=encoding utf8

=head1 NAME

Synopsis_02 - Bits and Pieces

=head1 AUTHOR

Larry Wall <larry@wall.org>

=head1 VERSION

  Maintainer: Larry Wall <larry@wall.org>
  Date: 10 Aug 2004
  Last Modified: 25 Feb 2006
  Number: 2
  Version: 17

This document summarizes Apocalypse 2, which covers small-scale
lexical items and typological issues.  (These Synopses also contain
updates to reflect the evolving design of Perl 6 over time, unlike the
Apocalypses, which are frozen in time as "historical documents".
These updates are not marked--if a Synopsis disagrees with its
Apocalypse, assume the Synopsis is correct.)

=head1 Atoms

=over 4

=item *

In the abstract, Perl is written in Unicode, and has consistent Unicode
semantics regardless of the underlying text representations.

=item *

Perl can count Unicode line and paragraph separators as line markers,
but that behavior had better be configurable so that Perl's idea of
line numbers matches what your editor thinks about Unicode lines.

=item *

Unicode horizontal whitespace is counted as whitespace, but it's better
not to use thin spaces where they will make adjoining tokens look like
a single token.  On the other hand, Perl doesn't use indentation as syntax,
so you are free to use any whitespace anywhere that whitespace makes sense.

=back

=head1 Molecules

=over 4

=item *

In general, whitespace is optional in Perl 6 except where it is
needed to separate constructs that would be misconstrued as a single
token or other syntactic unit.  (In other words, Perl 6 follows the
standard "longest-token" principle, or in the cases of large constructs,
a "prefer shifting to reducing" principle.)

This is an unchanging deep rule, but the surface ramifications of it
change as various operators and macros are added to or removed from
the language, which we expect to happen because Perl 6 is designed to
be a mutable language.  In particular, there is a natural conflict
between postfix operators and infix operators, either of which may
occur after a term.  If a given token may be interpreted as either a
postfix operator or an infix operator, the infix operator requires
space before it, and the postfix operator requires a lack of space
before it, unless it begins with a dot.  (Infix operators may not
start with a dot.)  For instance, if you were to add your own
C<< infix:<++> >> operator, then it must have space before it, and the
normal autoincrementing C<< postfix:<++> >> operator may not have
space before it, or must be written as C<.++> instead.  In standard Perl
6, however, it doesn't matter if you put a space in front of
C<< postfix:<++> >>.  To be future proof, though, you should omit
the space or use dot.

=item *

Single-line comments work as in Perl 5, starting with a C<#> character
and ending with the subsequent newline.  They count as whitespace for
purposes of separation.  Certain quoting tokens may make use of C<#>
characters as delimiters without starting a comment.

=item *

Multiline comments will be provided by extending the syntax of POD
to nest C<=begin COMMENT>/C<=end COMMENT> correctly without the need
for C<=cut>.  (Doesn't have to be "COMMENT"--any unrecognized POD
stream will do to make it a comment.  Bare C<=begin> and C<=end>
probably aren't good enough though, unless you want all your comments
to end up in the manpage...)

We have single paragraph comments with C<=for COMMENT> as well.
That lets C<=for> keep its meaning as the equivalent of a C<=begin>
and C<=end> combined.  As with C<=begin> and C<=end>, a comment started
in code reverts to code afterwards.

=item *

Intra-line comments will not be supported in standard Perl (but it would
be trivial to declare them as a macro).

=back

=head1 Built-In Data Types

=over 4

=item *

In support of OO encapsulation, there is a new fundamental datatype:
B<P6opaque>.  External access to opaque objects is always through method
calls, even for attributes.

=item *

Perl 6 has an optional type system that helps you write safer
code that performs better.  The compiler is free to infer what type
information it can from the types you supply, but will not complain
about missing type information unless you ask it to.

=item *

Perl 6 supports the notion of B<properties> on various kinds of
objects.  Properties are like object attributes, except that they're
managed by the individual object rather than by the object's class.

