NAME
    HTML::YaTmpl - Yet Another Template Processor

SYNOPSIS
     use HTML::YaTmpl;

     my $t=HTML::YaTmpl->new( file=>'template.tmpl' );
     $t->evaluate( key1=>$value1,
                   key2=>[$val21, $val22, ...]
                   ... );
     $t->evaluate_to_file( $outputfilename,
                           key1=>$value1,
                           key2=>[$val21, $val22, ...]
                           ... );

ABSTRACT
    "HTML::YaTmpl" aims mainly to provide a HTML template processor that
    saves the template writer typing.

    There are general template processors like "Text::Template" and tools to
    embed perl in HTML like "HTML::Embperl" or HTML template processors like
    "HTML::Template". Why have I decided to start yet another? Well,
    "Text::Template" is not really convenient when it comes to process
    repeating data records like HTML tables. With "HTML::Embperl" no
    professional "WEB Designer" will be able to "enhance" the pages. And
    "HTML::Template" enforces a strict division of design and programming.
    Thus, it enforces changes to the programming logic even if you only want
    to exchange a long number like 2835067264068365493 with a more human
    readable 2,835,067,264,068,365,493.

    "HTML::YaTmpl" attempts to make simple things easy but complexity
    feasible.

DESCRIPTION
    "HTML::YaTmpl" follows the object oriented paradigm, i.e. you have to
    create a template processor prior to using it. A template processor is
    not bound to any particular template. You can use one processor to
    evaluate various templates. But other properties like error handling are
    bound to the processor.

  Constructor
    new( attrname=>attrval, ... )
      creates a new "HTML::YaTmpl" object. These attributes can be set:

      template [optional]
        is the actual template text. It is used for programmatically
        generated templates. It is overridden by template() and open()
        methods.

      file [optional]
        sets a file to read the template from. Setting this attribute via
        new() causes the open() method to be called. If open() fails new()
        will return undef instead of an object.

      path [optional]
        is an ARRAY (passed as reference) to be used as template search
        path. If omitted the environment variable "HTML_TMPL_SEARCH_PATH" is
        split by your path separator (see the Config module documentation)
        (on UNIX a ':' on Windows ';'). The "path" attribute is used by
        open().

      package [optional]
        specify a package to evaluate template code fragments. If omitted
        the caller package of the constructor is used. Thus, the package can
        be used to define convenience functions to be called from within a
        template.

      no_eval_cache [optional]
        "HTML::YaTmpl" wraps all code fragments from templates into
        subroutines and calls then these subroutines. Normally the
        subroutines are cached to avoid multiple calls of the perl
        interpreter. If "no_eval_cache" is set the cache is turned off. This
        can be useful for long running applications that process user
        provided templates. On the other hand there is the method
        clear_cache() to clear that cache from time to time.

      onerror [optional]
      eprefix [optional]
      errors [optional]
        see ERROR HANDLING below

      If the constructor is called as instance method rather than as class
      method the new object inherits all attributes from the old instance.
      Naturally, explicitely named attributes are overridden.

  Attribute Access Methods
    These methods have got the ":lvalue" attribute. They can be assigned
    using "$self->method=$new_value" syntax. Called without parameter they
    return the actual attribute value. Calling with parameter sets the
    attribute value.

    template
    file
    path
    package
    no_eval_cache
      see new() above

    onerror
    eprefix
    errors
      see ERROR HANDLING below

  Template Evaluation Methods
    evaluate($private_data, key1=>$val1, key2=>$val2, ...)
      This function evaluates the current template (set via template() or
      open()) and returns the result. Note, simply setting the current file
      via "$self->file=$newfile" will not change the current template. Only
      after a call to the open() method the template is changed.

      $private_data is an optional argument that can be used to pass
      additional data to and from the template. From within the template it
      is accessible as $p.

      All other parameters are key=>value pairs that provide variables to
      the template. Internally they are gathered in a hash that is
      accessible from within the template as $h.

    evaluate_to_file($file, \%private_data, key1=>$val1, ...)
      This function calls evaluate() and writes the result to $file. If
      $file is a GLOB reference the evaluation result is written to that
      file handle. If it is a CODE reference the referenced function is
      called with the evaluation result as $_[0]. The return value is
      returned. If $file is an object and the object UNIVERSAL::can print()
      then that print method is called. Otherwise $file is interpreted as
      the name of a file where the evaluation result is to be written. If
      the template evaluation throws an exception $file remains untouched.
      evaluate_to_file() returns false if something went wrong, e.g. no
      space left on device or no permission to write the file.

