result:

    Now is the winter of our discontent / Made glorious summer
    by this sun of York; / And all the clouds that lour'd upon
    our house / In                             the deep  bosom
    of  the  ocean   +---------------------+   buried.  /  Now
    are our  brows   |                     |   bound      with
    victorious       | Eat at Mrs Miggins! |   wreaths; /  Our
    bruised   arms   |                     |   hung   up   for
    monuments;   /   +---------------------+   Our       stern
    alarums                                    changed      to
    merry meetings,  /  Our  dreadful  marches  to  delightful
    measures. Grim-visaged  war  hath  smooth'd  his  wrinkled
    front; / And now, instead of mounting barded steeds  /  To
    fright the souls  of  fearful  adversaries,  /  He  capers
    nimbly in a lady's chamber.

Notice that, in the third format line of the previous example, the two
overflow fields on either side of the advertisement are each overflowing
from the single field that's above both of them. This kind of multiple
overflow is fine, but it does require that we specify I<how> the various
fields overflow (i.e. as two separate columns of text, or E<ndash> as in
this case E<ndash> as a single, broken column across the page). That's
the purpose of the C<:layoutE<laquo>acrossE<raquo>> option on the
first line. This option is explained in detail L<below|"Lay out. Lay out.">.

The C<{VVVVVVVV}> fields only consumed as much data from C<$speech> as
was required to sandwich the output lines created by the verbatim
advertisement. This feature is important, because it means we can lay
out a series of block fields in one column and a single overflowed field
in another column without introducing ugly gaps. For example, because
the C<{VVVVVVVVV}> fields in:

    print form
        "Name:                                                  ",
        "  {[[[[[[[[[[[[}                                       ", $name,
        "                  Biography:                           ",
        "Status:             {<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<}", $bio,
        "  {[[[[[[[[[[[[}    {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}", $status,
        "                    {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}", 
        "Comments:           {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}",
        "  {[[[[[[[[[[[}     {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}", $comments;

only consume as much of the overflowing C<$bio> field as necessary,
the result is something like:

    Name:                                                  
      William                                             
      Shakespeare                                         
                      Biography:                          
    Status:             William Shakespeare was born on    
      Deceased (1564    April 23, 1564 in Strathford-upon- 
      -1616)            Avon, England; he was third of     
                        eight children from Father John    
    Comments:           Shakespeare and Mother Mary Arden. 
      Theories          Shakespeare began his education at 
      abound as to      the age of seven when he probably  
      the true          attended the Strathford grammar    
      author of his     school. The school provided        
      plays. The        Shakespeare with his formal        
      prime             education. The students chiefly    
      alternative       studied Latin rhetoric, logic, and 
      candidates        literature. His knowledge and      
      being Sir         imagination may have come from his 
      Francis           reading of ancient authors and     
      Bacon,            poetry. In November 1582,          
      Christopher       Shakespeare received a license to  
      Marlowe, or       marry Anne Hathaway. At the time of
      Edward de         their marriage, Shakespeare was 18 
      Vere              years old and Anne was 26. They had
                        three children, the oldest Susanna,
                        and twins- a boy, Hamneth, and a   
                        girl, Judith. Before his death on    
                        April 23 1616, William Shakespeare 
                        had written thirty-seven plays. He 
                        is generally considered the        
                        greatest playwright the world has  
                        ever known and has always been the 
                        world's most popular author.       

If C<{VVVVVVVVVVV}> fields ate their entire data E<ndash> the way
C<{[[[[[[[[[}> or C<{IIIIIIIIII}> fields do E<ndash> then the output would be
much less satisfactory. The first block overflow field for C<$bio> would
have to consume the entire biography, before the comments field was even
reached. So our output would be something like:

    Name:                                                                
      William                                                
      Shakespeare                                            
                      Biography:                          
    Status:             William Shakespeare was born on    
      Deceased (1564    April 23, 1564 in Strathford-upon- 
      -1616)            Avon, England; he was third of     
                        eight children from Father John    
                        Shakespeare and Mother Mary Arden. 
                        Shakespeare began his education at 
                        the age of seven when he probably  
                        attended the Strathford grammar    
                        school. The school provided        
                        Shakespeare with his formal        
                        education. The students chiefly    
                        studied Latin rhetoric, logic, and 
                        literature. His knowledge and      
                        imagination may have come from his 
                        reading of ancient authors and     
                        poetry. In November 1582,          
                        Shakespeare received a license to  
                        marry Anne Hathaway. At the time of
                        their marriage, Shakespeare was 18 
                        years old and Anne was 26. They had
                        three children, the oldest Susanna,
                        and twins- a boy, Hamneth, and a   
                        girl, Judith. Before his death on  
                        April 23 1616, William Shakespeare 
                        had written thirty-seven plays. He 
                        is generally considered the        
                        greatest playwright the world has  
                        ever known and has always been the 
                        world's most popular author.       

    Comments:                                               
      Theories                                               
      abound as to                                           
      the true                                               
      author of his                                          
      plays. The                                             
      prime                                                  
      alternative                                            
      candidates                                             
      being Sir                                              
      Francis                                                
      Bacon,                                                 
      Christopher                                            
      Marlowe, or                                            
      Edward de                                              
      Vere                                                   

Which is precisely why C<{VVVVVVVVVVV}> fields don't work that way.



=head1 Great floods have flown from simple sources...

When it comes to specifying the data source for each field in a format,
C<form> offers several alternatives as to where that data placed,
several alternatives as to the order in which that data is extracted, and 
an option that lets us control how the data is fitted into each field.


=head2 A man may break a word with you, sir...

Whenever a field is passed more data than it can
accommodate in a single line, C<form> is forced to "break" that data somewhere.

If the field in question is I<W>
columns wide, C<form> first squeezes any whitespace (as specified by
the L<user's C<:ws> option|"Within that space you may have drawn together...">) and then looks at the next I<W> columns of the string.
(Of course, that might actually correspond to less than I<W> I<characters>
if the string contains wide characters. However, for the sake of exposition
we'll pretend that all characters are one column wide here.)

C<form>'s breaking algorithm then searches for a newline, a carriage
return, any other whitespace character, or a hyphen. If it
finds a newline or carriage return within the first I<W> columns, it
immediately breaks the data string at that point. Otherwise it locates
the I<last> whitespace or hyphen in the first I<W> columns and breaks
the string immediately after that space or hyphen. If it can't find
anywhere suitable to break the string, it breaks it at the (I<W>-1)th
column and appends a hyphen.

So, for example:

    $data = "You can play no part but Pyramus;\nfor Pyramus is a sweet-faced man";

    print form "|{[[[[[}|",
                 $data;

prints:

    |You can|
    |play no|
    |part   |
    |but    |
    |Pyramu-|
    |s;     |
    |for    |
    |Pyramus|
    |is a   |
    |sweet- |
    |faced  |
    |man    |

Note the line-breaks after I<can> (at a whitespace), I<part> (after a
whitespace), I<sweet-> (after a hyphen), and I<s;> (at a newline). Note
too that I<Pyramus;> doesn't fit in the field, so it has to be chopped in two
and a hyphen inserted.

Of course, this particular style of line-breaking may not be suitable to all
applications, and we might prefer that C<form> use some other algorithm. For
example, if C<form> used the TeX breaking algorithm it would have broken
I<Pyramus;> less clumsily, yielding:

    |You can|
    |play no|
    |part   |
    |but    |
    |Pyra-  |
    |mus;   |
    |for    |
    |Pyramus|
    |is a   |
    |sweet- |
    |faced  |
    |man    |


To support different line-breaking strategies C<form> provides
the C<:break> option.  The C<:break> option's value must be
a closure/subroutine, which will then be called whenever a data string
needs to be broken to fit a particular field width.

That subroutine is passed three arguments: the data
string itself, an integer specifying how wide the field is, and a regex
indicating which (if any) characters are to be L<squeezed|
"Within that space you may have drawn together...">.
It is expected to return a list of two values: a string which is taken
as the "broken" text for the field, and a boolean value indicating
whether or not any data remains after the break (so C<form> knows when
to stop breaking the data string). The subroutine is also expected to
update the C<.pos> of the data string to point immediately after the
break it has imposed.

For example, if we always wanted to break at the exact width of the field
(with no hyphens), we could do that with:

    sub break_width ($data is rw, $width, $ws) {
        given $data {
            # Treat any squeezed or vertical whitespace as a single character
            # (since they'll subsequently be squeezed to a single space)
            my rule single_char { <$ws> | \v+ | . }

            # Give up if there are no more characters to grab...
            return ("", 0) unless m:cont/ (<single_char><1,$width>) /;

            # Squeeze the resultant substring...
            (my $result = $1) ~~ s:each/ <$ws> | \v+ /\c[SPACE]/;

            # Check for any more data still to come...
            my bool $more = m:cont/ <before: .* \S> /;

            # Return the squeezed substring and the "more" indicator...
            return ($result, $more);
        }
    }

    print form
        :break(&break_width),
        "|{[[[[[}|",
          $data;

producing:

    |You can|
    |play no|
    |part bu|
    |t Pyram|
    |us; for|
    |Pyramus|
    |is a sw|
    |eet-fac|
    |ed man |

Or we might prefer to break on every single whitespace-separated word:

    sub break_word ($data is rw, $width, $ws) {
        given $data {
            # Locate the next word (no longer than $width cols)
            my $found = m:cont/ \s* $?word:=(\S<1,$width>) /;

            # Fail if no more words...
            return ("", 0) unless $found{word};

            # Check for any more data still to come...
            my bool $more = m:cont/ <before: .* \S> /;

            # Otherwise, return broken text and "more" flag... 
            return ($found{word}, $more);
        }
    }

    print form
        :break(&break_word),
        "|{[[[[[}|",
          $data;

producing:

    |You    |
    |can    |
    |play   |
    |no     |
    |part   |
    |but    |
    |Pyramus|
    |;      |
    |for    |
    |Pyramus|
    |is     |
    |a      |
    |sweet-f|
    |aced   |
    |man    |


We'll see yet another application of user-defined breaking when
we discuss L<user-defined fields|"Define, define, well-educated infant.">.


=head2 He, being in the vaward, placed behind...

There are (at least) three schools of thought when it comes to setting
out a call to C<form> that uses more than one format. The
"traditional" way (i.e. the way Perl 5 formats do it) is to interleave
each format string with a line containing the data it is to
interpolate, with each datum aligned directly under the field into
which it is to be fitted. Like so:

    print form
        "Name:                                                  ",
        "  {[[[[[[[[[[[[}                                       ",
           $name,
        "                  Biography:                           ",
        "Status:             {<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<}",
                             $bio,
        "  {[[[[[[[[[[[[}    {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}",
           $status,
        "                    {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}", 
        "Comments:           {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}",
        "  {[[[[[[[[[[[}     {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}",
           $comments;

This approach has the advantage that it self-documents: to know what
a particular field is supposed to contain, we merely need to look
down one line.