According to S12, properties are actually implemented by a
kind of mixin mechanism, and such mixins are accomplished by the
generation of an individual anonymous class for the object (unless
an identical anonymous class already exists and can safely be shared).

=item *

Properties applied to compile-time objects such as variables and
classes are also called B<traits>.  Traits are not expected to change
at run time.  Changing run-time properties should be done via mixin
instead, so that the compiler can optimize based on declared traits.

=item *

Perl 6 is an OO engine, but you're not generally required to think
in OO when that's inconvenient.  However, some built-in concepts such
as filehandles will be more object-oriented in a user-visible way
than in Perl 5.

=item *

A variable's type is an interface contract indicating what sorts
of values the variable may contain. More precisely, it's a promise
that the object or objects contained in the variable are capable of
responding to the methods of the indicated "role".  See S12 for more
about roles.  A variable object may itself be bound to a container
type that specifies how the container works without necessarily
specifying what kinds of things it contains.

=item *

You'll be able to ask for the length of an array, but it won't be
called that, because C<length> does not specify units.  So
C<.elems> is the number of array elements.  You can also
ask for the length of an array in bytes or codepoints or graphemes.
The same methods apply to strings as well: there is no C<.length> on
strings either.

=item *

C<my Dog $spot> by itself does not automatically call a C<Dog> constructor.
The actual constructor syntax turns out to be C<my Dog $spot .= new;>,
making use of the C<.=> mutator method-call syntax.

=item *

If you say

    my int @array is MyArray;

you are declaring that the elements of C<@array> are integers,
but that the array itself is implemented by the C<MyArray> class.
Untyped arrays and hashes are still perfectly acceptable, but have
the same performance issues they have in Perl 5.

=item *

Built-in object types start with an uppercase letter: C<Int>, C<Num>,
C<Complex>, C<Str>, C<Bit>, C<Ref>, C<Scalar>, C<Array>, C<Hash>,
C<Rule> and C<Code>.  Non-object (value) types are lowercase: C<int>,
C<num>, C<complex>, C<str>, C<bit>, and C<ref>.  Value types are
primarily intended for declaring compact array storage.  However, Perl
will try to make those look like their corresponding uppercase types if
you treat them that way.  (In other words, it does autoboxing.  Note,
however, that sometimes repeated autoboxing can slow your program more
than the native type can speed it up.)

=item *

All Object types support the C<undefined> role, and may contain an
alternate set of attributes when undefined, such as the unthrown
exception explaining why the value is undefined.  Non-object types
are not required to support undefinedness, but it is an error to
assign an undefined value to such a location.

=item *

Regardless of whether they are defined, all objects support a C<.meta>
method that returns the class instance managing the current kind of object.
Any object (whether defined, undefined, or somewhere between) can be
used as a "kind" when the context requires it.

=item *

Perl 6 intrinsically supports big integers and rationals through its
system of type declarations.  C<Int> automatically supports promotion
to arbitrary precision.  (C<Num> may support arbitrary-precision
floating-point arithmatic, but is not required to unless we can do
so portably and efficiently.)  C<Rat> supports arbitrary precision
rational arithmetic.  Value types like C<int> and C<num> imply
the natural machine representation for integers and floating-point
numbers, respectively, and do not promote to arbitrary precision.
Untyped numeric scalars use C<Int> and C<Num> semantics rather than
C<int> and C<num>.

=item *

Perl 6 should by default make standard IEEE floating point concepts
visible, such as C<Inf> (infinity) and C<NaN> (not a number).
It should also be at least pragmatically possible to throw exceptions
on overflow.

=item *

A C<Str> is a Unicode string object.  A C<str> is a stringish view of
an array of integers, and has no Unicode or character properties without
explicit conversion to some kind of C<Str>.  Typically it's an array of bytes
serving as a buffer.

=back

=head1 Names and Variables

=over 4

=item *

The C<$pkg'var> syntax is dead.  Use C<$pkg::var> instead.