  Cache Management Methods
    "HTML::YaTmpl" uses a cache to avoid multiple compilation of the same
    code fragment or multiple parsing of the same template. Simply hashing
    each compiled code or template fragment can lead to memory leaks for
    long running processes that use user provided templates. To prevent that
    the cache is assigned a high and a low water mark. If the number of
    cache elements reaches the high water mark elements are deleted on a LRU
    basis so that it drops to the low water mark. The cache is not bound to
    a "HTML::YaTmpl" object but is shared by all objects.

    The cache is implemented as 2 independent HASHes for compiled code and
    template fragments respectively. The high and low water marks are use
    for both of them.

    clear_cache
      deletes all cache entries.

    cache_highwatermark
      returns and sets the high water mark. It is set by default to 10000.
      This function is assigned the :lvalue attribute. It can thus be called
      "HTML::YaTmpl-"cache_highwatermark=$newvalue>.

    cache_lowwatermark
      returns and sets the low water mark. It is set by default to 5000.
      This function is assigned the :lvalue attribute. It can thus be called
      "HTML::YaTmpl-"cache_lowwatermark=$newvalue>.

    cache_sizes
      returns the actual number of cached code fragments.

  Other Methods
    open
      opens the current file and sets it's content as the current template.
      You can construct a "HTML::YaTmpl" object without any template and
      later set the "file" and possibly the "path" attribute and call open()
      to set the template from the file's content.

      open() can be called with arguments: $self->open( file=>$filename,
      path=>$path ); to set the current file and template path in one call.
      Both parameters are optional.

      If the current file cannot be read for any reason open() returns
      undef. $! will indicate the reason. If a template file was read
      successfully $self is returned.

    clear_errors
      clears the objects error list returning it's content.

TEMPLATE SYNTAX AND EVALUATION
    As for other templare processors a template is a file containing normal
    content and some special template sequences that will be exchanged with
    computed values during template evaluation. Since this is mainly a HTML
    template engine these sequences are chosen similar to HTML tags. There
    are 3 kinds of sequences:

    Sequences starting with <=
      are used for variable expansion.

    Sequences starting with <:
      are used to control further evaluation, e.g. including other template
      files, evaluate only parts of the template depending on some
      variables, ...

    Sequences starting with <#
      are used as comments

    Template sequences as other HTML/XML sequences are opened with an
    opening tag and closed with a closing tag, e.g.

     <=variable_name parameter_list> body </=variable_name>

    But if "body" is empty this sequence can be abbreviated to

     <=variable_name parameter_list/>

    where the trailing "/>" instead of the simple ">" is essential. Thus,
    here come some valid template sequences:

    <=name/>
      this is probably the most used form. It simply inserts the value of
      the key "name" provided to evaluate() instead of "<=name/>".

    <=name type="array"/>
      now the parameter "type" is set. It specifies that this sequence
      should be expanded only if the value provided to evaluate as "name" is
      a reference to a non-empty array. In that case the string consisting
      of all elements of the array concatenated is inserted. Otherwise the
      template sequence is simply deleted from the output.

    <:include other_template.tmpl inherit a=b x=y/>
      this is an example of a control sequence. It reads and evaluates the
      template file "other_template.tmpl". This new evaluation inherits all
      variables from the current evaluation and adds 2 new variables "a" and
      "x" with the values "b" and "y".

    <:include other_template.tmpl><:set a>b</:set></:include>
      just the same but with closing tags. The only variable
      "other_template.tmpl" sees is "a".

  Simple Variable Substitution
   Basics
    The basic form of variable substitution is "<=name/>" or
    "<=name></=name>". Given the template:

     some text '<=var/>' other text

    If evaluate is called as

     $t->evaluate( var=>'computed text' );

    it will return

     some text 'computed text' other text

    But you can provide also an ARRAY reference to evaluate instead of the
    scalar to achieve the same result.

     $t->evaluate( var=>['com', 'pu', 'ted', ' ', 'text'] );

   Scalar, Array and Empty Processing
    How does this work? If evaluate receives an ARRAY ref as a variable's
    value it sort of evaluates "<=var/>" for each array element and
    concatenates the results. But you can specify that a variable
    substitution should be done only if the provided value is of a
    particular type. 3 such types are available

    scalar
      only if the provided value is a non-empty scalar (ref() does not
      return "ARRAY" and length() returns something not equal zero) it is
      substituted. Otherwise an empty string is substitued.

    array
      only if the provided value is a non-empty array (ref() returns "ARRAY"
      and it consists of at least one element) it is substituted. Otherwise
      an empty string is substitued.

    empty
      a value is empty if it is not a scalar nor an array as described
      above, i.e. if it is either an empty string or an array without any
      element.