It does, however, break up the "abstract picture" that the formats
portray, which can make it more difficult to envisage what the final
formatted text will look like. So some people prefer to put all the data
to the right of the formats:

    print form
        "Name:                                                  ",
        "  {[[[[[[[[[[[[}                                       ", $name,
        "                  Biography:                           ",
        "Status:             {<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<}", $bio,
        "  {[[[[[[[[[[[[}    {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}", $status,
        "                    {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}", 
        "Comments:           {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}",
        "  {[[[[[[[[[[[}     {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}", $comments;

And that's perfectly acceptable too.

Sometimes, however, the data to be interpolated doesn't come neatly
pre-packaged in separate variables that are easy to intersperse between the
formats. For example, the data might be a list returned by a
subroutine call (C<get_info($next_person)>) or might be stored in a hash
(S< C<%person{E<laquo> name biog stat comm E<raquo>}> >). In such
cases it's a nuisance to have to tease that data out into separate
variables (or hash accesses) and then sprinkle them through the formats:

    print form
        "Name:                                                  ",
        "  {[[[[[[[[[[[[}                                       ",%person{name},
        "                  Biography:                           ",
        "Status:             {<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<}",%person{biog},
        "  {[[[[[[[[[[[[}    {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}",%person{stat},
        "                    {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}", 
        "Comments:           {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}",
        "  {[[[[[[[[[[[}     {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}",%person{comm};

So C<form> has an option that lets us put a single, multi-line format
at the start of the argument list, place all the data together
after it, and have that data automatically interleaved as necessary.
Not surprisingly, that option is: C<:interleave>. It's normally used in
conjunction with a heredoc, since that's the easiest way to specify a
multi-line string in Perl:

    print form :interleave, <<'EOFORMAT',
           Name:                                                 
             {[[[[[[[[[[[[}                                      
                             Biography:                          
           Status:             {<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<}
             {[[[[[[[[[[[[}    {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
                               {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
           Comments:           {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
             {[[[[[[[[[[[}     {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
           EOFORMAT
         %person{E<laquo> name biog stat comm E<raquo>}

When C<:interleave> is in effect, C<form> grabs the first string
argument it's passed and breaks that argument up into individual lines.
It treats those individual lines as a series of distinct formats
and grabs as many of the remaining arguments as are required to
provide data for each format.

Of course, in this example we're also taking advantage of the new indenting
behaviour of heredocs. The "Name:", "Status:", and "Comments:"
titles are actually at the very beginning of their respective lines,
because the start of a Perl 6 heredoc terminator marks the left margin
of the entire heredoc string.


=head2 Would they were multitudes...

It's important to point out that, even when we're using C<form>'s
default B<non>-interleaving behaviour, it's still okay to use a format
that spans multiple lines. There I<is> however a significant (and useful)
difference in behaviour between the two alternatives.

The normal behaviour of C<form> is to take each format string,
fill in each field in the format with a substring from the
corresponding data source, and then repeat that process until all the
data sources have been exhausted. Which means that a multi-line format
like this:

    print form
         <<'EOFORMAT',
            Name:    {[[[[[[[[[[[[[[[}   Role: {[[[[[[[[[[}
            Address: {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[}
            _______________________________________________
            EOFORMAT
         @names, @roles, @addresses;

would normally produce this:

    Name:    King Lear           Role: Protagonist 
    Address: The Cliffs, Dover                     
    _______________________________________________
    Name:    The Three Witches   Role: Plot devices
    Address: Dismal Forest, Scotland               
    _______________________________________________
    Name:    Iago                Role: Villain     
    Address: Casa d'Otello, Venezia               
    _______________________________________________

because the entire three-line format is repeatedly filled in
as a single unit, line-by-line and datum-by-datum.

On the other hand, if we tell C<form> that it's supposed to automatically
interleave the data coming after the format, like so:

    print form :interleave,
         <<'EOFORMAT',
            Name:    {[[[[[[[[[[[[[[[}   Role: {[[[[[[[[[[}
            Address: {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[}
            _______________________________________________
            EOFORMAT
         @names, @roles, @addresses;

then the call produces:

    Name:    King Lear           Role: Protagonist 
    Name:    The Three Witches   Role: Plot devices
    Name:    Iago                Role: Villain     
    Address: The Cliffs, Dover                     
    Address: Dismal Forest, Scotland               
    Address: Casa d'Otello, Venezia               
    _______________________________________________

because that second version is really equivalent to:

    print form
         "Name:    {[[[[[[[[[[[[[[[}   Role: {[[[[[[[[[[}",
                   @names,                   @roles,
         "Address: {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[}",
                   @addresses,
         "_______________________________________________";


That's not much use in this particular example, but it was exactly what
was needed for the biography example earlier. It's just a matter of
choosing the right type of data placement to achieve the particular
effect we want.


=head2 Lay out. Lay out.

As we saw earlier, with L<follow-on fields|"And mark what way I make..."> and
L<overflow fields|"And now at length they overflow their banks.">, C<form>
is perfectly happy to have several fields in a single format that 
are all fed by the same data source. For example:

    print form
        "{[[[[[[[[]]]]]]]]]]E<hellip>}   {E<hellip>[[[[[[[]]]]]]]]]]E<hellip>}   {E<hellip>[[[[[[[[]]]]]]]]]]}",
             $soliloquy,             $soliloquy,              $soliloquy;

In fact, that kind of format is particularly useful for creating
multi-column outputs (like newspaper columns, for example).

But a small quandry arises. In what order should C<form> fill in these
fields? Should the data be formatted down the page, filling each column
completely before starting the next (and therefore potentially leaving
the last column "short"):

    Now is the winter  of   torious  wreaths;   /   front; / And now, in-
    our discontent / Made   Our bruised arms hung   stead of mounting ba-
    glorious  summer   by   up for  monuments;  /   rded steeds / To fri-
    this sun of  York;  /   Our stern alarums ch-   ght the souls of fea-
    And  all  the  clouds   anged to merry meeti-   rful  adversaries,  /
    that lour'd upon  our   ngs, /  Our  dreadful   He capers nimbly in a
    house / In  the  deep   marches to delightful   lady's chamber.
    bosom  of  the  ocean   measures.   /   Grim-   
    buried. / Now are our   visaged war hath smo-   
    brows bound with vic-   oth'd  his   wrinkled   

Or should the data be run line-by-line across all three columns (the
way a Perl 5 C<format> does it), filling one line completely before
starting the next:

    Now is the winter  of   our discontent / Made   glorious  summer   by
    this sun of  York;  /   And  all  the  clouds   that lour'd upon  our
    house / In  the  deep   bosom  of  the  ocean   buried. / Now are our
    brows bound with vic-   torious  wreaths;   /   Our bruised arms hung
    up for  monuments;  /   Our stern alarums ch-   anged to merry meeti-
    ngs, /  Our  dreadful   marches to delightful   measures.   /   Grim-
    visaged war hath smo-   oth'd  his   wrinkled   front; / And now, in-
    stead of mounting ba-   rded steeds / To fri-   ght the souls of fea-
    rful  adversaries,  /   He capers nimbly in a   lady's chamber.

Or should the text run down the columns, but in such a way as to leave
those columns as evenly balanced in length as possible:

    Now is the winter  of   brows bound with vic-   visaged war hath smo-
    our discontent / Made   torious  wreaths;   /   oth'd  his   wrinkled
    glorious  summer   by   Our bruised arms hung   front; / And now, in-
    this sun of  York;  /   up for  monuments;  /   stead of mounting ba-
    And  all  the  clouds   Our stern alarums ch-   rded steeds / To fri-
    that lour'd upon  our   anged to merry meeti-   ght the souls of fea-
    house / In  the  deep   ngs, /  Our  dreadful   rful  adversaries,  /
    bosom  of  the  ocean   marches to delightful   He capers nimbly in a
    buried. / Now are our   measures.   /   Grim-   lady's chamber.

Well, of course, there's no "right" answer to that; it depends entirely
on what kind of effect we're trying to achieve.

The first approach (i.e. lay out the text down each column first) works
well if we're formatting a news-column, or a report, or a description of
some kind. The second (i.e. lay out the text across each line first), is
excellent for putting diagrams or call-outs in the middle of a piece of
text (as we did for L<Mrs Miggins|"And now at length they overflow
their banks.">). The third approach (i.e. lay out the data downwards but
balance the columns) is best for presenting a single list of data in
multiple columns E<ndash> like C<ls> does.

So we need an option with which to tell C<form> which of these useful
alternatives we want for a particular format. That option is named
C<:layout> and can take one of three string values: C<"down">, C<"across">,
or C<"balanced">. So, for example, to produce three versions of Richard III's
famous monologue in the order shown above, we'd use:

    print form :layoutE<laquo>downE<raquo>,
        "{[[[[[[[[]]]]]]]]]]E<hellip>}   {E<hellip>[[[[[[[]]]]]]]]]]E<hellip>}   {E<hellip>[[[[[[[[]]]]]]]]]]}",
             $soliloquy,             $soliloquy,              $soliloquy;

then:

    print form :layoutE<laquo>acrossE<raquo>,
        "{[[[[[[[[]]]]]]]]]]E<hellip>}   {E<hellip>[[[[[[[]]]]]]]]]]E<hellip>}   {E<hellip>[[[[[[[[]]]]]]]]]]}",
             $soliloquy,             $soliloquy,              $soliloquy;

then:

    print form :layoutE<laquo>balancedE<raquo>,
        "{[[[[[[[[]]]]]]]]]]E<hellip>}   {E<hellip>[[[[[[[]]]]]]]]]]E<hellip>}   {E<hellip>[[[[[[[[]]]]]]]]]]}",
             $soliloquy,             $soliloquy,              $soliloquy;

By the way, the default value for the C<:layout> option is C<"balanced">
since formatting regular columns of data is more common than formatting
news or advertising inserts.


=head2 For the table, sir, it shall be served...

The C<:layout> option controls one other form of inter-column formatting:
tabular layout.

So far, all the examples of tables we've created (for example, our 
L<normalized scores|"Therefore, put you in your best array...">)
lined up nicely. But that was only because each item in each row
happened to take the same number of lines (typically just one).
So, a table generator like this:

    my @play = map {"$_\r"}  ( "Othello", "Richard III", "Hamlet"   );
    my @name = map {"$_\r"}  ( "Iago",    "Henry",       "Claudius" );

    print form 
         "Character       Appears in  ",
         "____________    ____________",
         "{[[[[[[[[[[}    {[[[[[[[[[[}",
          @name,          @play;

correctly produces:

    Character       Appears in
    ____________    ____________
    Iago            Othello     

    Henry           Richard III

    Claudius        Hamlet      

Note that we appended C<"\r"> to each element to add an extra
newline after each entry in the table. We can't use C<"\n"> to specify a
line-break within an array element, because C<form> uses C<"\n"> as an
L<"end-of-element" marker|"Therefore, put you in your best array...">.
So, to allow line breaks within a single element of an array datum,
C<form> treats C<"\r"> as "end-of-line-but-not-end-of-element"
(somewhat like Perl 5's C<format> does).

However, if we were to use the full titles for each character and each play:

    my @play = map {"$_\r"}  ( "Othello, The Moor of Venice",
                               "The Life and Death of King Richard III",
                               "Hamlet, Prince of Denmark",
                             );

    my @name = map {"$_\r"}  ( "Iago",
                               "Henry,\rEarl of Richmond",
                               "Claudius,\rKing of Denmark",
                             );

the same formatter would produce:

    Character       Appears in
    ____________    ____________
    Iago            Othello, The
                    Moor of     
    Henry,          Venice      
    Earl of
    Richmond        The Life and         
                    Death of    
    Claudius,       King Richard
    King of         III         
    Denmark         
                    Hamlet,     
                    Prince of   
                    Denmark     

The problem is that the two block fields we're using just grab all
the data from each array and format it independently into each column.
Usually that's fine because the columns I<are> independent (as we've
L<previously seen|"Able verbatim to rehearse...">).

But in a table, the data in each column specifically relates to data
in other columns, so corresponding elements from the column's data
arrays ought to remain vertically aligned. To achieve this, we simply
tell C<form> that the data in the various columns should be laid out
like a table:

    print form :layoutE<laquo>tabularE<raquo>,
         "Character       Appears in  ",
         "____________    ____________",
         "{[[[[[[[[[[}    {[[[[[[[[[[}",
          @name,          @play;

which then produces the desired result:

    Character       Appears in
    ____________    ____________
    Iago            Othello, The
                    Moor of     
                    Venice      

    Henry,          The Life and
    Earl of         Death of    
    Richmond        King Richard
                    III         

    Claudius,       Hamlet,     
    King of         Prince of   
    Denmark         Denmark     


=head1 Give him line and scope...

Sometimes we want to use a particular option or combination of options
in every call we make to C<form>. Or, more likely, in every call we make
within a specific scope.  For example, we might wish to default to
a different
L<line-breaking algorithm|"A man may break a word with you, sir...">
everywhere, or we might want to make repeated use of
L<a new type of field specifier|"Define, define, well-educated infant.">,
or we might want to L<reset the standard page length|
"Measure his woe the length and breadth of mine..."> from a
printable 60 to a screenable 24.

Normally in Perl 6, if we wanted to preset a particular optional
argument we'd simply make an assumption:

    my &down_form := &form.assuming(:layoutE<laquo>downE<raquo>);

But, of course, C<form> collects all of its arguments in L<a single slurpy
array|"Why, how now, ho! From whence ariseth this?">, so it
doesn't actually I<have> a C<$layout> parameter that we can prebind.

Fortunately, the C<.assuming> method is smart enough to recognize when it
being applied to a subroutine whose arguments are slurped. In such cases,
it just prepends any prebound arguments to the resulting subroutine's argument
list. That is, the binding of C<down_form> shown above is equivalent to:

    my &down_form :=
        sub (FormArgs *@args is context(Scalar)) returns Str {
            return form( :layoutE<laquo>downE<raquo>, *@args );
        };


=head2 This was your default...

C<form> provides one other mechanism by which options can be prebound.
To use it, we (re-)load the Form module with an explicit argument list:

    use Form :layoutE<laquo>downE<raquo>, :locale, :interleave;

This causes the module to export a modified version of C<form> in which the
specified options are prebound.  That modified version of C<form> is exported
lexically, and so C<form> only has the specified defaults preset for the
scope in which the C<use Form> statement appears.

These default options are handy if we have a series of calls
to C<form> that all need some consistent non-standard behaviour.
For example:

    use Form :layoutE<laquo>acrossE<raquo>,
             :interleave,
             :page{ :header("Draft $(localtime)\n\n") };

    print form $introduction_format, *@introduction_data;

    for @sections -> $format, @data {
        print form $format, *@data;
    }

    print form $conclusion_format, *@conclusion_data;

Another use is to set up a fixed formatting string into which different data
is to be interpolated (much in the way Perl 5 formats are typically used).
For example, we might want a standard format for errors in a C<CATCH> block:

    CATCH {
        use Form :interleave, <<EOFORMAT;
                     Error {<<<<<<<}: {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[}
                     ___________________________________________________
                     EOFORMAT

        when /Missing datum/ { warn form "EMISSDAT", $_.msg }
        when /too large/     { warn form "ETOOBIG",  $_.msg }
        when .core           { warn form "EINTERN",  "Internal error" }
        default              { warn form "EUNKNOWN", "Seek help" }
    }



=head1 And welcome to the wide fields...

All the fields we've seen so far have been exactly as wide as their
specifications. That's the whole point of having fields E<ndash> 
they allow us to lay out formats "by eye".

But C<form> also allows us to specify field widths in other ways. And better
yet, to avoid specifying them at all and let C<form> work out how big
they should be.


=head2 The measure then of one is easily told.

When specific field widths are required (perhaps by some design document
or data formatting protocol) laying out wide fields can be error-prone.
For example, most people can't visually distinguish between a
52-column field and a 53-column field and are therefore forced to manually
verify the width of the corresponding field specifier in some way.

When such fields are part of a larger format, errors like that can
easily result in a call to C<form> producing, say, 81-column lines. That
would merely be messy if the extra characters wrapped, but could
be disasterous if they happened to be chopped instead. Suppose,
for example, that the last 4 columns of output contain nuclear reactor
core temperatures and then consider the difference between an
apparently normal reading of 567 Celsius and what might actually be
happening if the reading were in fact a truncated 5678 Celsius.

To catch mistakes of this kind, fields can be specified with an
embedded integer in parentheses (with optional whitespace inside the
parens). For example:

    print form '{[[[( 15 )[[[[} {<<<<<(17)<<<<<<}  {]]](14)]]].[[}',
               *@data;

The integer in the parentheses acts like a checksum. Its value
must be identical to the actual width of the field (including the
delimiting braces and the embedded integer itself). Otherwise an
exception is thrown. For instance, running the above example produces
the error message:

    Inconsistent width for field 3.
    Specified as '{]]](14)]]].[[}' but actual width is 15
    in call to &form at demo.pl line 1

Numeric fields can be given a decimal checksum, which then also
specifies their number of decimal places.

    print form
        '{[[[( 15 )[[[[} {<<<<<(17)<<<<<<}  {]](14.2)]].[}',
         *@data;

Note that the digits before the decimal still indicate
the total width of the field. So the C<{]](14.2)]].[}> field
in the above example means I<must be 14 columns wide, including
2 decimal places>, in exactly the same way as a C<"%14.2f">
specifier would in a C<sprintf>.


=head2 What you will command me will I do...

Of course, in some instances it would be much more convenient if we
could simply I<tell> C<form> that we want a particular field to be
a particular width, instead of having to explicitly I<show> it.

So there's another type of integer field annotation that, instead of
acting like a checksum, acts like an...err..."tellsum". That is, we
can tell C<form> to ignore a field's physical width and instead
insist that it be magically expanded (or shrunk) to a nominated width. Such
a field is said to have an I<imperative width>. The integer specifying
the imperative width is placed in curly braces instead of parens.

For example, the format in the previous example could be specified
imperatively as:

    print form
        '{[{15}[} {<{17}<<}  {]]]]{14.2}]]]].[[}',
         *@data;

Note that the actual width of any field becomes irrelevant if it
contains an imperative width. The field will be condensed or expanded to
the specified width, with subsequent fields pushed left or right
accordingly.

Imperative fields disrupt the WYSIWYG layout of a format, so they're generally
only used when the format itself is being generated programmatically. For
example, when we were counting down the L<top ten reasons not to do one's
English Lit homework|"What a block art thou...">, we used a fixed-width
C<< {>} >> field to format each number:

    for @reasons.kv -> $n, $reason {
        my $n = @reasons - $index ~ '.';
        print form "   {>}  {[[[[[[[[[[[[[[[[[[[[[[[[[[[[}",
                       $n,  $reason,
                   "";
    }

But, of course, there's not reason (theoretically, at least) why we couldn't
find more than 99 reasons not to do our homework, in which case we'd
overflow the C<< {>} >> field.

So instead of limiting ourselves that way, we could just tell C<form> to make
the first field wide enough to enumerate however many reasons we come up with,
like so:

    my $width = length(+@reasons)+1;

    for @reasons.kv -> $n, $reason {
        my $n = @reasons - $index ~ '.';
        print form "   {>>{$width}>>}  {[[[[[[[[[[[[[[[[[[[[[[[[[[[[}",
                       $n,             $reason,
                   "";
    }

By evaluating C<@reasons> in a numeric context (C<+@reasons>) we determine the
number of reasons we have, and hence the largest number that need ever fit
into the first field. Taking the length of that number (C<length(+@reasons)>)
gives us the number of digits in that largest number and hence the width of a
field that can format that number. We add one extra column (for the dot
we're appending to each number) and that's our required width. Then we just
tell C<form> to make the first field that wide (C<<< {>>{$width}>>} >>>).

=head2 And every one shall share...

A special form of imperative width field is the I<starred field>.
A starred field is one that contains an imperative width
specification in which the number is replaced by a single asterisk.

The width of a starred field is not fixed, but rather is I<computed>
during formatting. That width is whatever is required to cause the
entire format to fill the current page width of the format (by default,
78 columns). Consider, for example:

    print form
        '{]]]]]]]]]]]]]]} {]]].[[}  {[[{*}[[}  ',
         @names,          @scores,  @comments;

The width of the starred comment field in this case is 49 columns E<ndash>
the default page width of 78 columns minus the 29 columns
consumed by the fixed-width portions of the format (including the other two
fields).

If a format contains two or more starred fields, the available space
is shared equally between them. So, for example, to create two equal columns
(say, to compare the contents of two files), we might use:

    print form 
         "{[[[[{*}[[[[}   {[[[[{*}[[[[}",
          slurp($file1),  slurp($file2);

And, yes, Perl 6 does have a built-in C<slurp> function that takes a filename,
opens the file, reads in the entire contents, and returns them as a single
string. For more details see the Perl6::Slurp module (now on the CPAN).