    The template

     some text '<=var type=scalar/>' other text

    will show the result seen above only if evaluate() is called as

     $t->evaluate( var=>'computed text' );

    The other case will produce

     some text '' other text

    If a variable substitution is valid for multiple types they can be
    concatenated with a comma (,):

     <=var type=scalar,array/>

    is evaluated if "var" is an array or a scalar but not if it's empty.

   Modifying the substituted value on the fly
    There are cases when it would be really useful to modify the substituted
    value a little from within the template. Imagine you want to make big
    numbers more readable (52345635476 should be displayed as
    52,345,635,476) or you want to substitute a list (passed as array
    reference, see above) and don't want the elements simply be concatenated
    but displayed as HTML list elements ("<li>element</li>"). "HTML::YaTmpl"
    provides 2 ways for doing this.

   Using "<:/>"
    Given the template

     fruits comprise
     <ul>
     <=fruits><li><:/></li>
     </=fruits></ul>

    the call

     $t->evaluate( fruits=>[qw{apples pears plums cherries}] );

    will generate

     fruits comprise
     <ul>
     <li>apples</li>
     <li>pears</li>
     <li>plums</li>
     <li>cherries</li>
     </ul>

    Now we are using the long form of variable substitution (<=var></=var>).
    The tag body describes the actual substitution. Within the tag body the
    control sequence "<:/>" stands for the actual value. Of course "<:/>"
    can be given several times within a substitution body:

     <=fruits><:/> and <:/> give <:/>
     </=fruits>but
     men and women give children

    generates

     apples and apples give apples
     pears and pears give pears
     plums and plums give plums
     cherries and cherries give cherries
     but
     men and women give children

   Using Perl code fragments
    The same result can be achieved using Perl inside the "<:/>" control
    sequence:

     <=fruits><:"$v and $v give $v\n"/></=fruits>but
     men and women give children

    Now we see the "<:/>" control sequence in action. It can contain perl
    code fragments that change the substituted value. The code fragment is
    called with $v set to the actual variable. If the evaluate() call was
    passed the optional $private_data parameter it is available as $p from
    within these code fragments. Otherwise $p points to a HASH that is
    created once for each evaluate() call. Thus, code fragments can
    communicate with each other using this hash:

     <=fruits><: ++$p->{fruitcounter} />. <:/>
     </=fruits>total: <:$p->{fruitcounter}/> fruits

    produces

     1. apples
     2. pears
     3. plums
     4. cherries
     total: 4 fruits

    Maybe you have noticed the total counter in the last line was generated
    without any surrounding "<=var></=var>" sequence. Yes, that works too.
    In this case $v is "undef".

   Supplying Perl fragments using "code=..."
    By now the most examples have used the long variable substitution form
    ("<=var>...</=var>"). In most cases this is probably the *right* thing
    but you can use the short form even when modifying the substitution
    value:

     <=fruits code="<: ++$p->{fruitcounter} />. <:/>
     "/>total: <:$p->{fruitcounter}/> fruits

    will produce exactly the same result as above but it's almost not
    readable. Whereas:

     <html><body>
     <table>
     <=fruits code="<tr><td><: ++$p->{counter} /></td><td><:/></td></tr>"/>
     <tr><td>total number of fruits</td><td><:$p->{counter}/></td></tr>
     </table>
     </body></html>

    generate perfectly valid (but not easy human readable) HTML text.

    You see, the "code=..." parameter to a "<=var .../>" sequence does the
    trick. It can comprehend any text but should in most cases be surrounded
    by double quote characters ("). Within this surrounding double quote and
    backslash characters must be quoted with backslashes, all other can.
    Thus:

     <=fruits code="<li><:\"\\u$v\"/></li>"/>

    generates a HTML list of capitalized fruits:

     <li>Apples</li><li>Pears</li><li>Plums</li><li>Cherries</li>

    However, I believe, this kind of code fragments is the wrong way. But it
    can be even worse. You can omit the surrounding double quotes but then
    you must quote almost anything except characters matching "\w" with
    backslashes. The previous example without surrounding quotes looks like:

     <=fruits code=\<li\>\<:\"\\u$v\"/\>\</li\>/>

   Special list processings
    When a list is to be substituted often special treatment is required for
    the first and the last list element or some text should be prepended or
    appended to the substitution result:

     <=fruits first="<: ucfirst $v/>" last=" and <:/>" code=", <:/>"/>
     are fruits.

    generates

     Apples, pears, plums and cherries
     are fruits.