There is one special case for starred fields: a starred verbatim field:

    {""""{*}""""}

It acts like any other starred field, growing according to the available
space, except that it will never grow any wider than the widest line
of the data it is formatting. For example, whereas a regular starred 
field:

    print form 
         '| {[[{*}[[} |',
            $monologue;

expands to the full page width:

    | Now is the winter of our discontent                           |
    | Made glorious summer by this sun of York;                     |
    | And all the clouds that lour'd upon our house                 |
    | In the deep bosom of the ocean buried.                        |
    | Now are our brows bound with victorious wreaths               |  
    | Our bruised arms hung up for monuments;                       |
    | Our stern alarums changed to merry meetings,                  |
    | Our dreadful marches to delightful measures.                  |
    | Grim-visaged war hath smooth'd his wrinkled front;            |
    | And now, instead of mounting barded steeds                    |  
    | To fright the souls of fearful adversaries,                   |  
    | He capers nimbly in a lady's chamber.                         |  


a starred verbatim field:

    print form 
         '| {""{*}""} |',
            $monologue;

only expands as much as is strictly necessary to accommodate the data:

    | Now is the winter of our discontent                |
    | Made glorious summer by this sun of York;          |
    | And all the clouds that lour'd upon our house      |
    | In the deep bosom of the ocean buried.             |
    | Now are our brows bound with victorious wreaths;   |  
    | Our bruised arms hung up for monuments;            |
    | Our stern alarums changed to merry meetings,       |
    | Our dreadful marches to delightful measures.       |
    | Grim-visaged war hath smooth'd his wrinkled front; |
    | And now, instead of mounting barded steeds         |  
    | To fright the souls of fearful adversaries,        |  
    | He capers nimbly in a lady's chamber.              |  


=head2 That we our largest bounty may extend...

By now you've probably noticed that there is quite a large overlap between the
functionality of C<form> and that of C<(s)printf>. For example, the call:

    for @procs {
        print form
            "{>>>}  {<<<<<<<(20)<<<<<<<}  {>>>>>>}  {>>.}%",
            .{pid}, .{cmd},               .{time},  .{cpu};
    }

has approximately the same effect as the call:

    for @procs {
        printf "%5d  %-20s  %8s  %5.1f%%\n",
               .{pid}, .{cmd}, .{time}, .{cpu};
    }

One is more WYSIWYG, the other more concise, but (placed in a suitable loop),
they would both print out lines like these:

     2461  vi -ii henry           0:55.83   11.6%
     2395  ex cathedra            0:06.59    3.5%
     2439  head anne.boleyn       0:00.18    0.1%
     2581  dig -short grave       0:01.04    0.0%

There is, however, a crucial difference between these two formatting
facilities; one that only shows up when one of our processes runs over 99
hours. For example, suppose our browser has been running continuously
for a few months (or, more precisely, for 1214:23.75 hours). Then the
calls to C<printf> would print:

     2461  vi -ii henry           0:55.83   11.6%
     2395  ex cathedra            0:06.59    3.5%
    27384  lynx www.divorce.com  1214:23.75    0.8%
     2439  head anne.boleyn       0:00.18    0.1%
     2581  dig -short grave       0:01.04    0.0%

whilst the calls to C<form> would print:

     2461  vi -ii henry           0:55.83   11.6%
     2395  ex cathedra            0:06.59    3.5%
    27384  lynx www.divorce.com  1214:23-    0.8%
     2439  head anne.boleyn       0:00.18    0.1%
     2581  dig -short grave       0:01.04    0.0%

In other words, field widths in a C<printf> represent I<minimal> spacing
(even if that throws off the overall layout), whereas field widths in a
C<form> represent I<guaranteed> spacing (even if that truncates some of
the data).

Of course, in a situation like this E<ndash> where we knew that the data might
not fit and we didn't want it truncated E<ndash> we could use a block field
instead:

    for @procs {
        print form
            "{>>>}  {<<<<<<<(19)<<<<<<}  {]]]]]]}  {>>.%}",
            .{pid}, .{cmd},              .{time},  .{cpu};
    }

in which case we'd get:

     2461  vi -ii henry           0:55.83   11.6%
     2395  ex cathedra            0:06.59    3.5%
    27384  lynx www.divorce.com  1214:23-    0.8%
                                      .75
     2439  head anne.boleyn       0:00.18    0.1%
     2581  dig -short grave       0:01.04    0.0%

That preserves the data, but the results are still ugly, and it also
requires some fancy footwork E<ndash> making the percentage sign part of
the field specification, as if it were L<a currency marker|
"Some tender money to me..."> E<ndash> to make the last field
work correctly. In other words: it's a kludge. The sad truth is that
sometimes variable-width fields are a better solution.

So C<form> provides them too. Any field specification may include a
plus sign (C<+>) anywhere between its braces, in which case it
specifies an I<extensible field>: a field whose width is minimal,
rather than absolute. So, in the above example, our call to C<form>
should actually look like this:

    for @procs {
        print form
            "{>>>}  {<<<<<<<(20)<<<<<<<}  {>>>>>+}  {>>.}%",
            .{pid}, .{cmd},               .{time},  .{cpu};
    }

and would produce this:

     2461  vi -ii henry           0:55.83   11.6%
     2395  ex cathedra            0:06.59    3.5%
    27384  lynx www.divorce.com  1214:23.75    0.8%
     2439  head anne.boleyn       0:00.18    0.1%
     2581  dig -short grave       0:01.04    0.0%

just like C<printf> does.

Likewise, if we thought the command names might exceed 20 columns we
could let that field stretch too:

    for @procs {
        print form
            "{>>>}  {<<<<<<<(20+)<<<<<<}  {>>>>>+}  {>>.}%",
            .{pid}, .{cmd},               .{time},  .{cpu};
    }

Note that the field width specifier would still warn us if the field's
"picture" was not exactly 20 columns wide, but the resulting field
would nevertheless stretch as necessary to accommodate longer data.


=head2 Within that space you may have drawn together...

When a field is being filled in, whitespace is normally left as-is
(except for justification, and wrapping of lines in block fields).
However, this behaviour can be altered by specifying a I<whitespace
squeezing> strategy. Squeezing replaces those substrings of the data
that match a specified pattern (for example: C</\s+/>), substituting
a single space character.

If we don't want the default (non-)squeezing strategy we can use
the C<:ws> option specify the particular pattern that is to be
used for squeezing:

    print form
        :ws(/\h+/),            # squeeze any horizontal whitespace
        $format1, *@data1,
        :ws(/<comment>|\s+/),  # now squeeze comments or whitespace
        $format2, *@data2;

For example, suppose we have a eulogy generator:

    sub eulogize ($who, $to, $blaming) {...}

that (rather poorly) drops the appropriate names into a pre-formatted template,
to produce strings like:

    Friends,   Romans  , countrymen, lend me your ears;
    I come to bury    Caesar   , not to praise him.
    The evil that men do lives after them;
    The good is oft interred with their bones;
    So let it be with    Caesar    . The noble    Brutus
    Hath told you     Caesar     was ambitious:
    If it were so, it was a grievous fault,
    And grievously hath    Caesar    answer'd it.

If we interpolate that string, with its extra spaces and its embedded
newlines, into a C<form> field:

    print form
         "| {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[} |",
            eulogize('Caesar', to=>'Romans', blaming=>'Brutus');

we'd get:

    | Friends,   Romans  , countrymen, lend me   |
    | your ears;                                 |
    | I come to bury    Caesar   , not to praise |
    | him.                                       |
    | The evil that men do lives after them;     |
    | The good is oft interred with their bones; |
    | So let it be with    Caesar    . The noble |
    | Brutus                                     |
    | Hath told you     Caesar     was           |
    | ambitious:                                 |
    | If it were so, it was a grievous fault,    |
    | And grievously hath    Caesar    answer'd  |
    | it.                                        |

Note that the extra spaces and the embedded newlines
are preserved in the resulting text.

But, if we told C<form> to squeeze all whitespaces:

    print form :ws(/\s+/),
         "| {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[} |",
            eulogize('Caesar', to=>'Romans', blaming=>'Brutus');

we'd get:

    | Friends, Romans , countrymen, lend me your |
    | ears; I come to bury Caesar , not to       |
    | praise him. The evil that men do lives     |
    | after them; The good is oft interred with  |
    | their bones; So let it be with Caesar .    |
    | The noble Brutus Hath told you Caesar was  |
    | ambitious: If it were so, it was a         |
    | grievous fault, And grievously hath Caesar |
    | answer'd it.                               |

with each sequence of characters that match C</\s+/> being reduced
to a single space.

On the other hand, if we wanted to preserve the newlines and squeeze
only horizontal whitespace, that would be:

    print form :ws(/\h+/),
         "| {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[} |",
            eulogize('Caesar', to=>'Romans', blaming=>'Brutus');

which produces:

    | Friends, Romans , countrymen, lend me your |
    | ears;                                      |
    | I come to bury Caesar , not to praise him. |
    | The evil that men do lives after them;     |
    | The good is oft interred with their bones; |
    | So let it be with Caesar . The noble       |
    | Brutus                                     |
    | Hath told you Caesar was ambitious:        |
    | If it were so, it was a grievous fault,    |
    | And grievously hath Caesar answer'd it.    |

Of course, for this particular text, none of these solutions is entirely
satisfactory since squeezing the whitespaces to a single space still leaves a
single space in places like C<"Caesar ."> and C<"Romans ,">.

To remove those blemishes we need to take advantage of a more
sophisticated aspect of C<form>'s whitespace squeezing behaviour. Namely
that, when squeezing whitespace using a particular pattern, C<form>
detects if that pattern captures anything and I<doesn't> squeeze the 
captured items.

More precisely, if the squeeze pattern matches but doesn't capture,
C<form> simply replaces the entire match with a single space character.
But if the squeeze pattern I<does> capture, C<form> doesn't insert a
space character, but instead replaces the entire match with the
concatenation of the captured substrings.

That means we can completely eliminate any whitespace before a punctuation
character with:

    print form :ws(/\h+ (<punct>)?/),
         "| {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[} |",
            eulogize('Caesar', to=>'Romans', blaming=>'Brutus');

which produces the desired:

    | Friends, Romans, countrymen, lend me your  |
    | ears;                                      |
    | I come to bury Caesar, not to praise him.  |
    | The evil that men do lives after them;     |
    | The good is oft interred with their bones; |
    | So let it be with Caesar. The noble Brutus |
    | Hath told you Caesar was ambitious:        |
    | If it were so, it was a grievous fault,    |
    | And grievously hath Caesar answer'd it.    |

This works because, in those instances where the pattern
C<< /\h+ (<punct>)?/ >> matches some whitespace followed by
a punctuation character, the punctuation character is captured,
and the captured character is then used to replace the entire
whitespace-plus-punctuator. On the other hand, if the
pattern matches whitespace but no punctuator (and it's allowed to do that
because the punctuator is optional), then nothing is captured, so
C<form> falls back to replacing the whitespace with a single space.


=head2 He doth fill fields with harness...

Fields are (almost) always of a fixed width. So, if there isn't
enough data to fill a particular field, the unused portions of that
field are filled in with spaces to preserve the vertical alignment of
other columns of formatted data. However, spaces are only the
default. The C<:hfill> (horizontal fill) option can be used to change
fillers. For example:

    print form
        :hfill("=-"),                   # Fill next fields with "=-"
        "{|{*}|}\n",                    # Full width field for title
        "[ Table of Contents ]",        # Title
        :hfill(" ."),                   # Fill next fields with spaced dots
        '   {[[[[[{*}[[[[[}{]]]}   ',   # Two indented block fields
            @contents,     @page;       # Data for those blocks

This fills the empty space either side of the centred title with a repeated
C<=-=-=-> sequence. It then fills the gaps to the right of the left-justified
the contents field, and to left of the right-justified pages field,
with spaced dots. Which, rather prettily, produces something like:

    =-=-=-=-=-=-=-[ Table of Contents ]-=-=-=-=-=-=-=

       Foreword. . . . . . . . . . . . . . . . . .i   
       Preface . . . . . . . . . . . . . . . . .iii   
       Glossary. . . . . . . . . . . . . . . . . vi   
       Introduction. . . . . . . . . . . . . . . .1   
       The Tempest . . . . . . . . . . . . . . . .7   
       Two Gentlemen of Verona . . . . . . . . . 17   
       The Merry Wives of Winsor . . . . . . . . 27   
       Twelfh Night. . . . . . . . . . . . . . . 39   
       Measure for Measure . . . . . . . . . . . 50   
       Much Ado About Nothing. . . . . . . . . . 62   
       A Midsummer Night's Dream . . . . . . . . 73   
       Love's Labour's Lost. . . . . . . . . . . 82   
       The Merchant of Venice. . . . . . . . . . 94   
       As You Like It. . . . . . . . . . . . . .105   

Note that the fill sequence doesn't have to be a single character and
that the fill pattern is consistent across multiple fields and between
adjacent lines. That is, it's as if every field is first filled with the
same fill pattern, then the actual data written over the top.
That's particularly handy in the above example, because it ensures that
the fill pattern seamlessly bridges the boundary between the adjacent
contents and pages fields.

It's also possible to specify separate fill sequences for the left-
and right-hand gaps in a particular field, using the C<:lfill> and C<:rfill>
options. This is particularly common for numerical fields. For example,
this call to C<form>:

    print form 
      'Name              Bribe (per dastardry)',
      '=============     =====================',
      '{[[[[[[[[[[[}         {]],]]].[[[}     ',
      @names,                @bribes;

would print something like:

    Name              Bribe (per dastardry)
    =============     =====================
    Crookback                  12.676
    Iago                        1.62
    Borachio               45,615.0
    Shylock                    19.0003

with the numeric field padded with whitespace and 
only showing as many decimal places as there are in
the data.

However, in order to prevent subsequent..err...creative calligraphy
(they I<are>, after all, villains and would presumably not hesitate
to add a few digits to the front of each number), we might prefer to
put stars before the numbers and show all decimal places.
We could do that like so:

    print form 
      'Name              Bribe (per dastardry)',
      '=============     =====================',
      '{[[[[[[[[[[[}         {]],]]].[[[}     ',
      @names,                :lfill('*'), :rfill('0'),
                             @bribes;

which would then print:

    Name              Bribe (per dastardry)
    =============     =====================
    Crookback             *****12.6760
    Iago                  ******1.6200
    Borachio              *45,615.0000
    Shylock               *****19.0003

Note that the C<:lfill> and C<:rfill> options are specified I<after> the
format string and, more particularly, before the data for the second
field. This means that those options only take effect for that
particular field and the previous fill behaviour is then reasserted
for subsequent fields. Many other C<form> options E<ndash> for example C<:ws>,
C<:height>, or C<:break> E<ndash> can be specified in this way, so as to
apply them only to a particular field.

There is also a general C<:fill> option that sets the default
sequence for any filling that isn't otherwise specified.


=head2 But say thou nought...

Filling numeric fields with zeros is so common that C<form> offers a
shorthand notation for it. If the first character inside a numeric field specification is a zero, then the left-fill string for that field is set to C<"0">.
Likewise if the last character in the field is a zero, it is right-filled
with zeros. For example:

    my @nums = (0, 1, -1.2345, 1234.56, -1234.56, 1234567.89);

    print form
        "{]]]].[[}     {]]]].[0}     {0]]].[[}     {0]]].[0}",
         @nums,        @nums,        @nums,        @nums;

prints:

        0.0           0.000     00000.0       00000.000
        1.0           1.000     00001.0       00001.000
       -1.234        -1.234     -0001.234     -0001.234
     1234.56       1234.560     01234.56      01234.560
    -1234.56      -1234.560     -1234.56      -1234.560
    #####.###     #####.###     #####.###     #####.###



=head2 Up and down, up and down, I will lead them up and down...

Formatted text blocks are also filled vertically. Empty lines at the end
of the block are normally filled with spaces (so as to preserve the
alignment of any other fields on the same line). However, this too can
be controlled, with the C<:vfill> option. Alternatively E<ndash> as with
horizontal filling E<ndash> separate fill sequences can be specified for
above and below the text using the C<:tfill> and C<:bfill> ("top" and
"bottom" fill) options.

For example, if we had six elements in C<@task>, but only four processors:

    print form
        :bfill('[unallocated]'),
        'Task                      Processor',
        '====                      =========',
        '{[[[[[[[[[[[[[[[[[[[[}  {]]]]]][[[[[}',
         @task,                      [1..4];

we'd get:

    Task                      Processor
    ====                      =========
    Borrow story                  1      
    Rename characters             2      
    Subdivide into scenes         3      
    Write dialogue                4      
    Check rhythm and meter  [unallocated]
    Insert puns and japes   [unallocated]


=head2 I have got strength of limit...

It is possible to constrain the minimum and maximum number of lines
that a particular format or block field must cover, regardless of how much
data it contains. We do that using the C<:height> option. For example:

    print form
        :height(3),
        '{[[[[}{IIII}{]]]]}',
         $l,   $c,   $r;

This will cause the call to C<form> to generate exactly three output lines,
even if the contents of the data variables would normally fit in fewer lines
or would actually require more.

To specify a range of heights we can use the C<:min> and C<:max> suboptions:

    print form
        :height{ :min(3), :max(20) },
        '{[[[[}{IIII}{]]]]}',
         $l,   $c,   $r;

This specifies that, no matter how much data is available, the output will be
no less than three lines and no more than 20.

Note, however, that the C<height> option refers to the height of individual
fields, not of entire output pages. we'll see how to control the
latter L<anon|"Measure his woe the length and breadth of mine...">.


=head2 Forbear, and eat no more.

As we saw L<earlier|"And now at length they overflow their banks.">,
a block overflow field (C<{VVVVVVVVV}>)
has the special property that it only overflows as much as necessary to
fill the output lines generated by other block fields. That enabled us
to create an overflowing column of text like so:

    print form
        :interleave, <<EOFORMAT,
            Name:
              {[[[[[[[[[[[[}
                              Biography:
            Status:             {<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<}
              {[[[[[[[[[[[[}    {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
                                {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
            Comments:           {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
              {[[[[[[[[[[[}     {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
            EOFORMAT
        $name,
        $biography,
        $status,
        $comments;

without the first C<{VVVVVVVVV}> field eating all the data out of C<$bio> and
leaving a large gap between the Status and the Comments.

That's a very handy feature, but restricting the "minimal height" feature
to overflow fields turns out to be not good enough in the general case.
For instance, suppose we had wanted the biography field to start at the
first line of the output text:

    Name:             Biography:                            
      William           William Shakespeare was born on    
      Shakespeare       April 23, 1564 in Strathford-upon- 
                        Avon, England; he was third of     
    Status:             eight children from Father John    
      Deceased (1564    Shakespeare and Mother Mary Arden. 
      -1616)            Shakespeare began his education at 
                        the age of seven when he probably  
    Comments:           attended the Strathford grammar    
      Theories          school. The school provided        
      abound as to      Shakespeare with his formal        
      the true          education. The students chiefly    
      author of his     studied Latin rhetoric, logic, and 
      plays. The        literature. His knowledge and      
      prime             imagination may have come from his 
      alternative       reading of ancient authors and     
      candidates        poetry. In November 1582,          
      being Sir         Shakespeare received a license to  
      Francis           marry Anne Hathaway. At the time of
      Bacon,            their marriage, Shakespeare was 18 
      Christopher       years old and Anne was 26. They had
      Marlowe, or       three children, the oldest Susanna,
      Edward de         and twins- a boy, Hamneth, and a   
      Vere              girl, Judith. Before his death on
                        April 23 1616, William Shakespeare
                        had written thirty-seven plays. He
                        is generally considered the greatest
                        playwright the world has ever known
                        and has always been the world's most
                        popular author.

To do that, we would have required a call to C<form> like this:

    print form
        :interleave, <<EOFORMAT,
            Name:             Biography:
              {[[[[[[[[[[[[}    {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[}
                                {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
            Status:             {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
              {[[[[[[[[[[[[}    {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
                                {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
            Comments:           {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
              {[[[[[[[[[[[}     {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
            EOFORMAT
        $name,
        $biography,
        $status,
        $comments;

Note that the first line of the Biography field now has to be a block field,
not a single-line field (as in previous versions). It can't be a single-line,
because the Name field is a block field and that would leave a gap in the 
Biography column:

    Name:             Biography:                            
      William           William Shakespeare was born on    
      Shakespeare       
                        April 23, 1564 in Strathford-upon- 
    Status:             Avon, England; he was third of     
                        etc.

So it has to be a block field, to "keep up" with however much output the
multi-line Name field produces. Unfortunately, starting the Biography column
with a normal block field doesn't solve the problem either. In fact we get:

    Name:             Biography:                            
      William           William Shakespeare was born on    
      Shakespeare       April 23, 1564 in Strathford-upon- 
                        Avon, England; he was third of     
                        eight children from Father John    
                        Shakespeare and Mother Mary Arden. 
                        Shakespeare began his education at 
                        the age of seven when he probably  
                        attended the Strathford grammar    
                        school. The school provided        
                        Shakespeare with his formal        
                        education. The students chiefly    
                        studied Latin rhetoric, logic, and 
                        literature. His knowledge and      
                        imagination may have come from his 
                        reading of ancient authors and     
                        poetry. In November 1582,          
                        Shakespeare received a license to  
                        marry Anne Hathaway. At the time of
                        their marriage, Shakespeare was 18 
                        years old and Anne was 26. They had
                        three children, the oldest Susanna,
                        and twins- a boy, Hamneth, and a   
                        girl, Judith. Before his death on  
                        April 23 1616, William Shakespeare 
                        had written thirty-seven plays. He 
                        is generally considered the        
                        greatest playwright the world has  
                        ever known and has always been the 
                        world's most popular author.       