    You see, if there are "first=..." and "last=..." parameters to a
    variable substitution they affect the first respectively last list
    element.

    On the other side there are "pre=..." and "post=..." parameters. They
    are evaluated before the first respectivly after the last list element:

     <=fruits pre="<select name=\"fruit\"><:\"\\n\"/>"
              code="<option><:/></option><:\"\\n\"/>"
              post="</select><:\"\\n\"/>"/>

    produces

     <select name="fruit">
     <option>apples</option>
     <option>pears</option>
     <option>plums</option>
     <option>cherries</option>
     </select>

    Using the long substitution form these examples would look like:

     <=fruits>
     <:first><:"\u$v"/></:first>
     <:last> and <:/></:last>
     <:code>, <:/></:code>
     </=fruits>
     are fruits.

    respectively:

     <=fruits>
     <:pre><select name="fruit">
     </:pre>
     <:post></select>
     </:post>
     <:code><option><:/></option>
     </:code>
     </=fruits>

    I think, the "<:first>", "<:last>", "<:prev>" and "<:post>" semantics
    are intuitively clear. "<:code>" needs some explanation. In the previous
    examples using the long form the text between "<=var>" and "</=var>" has
    described what to substitute. That will remain to work. If no "<:code>"
    section is found all text between "<=var>" and "</=var>" save the
    control sequences will be used. But this results in many unnecessary
    newlines since the previous example without "<:prev>" and "<:post>" look
    like:

     <=fruits>
                                                   # empty line
                                                   # empty line
     <option><:/></option>
     </=fruits>

    Thus, "<:code>" can be used for convenience.

   More special list processings
    Perl knows list operations such as "map", "grep" and "sort". These are
    also useful in templates. Why the program logic should know about the
    order in which a list is displayed? It must supply the list. That's it.

    Just to arrange our fruits alphabetically we can write:

     <=fruits sort="$a cmp $b">
     <:code><:/>, </:code>
     <:last><:/></:last>
     </=fruits>

    and get

     apples, cherries, pears, plums

    But can we order reverse fruits, i.e. selppa instead of apples?

     <=fruits map="scalar reverse $_" sort="$a cmp $b">
     <:code><:/>, </:code>
     <:last><:/></:last>
     </=fruits>

    give

     seirrehc, selppa, smulp, sraep

    But I want:

     cherries, apples, plums, pears

    ok, here comes the template:

     <=fruits map="scalar reverse $_"
              sort="$a cmp $b"
              map="scalar reverse $_">
     <:code><:/>, </:code>
     <:last><:/></:last>
     </=fruits>

    But I don't like plums! Ok then:

     <=fruits grep="!/plum/i"
              map="scalar reverse $_"
              sort="$a cmp $b"
              map="scalar reverse $_">
     <:code><:/>, </:code>
     <:last><:/></:last>
     </=fruits>

    results in

     cherries, apples, pears

    Of course these parameters can also be written in long form and even
    mixed:

     <=fruits grep="!/plum/i">
     <:map>scalar reverse $_</:map>
     <:sort>$a cmp $b</:sort>
     <:code><:/>, </:code>
     <:last><:/></:last>
     </=fruits>

    produces

     seirrehc, selppa, sraep

    There can be as many as you like grep/sort/map fragments. The source
    list is first processed by the fragments passed as parameters ("<=var
    grep=... map=... sort=... ...>") in left to right order and then by the
    fragments given in long form ("<:sort>...</:sort>") in top down order.

   Real World Example
    Suppose you want to create a WEB application with an input field to put
    the name of a town. The user would first see a simple "<input
    type="text">" field. He types in some characters and submits the form.
    Now your program matches the user input with a database. There can be 3
    results. The user input can non-ambiguously match a database record or
    there are several matches or no match at all. In the first case your
    program should answer the user with a page simply showing the matching
    record. In the second case it should display a select field and in the
    3rd the "<input type="text">" field. Your template would look like:

     <=town type=empty><input type="text" name="town"></=town>
     <=town type=array pre="<select name=\"town\">"
                       post="</select>">
     <option><:/></option>
     </=town>
     <=town type=scalar><b><:/></b></=town>

    and you get a text field "town" is passed as "undef", an empty string or
    an empty array. A selection field is generated if "town" is passed as a
    non-empty array and the town set in bold is produced if it is passed as
    non-empty string.