    Status:                                                
      Deceased (1564                                       
      -1616)                                               

    Comments:                                              
      Theories                                             
      abound as to                                         
      the true                                             
      author of his                                        
      plays. The                                           
      prime                                                
      alternative                                          
      candidates                                           
      being Sir                                            
      Francis                                              
      Bacon,                                               
      Christopher                                          
      Marlowe, or                                          
      Edward de                                            
      Vere                                                 

Normal block fields are remorseless in consuming all of their data.
So the first Biography field absolutely will not stop formatting, ever,
until your entire C<$biography> string is gone.

What we really need here, is a kinder, gentler block field; a 
block field that formats minimally, like an overflow field.
And we get that with yet another C<:height> option:
C<:heightE<laquo>minimalE<raquo>>. Like so:

    print form
        :interleave, <<EOFORMAT,
            Name:             Biography:
              {[[[[[[[[[[[[}    {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[}
                                {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
            Status:             {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
              {[[[[[[[[[[[[}    {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
                                {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
            Comments:           {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
              {[[[[[[[[[[[}     {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}
            EOFORMAT
         $name,
         :heightE<laquo>minimalE<raquo>, $biography,
         $status,
         $comments;

When this option is applied to a particular field (by placing it
immediately before the field's data), that field only consumes
as much of its data is is required to fill the output lines created by
the other (non-minimal) fields in the same format. In this case, that means
that the first Biography field only extracts as much data from C<$biography>
as is needed to fill the text lines created by the Name field.

Note that any kind of block field can be modified in this way:
justified, numeric, currency, or verbatim.


=head2 Now is the jerkin under the line...

As some of the examples we've seen so far illustrate, formats frequently
consist of a set of column titles, followed by the corresponding columns
of data. And, typically, those column titles are underlined to make them
stand out:

    print form 
      'Name              Bribe (per dastardry)',
      '=============     =====================',
      '{[[[[[[[[[[[}         {]],]]].[[[}     ',
      @names,                @bribes;

So C<form> has an option that automates that process. For
example, the payments example above could also have been written:

    print form 
      'Name              Bribe (per dastardry)',
      :under("="),
      '{[[[[[[[[[[[}         {]],]]].[[[}     ',
      @names,                @bribes;

The C<:under> option takes a string and uses it to underline the most
recently formatted line. It does this by examining the formats
immediately before and after the C<:under>. It then generates a
series of underlines by repeating the specified underlining string
as many times as required. The underlines are generated such that
every field and every other non-whitespace literal in the preceding
format has a underline under it and every field/non-whitespace in
the next format has an "overline" above it.

For example, this call to C<form>:

    print form
        '      Rank Name         Serial Number',
        :under("_"),
        '{]]]]]]]]} {[[[[[[[[[}     {IIIII}',
             @ranks,@names,          @nums;

prints:

          Rank Name         Serial Number
    __________ ___________  _____________
      Corporal Nym              CMXVII
    Lieutenant Bardolph          CCIV
       Captain Spurio           MMMCDX
       General Pompey             XI  

The usual effect is that the auto-generated underlines always
extend to the edges of both the preceding title and the following
field, whichever is wider.

Many people, of course, prefer to draw the underlines themselves, as
the results are then much easier to visualize when looking at the code.
The C<:under> option is most useful when we're constructing tables 
programmatically, with columns and column titles that are only known
at run-time.


=head2 They come like sacrifices in their trim...

The default L<fill-with-spaces behaviour|"He doth fill fields with harness...">
of fields is useful to preserve
the vertical alignment of columns within a formatted text, but it could
also potentially increase the size of C<form>'s output unnecessarily.
For example, the following:

    print form 
        'To Do:',
        '   {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[}',
            @todo;

might produce something like:

    To Do:
       Dissemble                                         
       Deceive                                           
       Dispute                                           
       Defy                                              
       Duel                                              
       Defeat                                            
       Dispatch                                         E<nbsp>

That looks fine but, because each line is produced by the large
left-justified field that is automatically filled with whitespace, the
output contains several hundred more space characters than are strictly
necessary (you probably didn't notice them, but they're all there
E<ndash> hanging off the right sides of the individual To-Do items).

Fortunately, however, C<form> is smarter than that. Extraneous trailing
whitespace on the right-hand side of any output line is automatically
trimmed. So the above example actually produces:

    To Do:
       Dissemble
       Deceive
       Dispute
       Defy
       Duel
       Defeat
       Dispatch

Of course, if you really do need those "invisible" trailing whitespaces
for some reason, C<form> provides a way to keep them E<ndash> the
C<:untrimmed> option:

    print form :untrimmed,
        'To Do:',
        '   {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[}',
            @todo;


=head1 Master Page, I am glad to see you...

Normally, C<form> assumes that whatever data it is formatting is supposed
to produce a single, arbitrarily long, unbroken piece of text. But C<form>
can also format data into multiple pages of fixed length and width,
inserting customized, page-specific headers, footers, and pagefeeds
for each page.

All these features are controlled by the the C<page> option (or more
precisely, by its various suboptions):

    print form
        :page{ :length( $page_len ),       # Default: 60 lines
               :width( $page_width ),      # Default: 78 columns
               :number( $first_page_num ), # Default: 1
               :header( &make_header ),    # Default: no header
               :footer( &make_footer ),    # Default: no footer
               :feed( &make_pagefeed ),    # Default: no pagefeed
               :body( &adjust_body ),      # Default: no chiropracty
             },
        $format,
        *@args;


=head2 Measure his woe the length and breadth of mine...

The C<:page{ :length(...) }> suboption determines the number of output
lines per page (including headers and footers). Normally,
this suboption is set to infinity, which produces that single, arbitrarily
long, unbroken page of text. But the suboption can be set to any
positive integer value, to cause C<form> to generate distinct
pages of that many lines each.

The value of the C<:page{ :width(...) }> suboption is
used to determine the width of L<distributive fields|
"And every one shall share..."> and
in some L<page body postprocessors|
"Do to this body what extremes you can...">.
By default, this suboption is set to 78 (columns), but it may
be set to any positive integer value.

The C<:page{ :number(...) }> suboption specifies the current page number.
By default it starts at 1, but may be set to any numeric value.
This suboption is generally only of use in headers and footers (see below).


=head2 From the crown of his head to the sole of his foot...

The C<:page{ :header(...) }> suboption specifies a hash containing 
a set of strings or subroutines that are to be used to create page headers.
Each key of the hash indicates a particular kind of page that the
corresponding value will provide the header for. For example:

    :header{ first => "           'The Tempest' by W. Shakespeare          ",
             last  => "                   -- The End --                    ",
             odd   => "Act $act, Scene $scene                              ",
             even  => "                                                    ",
             other => "          [Thys hedder intenshunally blanke]        ",
           }

Given the above specification, C<form> will:

=over

=item *

use the full title and author as the header of the first page,

=item *

write C<"-- The End --"> across the top of the last page,

=item * 

prepend the act and scene information to the start of any odd page
(except, of course, the first or the last), and

=item * 

provide an empty line as the header of any even page (except the last,
if it happens to be even).

=back 

Note that, in this case, since we've provided specific headers for every
odd and even page, the C<"other"> header will never be used. On the other
hand, if we'd specified:

    :header{ first => "           'The Tempest' by W. Shakespeare          ",
             other => "                                       'The Tempest'",
           }

then every page except the first would have just a right-justified title at
the top. 

Of course, if we want every page to have the same header, we can just write:

    :header{ other => "                                       'The Tempest'"}

But that's a little klunky, so C<form> also accepts a single string instead of
a hash, to specify a header to be used for every page:

    :header("                                       'The Tempest'")

Headers don't all have to be the same size either. For example, we might
prefer a more imposing first header:

    :header{ first => "                  'The Tempest'                   \n"
                    ~ "                        by                        \n"
                    ~ "                  W. Shakespeare                  \n"
                    ~ "____________________________________________________",

             other => "                                       'The Tempest'",
           }

C<form> simply notes the number of lines each header requires and then
reduces the available number of lines within the page accordingly,
so as to preserve the exact overall page length.

Often we'll need headers that aren't fixed strings. For example, we might
want each page to include the appropriate page number. So instead of a string,
we're allowed to specify a particular header as a subroutine. That subroutine
is then called each time that particular header is required, and its return
value is used as the required header.

When the subroutine is called, the current set of active formatting
options are passed to it as a list of pairs. Typically, then, the
subroutine will specify one or more named-only parameters corresponding
to the options it cares about, followed by a starred hash parameter to
collect the rest. For example if every page should have its
(left-justified) page number for a header:

    :header( sub (+$page, *%_) { return $page{number}; } )

Of course, this is also an excellent candidate for the cleaner (but
equivalent) syntax of placeholder variables in raw blocks:

    :header{ $^page{number}; }

And, naturally, we can mix-and-match static and dynamic headers:

    :header{ odd  => "                                        'The Tempest'",
             even => { $^page{number}; },
           }


Footers work in exactly the same way in almost all respects; the obvious
exception being that they're placed at the end of a page, rather than the
start.

Pagefeeds work the same way too. A pagefeed is a string that is placed
between the footer of one page and the header of the next. They're like
formfeeds, except they can be any string we choose (not just C<\c[FF]>).
They're called "pagefeeds" instead of "formfeeds" because they're
placed between pages, not between calls to C<form>. 

Here's a complete example to illustrate the full set of features:

    my @tobe = slurp 'Soliloquy.txt'  err die;

    my %page = (
        :length(15),
        :header{ first => "Hamlet's soliloquy begins...\n\n",
                 odd   => "Hamlet's soliloquy continues...\n\n",
                 even  => { form '{>>{*}>>}', "Hamlet's soliloquy continues...\n\n"; },
                 last  => "Hamlet's soliloquy concludes...\n\n",
               },
        :footer{
                 last  => { form "\n{||{*}||}", "END OF TEXT"; }
                 other => { form "\n{>>{*}>>}", "../"~($^page{number}+1); },
               },
        :feed("\f"),
    );

    print form
        :page(%page),
        '{]]]]]}  {"{*}"}  {[[[[[}',
        [1..@tobe], @tobe,  [1..@tobe];


which prints:

    Hamlet's soliloquy begins...

          1  To be, or not to be -- that is the question:          1
          2  Whether 'tis nobler in the mind to suffer             2
          3  The slings and arrows of outrageous fortune           3
          4  Or to take arms against a sea of troubles             4
          5  And by opposing end them. To die, to sleep --         5
          6  No more -- and by a sleep to say we end               6
          7  The heartache, and the thousand natural shocks        7
          8  That flesh is heir to. 'Tis a consummation            8
          9  Devoutly to be wished. To die, to sleep --            9
         10  To sleep -- perchance to dream: ay, there's the rub,  10
         11  For in that sleep of death what dreams may come       11

                                                                      ../2
    ^L
                                           Hamlet's soliloquy continues...

         12  When we have shuffled off this mortal coil,           12
         13  Must give us pause. There's the respect               13
         14  That makes calamity of so long life.                  14
         15  For who would bear the whips and scorns of time,      15
         16  Th' oppressor's wrong, the proud man's contumely      16
         17  The pangs of despised love, the law's delay,          17
         18  The insolence of office, and the spurns               18
         19  That patient merit of th' unworthy takes,             19
         20  When he himself might his quietus make                20
         21  With a bare bodkin? Who would fardels bear,           21
         22  To grunt and sweat under a weary life,                22

                                                                      ../3
    ^L
    Hamlet's soliloquy continues...

         23  But that the dread of something after death,          23
         24  The undiscovered country, from whose bourn            24
         25  No traveller returns, puzzles the will,               25
         26  And makes us rather bear those ills we have           26
         27  Than fly to others that we know not of?               27
         28  Thus conscience does make cowards of us all,          28
         29  And thus the native hue of resolution                 29
         30  Is sicklied o'er with the pale cast of thought,       30
         31  And enterprise of great pitch and moment              31
         32  With this regard their currents turn awry             32
         33  And lose the name of action. -- Soft you now,         33

                                                                      ../4
    ^L
    Hamlet's soliloquy concludes...

         34  The fair Ophelia! -- Nymph, in thy orisons            34
         35  Be all my sins remembered.                            35










                                 END OF TEXT
    ^L

Note, in particular, the nested calls to C<form> within some of the
subroutines E<ndash> to center or right-justify a particular header or footer.
Permitting just this kind of "recursive" formatting is one of the main
reasons Perl 5's built-in C<format> has become the (reentrant) C<form>
subroutine in Perl 6.


=head2 Do to this body what extremes you can...

Sometimes it's useful to be able to grab the entire body of a page
(i.e. the contents of the page between the header and footer)
I<after> it's been formatted together. For example, we might wish to
centre those contents, or to crop them at a particular column.

To this end, the C<:page{ :body(...) }> suboption allows us to specify
a page body post-processor. That is, a subroutine or format that
lays out the page's formatted text between the page's header and footer.
Like the C<:header>, C<:footer>, and C<:feed> suboptions, the
C<:body> suboption can take either a closure, a hash, or a string.

If the value of the C<:body> suboption is a string or a hash of
pairs, the text of the body is (recursively) C<form>'ed using that
string (or those string values) as its format. A very common usage is to
arrange for the formatted text to be horizonally and vertically
centred on each page:

    :body('{=I{*}I=}')

A more sophisticated variation on this is to use a hash to insert a
left or right "gutter" for each page:

    $gutter = " " x $gutter_width;

    :body{ odd   =>  $gutter ~ '{"""{*}"""}',
           even  =>  '{"""{*}"""}' ~ $gutter,
         }

On the other hand, if the value of the C<:body> suboption is closure,
the body text is passed to that closure as an array of lines. A
second array is also passed in, containing as many newlines as would
be needed to pad out the body text to the correct number of lines
for the page. Finally, the current formatting options are passed as
a list of pairs. As with the C<:header> etc. suboption, the closure
is expected to return a single string (representing the final
formatting of the page body).

For example, to add line numbers to the text each page (but I<not>
to the headers or footers or filler lines):

    sub numerate (@lines, @fill, +$page, *%_) {

        # Remember line numbers from call to call...
        state $linenum = 1;

        # Compute range of line numbers
        my @linenums = ($linenum .. $linenum+@lines-1);

        # Reformat body lines verbatim,
        # with a left-justified line number before each...
        my $body = form '{[[[[} {"""{*}"""}',
                         @linenums, @lines,
                        *@fill;

        # Update the final line number and return the new body text...
        $linenum += @lines;
        return $body;
    }

    print form
        :page{ :body(&numerate),
               :header("\n==========\n\n"),
               :length(12),
             },
        # Left-justify the Briton...
        "{[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[}",
        %soliloquy{RichardIII},
                         # Right-justify the Dane...
        "                 {]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]}",
                          %soliloquy{Hamlet};

which produces:

    ==========

    1      Now is the winter of our discontent /  
    2      Made glorious summer by this sun of    
    3      York; / And all the clouds that lour'd 
    4      upon our house / In the deep bosom of  
    5      the ocean buried. / Now are our brows  
    6      bound with victorious wreaths; / Our   
    7      bruised arms hung up for monuments; /  
    8      Our stern alarums changed to merry     
    9      meetings, / Our dreadful marches to    

    ==========

    10     delightful measures. Grim-visaged war  
    11     hath smooth'd his wrinkled front; / And
    12     now, instead of mounting barded steeds 
    13     / To fright the souls of fearful       
    14     adversaries, / He capers nimbly in a   
    15     lady's chamber.                        




    ==========

    16                      To be, or not to be -- that is the question: /
    17                         Whether 'tis nobler in the mind to suffer /
    18                       The slings and arrows of outrageous fortune /
    19                         Or to take arms against a sea of troubles /
    20                       And by opposing end them. To die, to sleep --
    21                         / No more -- and by a sleep to say we end /
    22                      The heartache, and the thousand natural shocks
    23                      / That flesh is heir to. 'Tis a consummation /
    24                        Devoutly to be wished. To die, to sleep -- /

    ==========

    25                         To sleep -- perchance to dream: ay, there's
    26                          the rub, / For in that sleep of death what
    27                         dreams may come / When we have shuffled off
    28                             this mortal coil, / Must give us pause.
    29                        There's the respect / That makes calamity of
    30                                                       so long life.






                   E<nbsp>



=head1 Define, define, well-educated infant.

Laziness is, of course, a major virtue. And one of the Laziest
approaches to programming is never to repeat oneself. Which is why Perl
6 has subroutines and macros and classes and constants and dozens of
other ways for us to factor out commonalities.

Occasionally, the same need for factoring arises in formatting. For
example, suppose we want a field that masks its data in some way.
Perhaps a field that blanks out certain words by replacing them with the
corresponding number of X's.

We could always do that by writing a subroutine that generates the
appropriate filter:

    sub expurgate (Str *@hidewords) {
        return sub (Str $data is rw) {
            $data ~~ s:ei/(@hidewords)/$( 'X' x length $1 )/;
            return $data;
        }
    }

We could then apply that subroutine to the data of any field that needed
bowdlerization:

    my &censor := expurgate E<laquo>villain plot libel treacherous murderer false deadly 'G'E<raquo>;

    print form
        "[Ye following tranfcript hath been cenfored by Order of ye King]\n\n",
        "         {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[}",
                  censor($speech);

to produce:

    [Ye following tranfcript hath been cenfored by Order of ye King]

             And therefore, since I cannot prove a lover,  
             To entertain these fair well-spoken days,     
             I am determined to prove a XXXXXXX            
             And hate the idle pleasures of these days.    
             XXXXs have I laid, inductions dangerous,      
             By drunken prophecies, XXXXXs and dreams,     
             To set my brother Clarence and the king       
             In XXXXXX hate the one against the other:     
             And if King Edward be as true and just        
             As I am subtle, XXXXX and XXXXXXXXXXX,        
             This day should Clarence closely be mew'd up, 
             About a prophecy, which says that XXX         
             Of Edward's heirs the XXXXXXXX shall be.

Of course, if this were Puritanism and not Perl, we might have a long
list of proscribed words that we needed to excise from I<every> formatted text.
In that case, rather that explicitly running every data
source through the same censorious subroutine, it would be handy if C<form>
had a built-in field that did that for us automatically.

Naturally, C<form> doesn't have such a field built-in...but we
can certainly give it one.

User-defined field specifiers can be declared using the C<:field> option,
which takes as its value an array of pairs. The key of each pair
is a string or a rule (i.e. regex) that specifies the syntax of the
user-defined field. The value of each pair is a closure/subroutine that
constructs a standard field specifier to replace the user-defined
specifier. Alternatively, the value of a pair may be a string, which is
taken as the (static) field specifier to be used instead of the
user-defined field.

In other words, each pair is a macro that maps a user-defined field
(specified by the pair's key) onto a standard C<form> field (specified by
the pair's value). For example:

    :field[ /\{ X+ \}/ => &censor_field ]

This tells C<form> that whenever it finds a brace-delimited field consisting
of one or more X's, it should call a subroutine named C<censor_field> and
use the return value of that call instead of the all-X field.

When the key of a C<:field> pair matches some part of a format,
its corresponding subroutine is called. That subroutine is passed
the result (i.e. C<$0>) of the rule
match, as well as the hash of active options for that field. Changes
to the options hash will affect the subsequent formatting behaviour of
that field.

So C<censor_field> could be implemented like so:

        # Constructor subroutine for user-defined censor fields...
        sub censor_field ($field_spec, %opts) {

            # Set up the field's 'break' option with a censorious break...
            %opts{break} = break_and_censor(%opts{break});

            # Construct a left-justified field with the appropriate width
            # specified imperatively...
            return '{[[{' ~ length($field_spec) ~ '}[[}';
        }

The C<censor_field> subroutine has to change the field's C<:break>
option, creating a new line breaker that also expurgates unsuitable
words. To do this it calls C<break_and_censor>, which returns a new line
breaker subroutine:

        # Create a new 'break' sub...
        sub break_and_censor (&original_breaker) {
            return sub (*@args) {

                # Call the field's original 'break' sub...
                my ($nextline, $more) = original_breaker(*@args);

                # X out any doubleplus ungood words
                $nextline ~~ s:ei/(@proscribed)/$( 'X' x length $1 )/;

                # Return the "corrected" version...
                return ($nextline, $more);
            }
        }

Having created a subroutine to translate censor fields and another to
break-and-expurgate the data placed in them, we are now in a position
to create a module that encapsulates the new formatting functionality:

    module Ministry::Of::Truth {

        # Internal mechanism (as above)...
        my @proscribed = E<laquo>villain plot libel treacherous murderer false deadly 'G'E<raquo>;
        sub break_and_censor (&original_breaker) {...}
        sub censor_field ($field_spec, %opts) {...}

        # Make the new field type standard by default in this scope...
        use Form :field[ /\{ X+ \}/ => &censor_field ];

        # Re-export the specialized &form that was imported above...  
        sub form is exported {...}

    }

Okay, admittedly that's quite a lot of work. But the pay-off is huge: we can now
trample on free speech I<much> more easily:

    use Ministry::Of::Truth;

    print form 
        "[Ye following tranfcript hath been cenfored by Order of ye King]\n\n",
        "        {XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX}",
                  $speech;

And we'd get the same carefully XXXX'ed output as before.