  Control Statements
    Many control sequences have been shown in the previous chapter. Here the
    are listed again:

    <:/> or <:></:>
      is used to execute perl code. "<: some perl code />" and "<:> some
      perl code </:>" are equivalent. Within the perl code the variables $v,
      $p and $h can be used. $v contains the current variable to be
      substituted. $p holds the "private_data" parameter. $h points at a
      HASH containing all parameters passed to the current scope (see
      "<:eval>" for an example of using it).

    <:code></:code>
      is used to mark the actual template code within variable substitution
      or "<:eval>" or "<:for>" blocks. This can be used for convenience.

    <:pre></:pre>
    <:post></:post>
    <:first></:first>
    <:last></:last>
    <:map></:map>
    <:grep></:grep>
    <:sort></:sort>
      see previous chapter.

    <:for ...>...</:for>
      The "<:for>" statement is used to evaluate a part of the template with
      changed parameters. For example you want to make up a HTML table that
      is passed as list of lists:

       $t->evaluate( fruit_colors=>[[qw{apples red/green}],
                                    [qw{pears green}],
                                    [qw{plums blue/yellow}],
                                    [qw{cherries red}]] );

      with the template

       <table>
       <=fruit_colors><:for f="<:/>">
       <:code><tr><=f><td><:/></td></=f></tr>
       </:code>
       </:for></=fruit_colors></table>

      generates

       <table>
       <tr><td>apples</td><td>red/green</td></tr>
       <tr><td>pears</td><td>green</td></tr>
       <tr><td>plums</td><td>blue/yellow</td></tr>
       <tr><td>cherries</td><td>red</td></tr>
       </table>

      Here the opening "<=fruit_colors>" begins a scope of substituting
      "fruit_colors". As "fruit_colors" is an array the tag body is
      evaluated for each element. The tag body contains a single "<:for>"
      statement used to assign "f" temporarily the current value of
      "fruit_colors", i.e. "f" is assigned in turn each element of the
      "fruit_colors" list. Within to "<:for>" statement we see the
      evaluation of "<=f>" surrounded by "<tr>". As "f" is also an array it
      generated "<td>"'s for each element.

      The "<:for>" statement features a template within a template. The
      inner template can be given as a "<:code>" statement or if omitted the
      body of the tag is used. Thus, the following template is almost
      equivalent to the previous one:

       <table>
       <=fruit_colors><:for f="<:/>">
       <tr><=f><td><:/></td></=f></tr>
       </:for></=fruit_colors></table>

      In fact it generates an empty line before each table row.

      The parameter list for the inner template (the one that is passed to
      the evaluate() function) is completely made anew. In our example the
      inner template receives only one variable: "f". But there is a way to
      bequeath all current variables to the inner template. Just put the
      reserved word ":inherit" or ":inheritparms" in the parameter list of
      the "<:for>" statement. With

       <:for f="<:/>" :inherit>

      the inner template would see also "fruit_colors" and all other outer
      variables.

      All parameters containing an unquoted equal sign (=) are used to set
      up the parameter list for the include template. The simplest form of a
      parameter is a string like

       <:for some_fruit="plum">

      Slightly more complex is passing an outer variable with an other name:

       <:for inner_fruits="<=fruits/>">

      This evaluates "fruits" and stores the result as "inner_fruits". If
      "fruits" is an array so does "inner_fruits". If the substitution
      statement of "fruits" contains "pre" or "post" components
      "inner_fruits" will contain them as first / last list element. You
      also can surround the variable substitution with plain text. In this
      case each element of "inner_fruits" gets surrounded by this text. And
      you can include in the definition of one inner variable multiple outer
      variables. In this case each outer list variable is expanded and the
      resulting number of list elements is the mathematical product of the
      numbers of elements of all outer lists. The template (Note: The
      sequence "<#.../>" is a comment and used in this example to hide the
      newline in the template. For more details on comments see below.):

       <:for inner_fruits="PRE <=fruits pre=pre1
                                        post=post1
                                        grep=\"/l/\"/> <#
                         />BETWEEN <=fruits pre=pre2
                                            post=post2
                                            grep=\"/r/\"/> POST">
       <:code><=inner_fruits last="(<:/>)">(<:/>)
       </=inner_fruits></:code>

      generates a total of 16 lines:

       (PRE pre1 BETWEEN pre2 POST)
       (PRE pre1 BETWEEN pears POST)
       (PRE pre1 BETWEEN cherries POST)
       (PRE pre1 BETWEEN post2 POST)
       (PRE apples BETWEEN pre2 POST)
       (PRE apples BETWEEN pears POST)
       (PRE apples BETWEEN cherries POST)
       (PRE apples BETWEEN post2 POST)
       (PRE plums BETWEEN pre2 POST)
       (PRE plums BETWEEN pears POST)
       (PRE plums BETWEEN cherries POST)
       (PRE plums BETWEEN post2 POST)
       (PRE post1 BETWEEN pre2 POST)
       (PRE post1 BETWEEN pears POST)
       (PRE post1 BETWEEN cherries POST)
       (PRE post1 BETWEEN post2 POST)

      Due to the "grep" statements the first expansion of "fruits" contains
      only elements with an "l" letter, i.e. "apples" and "plums", whereas
      the second expansion consists of elements with an "r" letter, i.e
      "pears" and "cherries". As shown each "fruits" list is expanded with a
      "pre" and "post" elements. Thus, the total number of elements of the
      "inner_fruits" list is 4*4=16.

      By now we have seen in this chapter assigning of simple strings and
      expanded arrays to inner variables. But what happens if "<:for>" is
      placed within a variable substitution scope like in the prefacing
      example to this chapter? The template (provided with line numbers)

        1  <:>
        2  sub fac {
        3    use Math::BigInt;
        4    my $x=shift;
        5    my $res=Math::BigInt->new(1);
        6    for( my $i=1; $i<=$x; $i++ ) {
        7      $res*=$i;
        8    }
        9    return $res;
       10  }
       11  </:><html><body>
       12  <table>
       13  <:for x="<:[map {[$_, $_**2, $_**3, fac $_]} 1..30]/>"
       14        h="<:[qw{n n^2 n^3 n!}]/>">
       15  <:code>
       16      <tr>
       17        <=h><th><:/></th> </=h>
       18      </tr>
       19    <=x>
       20      <:code>
       21        <tr>
       22          <:for y="<:/>">
       23            <:code><=y><td><:/></td> </=y></:code>
       24          </:for>
       25        </tr>
       26      </:code>
       27    </=x></:code>
       28  </:for></table>
       29  </body></html>

      generates a HTML page containing a table of the first 30 square and
      cube numbers and factorials. The first control sequence (lines 1-11)
      is substituted with nothing as the value of this code fragment is
      "undef". It just defines a "fac" function. Lines 13 and 14 opens a
      "<:for>" scope that is closed at line 28. Within this scope the
      variables "x" and "h" are valid. Both are list variables since the
      "<:/>" control sequences return ARRAY references. Within that scope at
      line 17 "h" is evaluated to a list of "<th>" statements. That is more
      or less what we have seen before. But now in line 19 a variable
      substitution scope is opened. That means for each element of the "x"
      list lines 21 to 25 are evaluated repeatedly. Since "x" consists of
      arrays the evaluation of "<:/>" produces an array. That is used in a
      nested "<:for>" scope at lines 22 to 24 to subsequently assign to a
      variable "y" each of them. Now the body of our nested "<:for>" can
      evaluate "y" and create a list of "<td>" statements.

      By now we have seen parameter lists formed like

       <:for a="b" c="d" ... />

      This can lead to a lot of quoting backslashes within strings. A little
      foreboding give the last "inner_fruits" example. There we had to write
      "grep=\"/l/\"" to quote the double quotes. This can be avoided using
      the "<:set>" control sequence.

       <:for>
       <:set inner_fruits>PRE <=fruits pre=pre1
                                       post=post1
                                       grep="/l/"/> <#
                        />BETWEEN <=fruits pre=pre2
                                           post=post2
                                           grep="/r/"/> POST</:set>
       <:code><=inner_fruits last="(<:/>)">(<:/>)
       </=inner_fruits></:code>
       </:for>

      generates exactly the same result without quoting any character.

      Well, I believe, these examples are enough to show what can be done
      with "<:for>". BTW, all these examples are contained in the file
      t/5_fruits.t of the distribution. The module is tested against them.

    <:eval ...>...</:eval>
      works exactly the same as the "<:for>" statement but resulting text is
      evaluated again. This can be useful in some rare cases, e.g.:

        1  <:for fruits="<:[qw/apple pear/]/>"
        2        books="<:['The Silmarillion',
        3                  'The Lord of the Rings',
        4                  'The Hobbit or There And Back Again']/>"><#
        5  /><:eval what="<:[qw/book fruit/]/>"><#
        6  /><=what><#
        7  /><=<:/>s>
        8  <<#/>:pre>Select a <: ucfirst $v />:
        9  <select name="<:/>"><<#/>/:pre>
       10  <<#/>:post></select>
       11  <<#/>/:post>
       12  <<#/>:code><option><<#/>:/></option>
       13  <<#/>/:code>
       14  </=<:/>s><#
       15  /></=what><#
       16  /></:eval><#
       17  /></:for>

      Consider you get a lot of specialized variables that need to be
      treated all the same way. In the example above the outer "<:for>"
      scope built with lines 1-4 and 17 creates 2 variables. For both of
      them a "<select>" box should be generated. Of course one could write

       <=books>...</=books><=fruits>...</=fruits>

      but it's also possible to let the template system generate the actual
      template and then evaluate it. That's what "<:eval>" is for. Within
      the "<:eval>" scope between lines 5 and 16 the variable "what"
      contains a list of 2 elements "book" and "fruit". Variable
      substitution then occurs at lines 7, 14, 8 and 9. Lines 7 and 14 are
      evaluated to "<=books>" and "<=fruits>" and "</=books>" and
      "</=fruits>" respectively thus creating something looking very similar
      to a template. Further, all comments are eliminated. Here the "<#/>"
      comments are important. They prevent constructs like "<<#/>:/>" to be
      evaluated as "<:/>" in the first run. They sort of quote template
      sequences. In fact the generated template is:

       <=books>
       <:pre>Select a Book:
       <select name="book">
       </:pre>
       <:post></select>
       </:post>
       <:code><option><:/></option>
       </:code>
       </=books><=fruits>
       <:pre>Select a Fruit:
       <select name="fruit">
       </:pre>
       <:post></select>
       </:post>
       <:code><option><:/></option>
       </:code>
       </=fruits>

      This template is then evaluated with the parameter list of the
      "<:eval>"'s outer scope, i.e. it sees "fruits" and "books" but not
      "what".

      One could argument that the same result can be achieved avoiding an
      intermediate template. In fact

        1  <:for fruits="<:[qw/apple pear/]/>"
        2        books="<:['The Silmarillion',
        3                  'The Lord of the Rings',
        4                  'The Hobbit or There And Back Again']/>"><#
        5  /><:for what="<:[qw/book fruit/]/>" :inherit><#
        6  /><=what><#
        7  />Select a <: ucfirst $v />:
        8  <select name="<:/>">
        9  <:for el="<:$h->{$v.'s'}/>" :inherit><#
       10  /><=el>
       11  <:code><option><:/></option>
       12  </:code>
       13  </=el></select>
       14  </:for><#
       15  /></=what><#
       16  /></:for><#
       17  /></:for>

      produces exactly the same output. But it needs the special variable $h
      to do the trick. Anyhow, "<:eval>" is useful if parts of a template
      are fetched from a database for example.

    <:include file .../> or <:include file>...</:include>
      evaluates another template and inserts the result. The include
      directive expects a parameter list containing at least on item without
      an unquoted equal sign (=). The first such element is used as name of
      the template file. Templates are searched according to the given
      search path.

      All other parameters are used to make up the parameter list for the
      include template, see <:for> above. The tag body is ignored save
      "<:set>...</:set>" statements which are also used to form the
      parameter list.

    <:cond ...><:case ...>...</:case>...</:cond>
      is used to allow conditionally evaluation of templates. The statement
      somehow resembles Lisp's "cond" statement or C's "switch". The
      template

        1  <:for>
        2  <:set goods><:
        3  [
        4   [apple=>'300'],
        5   [pear=>'90'],
        6   [cherry=>'82'],
        7   [plum=>'120'],
        8  ]
        9  /></:set>
       10  <:code><=goods pre="<table>" post="
       11  </table>">
       12  <tr><:for x="<:/>"><=x><td><:/></td></=x></:for><:cond>
       13  <:case "$v->[1]>150"><b>very expensive</b></:case>
       14  <:case "$v->[1]<100"><b>bargain</b></:case>
       15  <:case 1>normal prize</:case>
       16  </:cond></tr></=goods></:code>
       17  </:for>

      generates a table of goods marking a few of them as very expensive or
      bargain buy depending on their prizes. The "<=goods>" statement at
      line 10 opens a variable substitution scope. Within this scope at line
      12 starts a "<:cond>" statement thus $v references to one list
      element. The "<:case>" statements at lines 13, 14 and 15 are then
      evaluated top down. The first "<:case>"'es body whose condition
      evaluates to true builds the value of the whole "<:cond>" statement.