=head2 Put thyself into the trick of singularity...

User-defined fields are also a handy way to create single-character
markers for single-column fields (in order to preserve the
one-to-one spacing of a format). For example:

    print form
        :field{ '^' => '{<III{1}III}',   # 1-char-wide, top-justified block
                '=' => '{<=II{1}II=}',   # 1-char-wide, middle-justified block
                '_' => '{<_II{1}II_}',   # 1-char-wide, bottom-justified block
              },
        '~~~~~~~~~',
        '^ _ = _ ^',   *E<laquo>like round and orient perlsE<raquo>,
        '~~~~~~~~~';

prints:

    ~~~~~~~~~
    l     o p
    i r a r e
    k o n i r
    e u d e l
      n   n s
      d   t  
    ~~~~~~~~~

Note that we needed to use a unary C<*> to flatten the 
S<C<E<laquo>like round and orient perlsE<raquo>>>
data list. That's because every argument of C<form> is evaluated in scalar
context, and an unflattened C<E<laquo>...E<raquo>> list in scalar context
becomes an array reference, rather than the five separate strings we needed
to fill our five single-character fields.

Single fields are particularly useful for labelling the vertical axes of a
graph:

    use Form :field[ '=' => '{<=II{1}II=}' ];

    @vert_label = E<laquo>Villain's fortunesE<raquo>
    $hor_label  = "Time";

    print form 
       '     ^                                        ',
       ' = = | {""""""""""""""""""""""""""""""""""""} ', *@vert_label, @data,
       '     +--------------------------------------->',
       '      {|||||||||||||||||||||||||||||||||||||} ', $hor_label;

which produces:

         ^                                        
         |                                        
     V   |       *                                
     i f |     *   *                              
     l o |    *     *                             
     l r |                                        
     a t |   *       *                            
     i u |                                        
     n n |  *         *                           
     ' e |                                        
     s s |                                        
         |                                        
         | *           *                          
         +--------------------------------------->
                           Time                   

Specifying these kinds of single-character block markers is perhaps the
commonest use of user-defined fields. But the:

    :field[ '=' => '{<=II{1}II=}' ]

syntax is uncomfortably verbose for that purpose. So calls to
C<form> can also accept a short-hand notation to define a
single-character field:

    :single('=')

or to define several at once:

    :single['#', '*', '+']

The C<:single> option does exactly the same thing as the C<:field> options
shown above. It takes a single-character string, or a reference
to an array of such strings, as its value. It then turns each of those
strings into a single-column field marker. If the character is C<'='>
then the field is vertically "middled" within its block. If the
character is C<'_'> then the field is "bottomed" within its block. If
the single character is anything else, the resulting block is top-justified.
So our previous example could also have been written:

    print form
        :single("="),
        '     ^                                        ',
        ' = = | {""""""""""""""""""""""""""""""""""""} ', *@vert_label, @data,
        '     +--------------------------------------->',
        '      {|||||||||||||||||||||||||||||||||||||} ', $hor_label;


=head1 These paper bullets of the brain...

Bulleted lists of items are a very common feature of reports, but as we saw
L<earlier|"Therefore, put you in your best array..."> they're surprisingly
hard to get right.

Suppose, for example, we want a list of items bulleted by "diamonds":

    <> A rubber sword (laminated with mylar to
       look suitably shiny).                   
    <> Cotton tights (summer performances).   
    <> Woolen tights (winter performances or  
       those actors who are willing to admit
       to being over 65 years of age).                 
    <> Talcum powder.                         
    <> Codpieces (assorted sizes).            
    <> Singlet.                               
    <> Double.                                
    <> Triplet (Kings and Emperors only).     
    <> Supercilious attitude (optional).      

Something like this works well enough:

    for @items -> $item {
        print form
            '<> {<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<}', $item;
            '   {VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV}';
    }

The first format produces the bullet plus the first line of text for the item,
then the second format handles any overflow of the item data.

Alternatively, we could achieve the same result with a single format string
by interpolating the bullet as well:

    my $bullet = "<>";

    for @items -> $item {
        print form
            "{''{*}''} {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[}",
             $bullet,  $item;
    }

Here we use a single-line starred verbatim field (C<{''{*}''}>),
so that the bullet is interpolated "as-is" and the field
is only as wide as the bullet itself.
Then for the item itself we use a block field, which will format the item
data over as many lines as necessary. Meanwhile, because the bullet's
field is single-line, after the first line the bullet field will be
filled with spaces (instead of a "diamond"), leaving a bullet only on
the first line.

This second approach also has the advantage that we could change the bullet
string at run-time and the format would adapt automatically.

However, it's still a little irritating that we have to set up a loop and
call C<form> separately for each element of C<@items>. After all, if we
didn't need to bullet our list we could just write:

    print form
        "{[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[}",
        @items;

and C<form> would take care of iterating over the C<@items> for us. It
seems that things ought to be that easy for bulleted lists as well.

And, of course, things I<are> that easy. 

All we need to do is tell C<form> that whenever the string C<< "<>" >>
appears in a format, it should be treated as a bullet. That is, it should
appear only beside the I<first> line of text produced when formatting each
element of the adjacent field's data.

To tell C<form> all that we use the C<:bullet> option:

    print form
        :bullet("<>"),
        "<> {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[}",
            @items;

or, more permanently:

    use Form :bullet("<>");

    # and later...

    print form
        "<> {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[}",
            @items;

The presence of this C<:bullet> option causes C<form> to treat the sequence
C<< "<>" >> as a special field. That special field interpolates the 
string C<< "<>" >> when the field immediately to its right begins to
format a new data element, but thereafter interpolates only spaces until
the adjacent field finishes formatting that data element.

Or, more simply, if we tell C<form> that  C<< "<>" >> is a bullet,
C<form> treats it like a bullet that's attached to the very next field.

So we could finally fix our L<Shakespearean roles example|
"Therefore, put you in your best array...">, like so:

    print "The best Shakespearean roles are:\n\n";

    print form
        :bullet("* "),
        "   * {[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[}   *{[[[[[[[[]]]]]]]]}*",
              @roles,                                 $disclaimer;

This would then produce:

   The best Shakespearean roles are:

      * Either of the 'two foolish             *WARNING:          *
        officers': Dogberry and Verges         *This list of roles*
      * That dour Scot, the Laird              *constitutes      a*
        Macbeth                                *personal   opinion*
      * The tragic Moor of Venice,             *only and is in  no*
        Othello                                *way  endorsed   by*
      * Rosencrantz's good buddy               *Shakespeare'R'Us. *
        Guildenstern                           *It   may   contain*
      * The hideous and malevolent             *nuts.             *
        Richard III                            *                  *

Notice too that the asterisks on either side of the disclaimer I<aren't>
treated as bullets. That's because we defined a bullet to be C<"* ">, and
neither of the disclaimer asterisks has a space after it.

Bullets can be any string we like, and there can be more than one of them
in a single format. For example:

    print form
        :bullet('+'),
        "+ {[[[[[[[[[[[[[[[[[[[E<hellip>}       + {E<hellip>[[[[[[[[[[[[[[[[[[[}",
            @items,                         @items;

would print:

    + A rubber sword,                65 years of age).     
      laminated with mylar         + Talcum powder.        
      to look suitably             + Codpieces (assorted   
      shiny.                         sizes).               
    + Cotton tights (summer        + Singlet.              
      performances).               + Double.               
    + Woolen tights (winter        + Triplet (Kings and    
      performances or those          Emperors only).       
      actors who are willing       + Supercilious attitude 
      to admit to being over         (optional).           

We can even change bullets in mid-C<form>, which is useful for 
multi-level bulleting. Of course, in that case we're going
to need a loop again, since C<form> itself has only
one level of intrinsic looping:

    %categories = (
       Animal    => ["The mighty destrider, ship of the knight",
                     "The patient cat, warden of the granary",
                     "Our beloved king, whom we shall soon have to kill"],
       Vegetable => ["The lovely peony, garland of Eddore",
                     "The mighty oak, from which tiny acorns grow",
                     "The humble cabbage, both food and metaphor for the fool"],
       Mineral   => ["Gold, for which men thirst",
                     "Salt, by which men thirst",
                     "Sand, on which men thirst"],
    );

    for %categories.kv -> $category, @examples {
        print form
            :bullet('*'),  "* {<<<<<<<<<<<<<<<<<<<<<<<<<<<<}",  $category,
            :bullet('-'),  "    - {[[[[[[[[[[[[[[[[[[[[[[[[}",  @examples;
    }

This would produce:

    * Mineral                       
        - Gold, for which men thirst
        - Salt, by which men thirst 
        - Sand, on which men thirst 
    * Animal                        
        - The mighty destrider, ship
          of the knight             
        - The patient cat, warden of
          the granary               
        - Our beloved king, whom we 
          shall soon have to kill   
    * Vegetable                     
        - The lovely peony, garland 
          of Eddore                 
        - The mighty oak, from which
          tiny acorns grow          
        - The humble cabbage, both  
          food and metaphor for the 
          fool                      



=head1 All's well that ends...

Report generation was one of Perl's original I<raisons d'etre>. Over the
years we've found out what C<format> does well, and where its
limitations lurk. The new Perl 6 C<form> function aims to preserve
C<format>'s simple approach to report generation and build on its strengths
by adding:

=over

=item *

independence from the I/O system;

=item *

run-time specifiable format strings;

=item *

a wider range of useful field types, including fully justified, verbatim,
and overflow fields;

=item *

the ability to define new field types;

=item * 

sophisticated formatting of numeric/currency data;

=item *

declarative, imperative, distributive, and extensible field widths;

=item *

more flexible control of headers, footers, and page layout;

=item *

control over line-breaking, whitespace squeezing, and
filling of empty fields; and

=item *

support for creating plaintext lists, tables, and graphs.

=back

And because it's now part of a module, rather than a core component,
C<form> will be able to evolve more easily to meet the needs of its
community. For example, we are currently investigating how we might
add facilities for specifying numerical bullets, for formatting
text using variable-width fonts, and for outputting HTML instead
of plaintext.

If you're a regular user of Perl 5's C<format> you might like to try the
C<form> function instead. It's available right now in the Perl6::Form
module, which waits upon thy pleasure at the CPAN.