      The example above shows "<:cond>" without parameters. If it is called
      with parameters they name variables that are to be used in the
      "<:case>" conditions. Thus you can compare more than one variable in
      "<:case>"es.

      Consider you often want to display thumbnails and only some of them
      should link to the originals. It would be useful to write an include
      template that can be called:

       <:include thumb.tmpl name=img321 link=yes/>

      to create a thumbnail that links to it's original or

       <:include thumb.tmpl name=img322/>

      to create a thumbnail without a link.

      thumb.tmpl could look like:

        1  <:cond link>
        2  <:case "$link eq 'yes'">
        3    <a href="orig/<=name/>.jpg><img src="<=name/>.jpg"></a>
        4  </:case>
        5  <:case 1>
        6    <img src="<=name/>.jpg">
        7  </:case>
        8  </:cond>

      Now the "<:cond>" at line 1 names "link". Thus $link can be used at
      line 2 in the "<:case:" condition.

    <:set></:set>
      see "<:for>" statement above.

Commenting templates
    Writing templates is very similar to writing programs. And as programs
    should contain comments to be maintainable so do templates. You can even
    put POD sections into a template and generate documentation using POD
    translators.

    When thinking about comments within templates I first thought that using

     <:# any comment/>

    or

     <:># any comment</:>

    would be appropriate. It invokes the perl interpreter to evaluate just

     # any comment

    which is a perl comment and evaluates to "undef". Although it works it
    can slow down template eveluation. Hence I decided to let comments look
    like

     <# this is a comment />

    or

     <#> this is a comment </#>

    Thus, to put a longer comment (POD section) into your template surround
    it with "<#>" and "</#>":

     <#>
 
     =head1 NAME
 
     My very cute template
 
     ...
 
     </#>

ERROR HANDLING
    There are various error conditions that can occur while evaluating a
    template. Operating system errors like *Template File not found* or *No
    space left on device* can be recognized by the return codes of open() or
    evaluate_to_file(). But what to do if the evaluation of a template code
    fragment dies. Abort the whole process, insert nothing in place of the
    failed fragment, ...? Well, that's the subject of this chapter.

    There are 3 object attributes to specify what to do in case of an error
    during template evaluation.

    onerror
      defines what to do if a template code fragment dies during compilation
      or at runtime. "onerror" can be assigned the following values:

      "warn"
        report the error via a "warn" statement, continue template
        evaluation and insert nothing instead of the failed fragment.

      "die"
        report the error via a "die" statement aborting further temlpate
        evaluation.

      "output"
        continue template evaluation and insert the error message instead of
        the code fragment.

      a CODE reference
        the specified function is called in scalar context receiving the
        actual error message as $_[0]. It's return value is inserted instead
        of the code fragment. If the function dies the whole template
        evaluation is aborted.

      In either case the error message is appended to list of errors occured
      during template evaluation. This list can be retrieved after
      evaluate() has returned via the errors() method.

    eprefix
      That string will be prepended to each error message. This can be used
      to distinguish between errors from different object using all the same
      error list.

    errors
      contains the reference to the object's error list. This list is empty
      after a new object is created. If an error occur while evaluating a
      template with that object the error message is appended to that array.
      The error list is not cleared between calls to evaluate(). Thus, if an
      object is used to evaluate multiple templates or one templates several
      times it is recommended to clear this list between evaluate() using
      either clear_errors() or "$self->errors=[]".

TODO
    * Since our parse function can parse our templates why not use it to
      read configuration files or even files that contain data to be filled
      in other templates?

    * more control elements. Particularly it is useful to have a macro
      definition and invocation functionality and set, change and delete
      values from within a template.

    * output options: filters, gzip compression, ...

    * plugins. fetch variable content directly from a database

    * more testing

    * mod_perl support. (maybe it is only a matter of testing)

    * optional using Safe compartments to evaluate template code fragments.

    * It would be nice if some native speaker could correct my english.

CAVEATS
    The template parser is completely based on perl regular expressions.
    Though I have spent some effort to let the parsing process finish
    quickly even if the template is erroneous, it is certainly possible to
    construct malicious templates that will be parsed infinitely. Please let
    me know if you find such a template. I will try to fix it.

AUTHOR
    Torsten Förtsch <Torsten.Foertsch@gmx.net>

COPYRIGHT
    Copyright 2003 Torsten Förtsch.

    This library is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself.

INSTALLATION
     perl Makefile.PL
     make
     make test
     make install

DEPENDENCIES
    Class::Member 1.2