=item *

Perl 6 includes a system of B<sigils> to mark the fundamental
structural type of a variable:

    $	scalar
    @	ordered array
    %	unordered hash (associative array)
    &	code
    ::	package/module/class/role/subset/enum/type

Within a declaration, the C<&> sigil also declares the visibility of the
subroutine name without the sigil within the scope of the declaration.

Within a signature or other declaration, the C<::> sigil followed by an
identifier marks a parametric type that also declares the visibility
of a package/type name without the sigil within the scope of the
declaration.  The first such declaration within a scope is assumed
to be an unbound type, and takes the actual type of its associated
argument.  With subsequent declarations in the same scope the use of
the sigil is optional, since the bare type name is also declared.
A declaration nested within must not use the sigil if it wishes to
refer to the same type, since the inner declaration would rebind
the type.  (Note that the signature of a pointy block counts as part
of the inner block, not the outer block.)

=item *

Unlike in Perl 5, you may no longer put whitespace between a sigil
and its following name or construct.

=item *

Ordinary sigils indicate normally scoped variables, either lexical
or package scoped.  Oddly scoped variables include a secondary sigil
(a B<twigil>) that indicates what kind of strange scoping the variable
is subject to:

    $foo	ordinary scoping
    $.foo	object attribute accessor
    $^foo	self-declared formal parameter
    $*foo	global variable
    $+foo	environmental variable
    $?foo	compiler hint variable
    $=foo	pod variable
    $<foo>	match variable, short for $/{'foo'}
    $!foo	explicitly private attribute (mapped to $foo though)
    @;foo	multislice

Most variables with twigils are implicitly declared or assumed to
be declared in some other scope, and don't need a "my" or "our".
Attribute variables are declared with C<has>, though, and environment
variables are declared somewhere in the dynamic scope with the C<env>
declarator.

=item *

Sigils are now invariant.  C<$> always means a scalar variable, C<@>
an array variable, and C<%> a hash variable, even when subscripting.
Array and hash variable names in scalar context automatically produce
references.

=item *

In string contexts container references automatically dereference
to appropriate (white-space separated) string values.  In numeric
contexts, the number of elements in the container is returned.
In boolean contexts, a true value is returned if and only if there
are any elements in the container.

=item *

To get a Perlish representation of any data value, use the C<.perl>
method.  This will put quotes around strings, square brackets around
list values, curlies around hash values, constructors around objects,
etc., such that standard Perl could reparse the result.

=item *

To get a formatted representation of any scalar data value, use
the C<.as('%03d')> method to do an implicit sprintf on the value.
To format an array value separated by commas, supply a second argument:
C<.as('%03d', ', ')>.  To format a hash value or list of pairs, include
formats for both key and value in the first string: C<< .as('%s: %s', "\n") >>.

=item *

Subscripts now consistently dereference the reference produced by
whatever was to their left.  Whitespace is not allowed between a
variable name and its subscript.  However, there is a corresponding
B<dot> form of each subscript (C<@foo.[1]> and C<%bar.{'a'}>) which
allows optional whitespace before the dot (except when interpolating).
Constant string subscripts may be placed in angles, so C<%bar.{'a'}>
may also be written as C<< %bar<a> >> or C<< %bar.<a> >>.

=item *

Slicing is specified by the nature of the subscript, not by
the sigil.

=item *

The context in which a subscript is evaluated is no longer controlled
by the sigil either.  Subscripts are always evaluated in list context
on the assumption that slicing behavior is desired.  If you need to
force inner context to scalar, we now have convenient single-character
context specifiers such as + for numbers and ~ for strings.

=item *

There is a need to distinguish list assignment from list binding.
List assignment works exactly as it does in Perl 5, copying the
values.  There's a new C<:=> binding operator that lets you bind
names to array and hash references without copying, just as subroutine
arguments are bound to formal parameters.  See S06 for more about
parameter binding.

=item *

An argument list object (C<List>) may be created with backslashed parens:

    $args = \(1,2,3,:mice<blind>)

A C<List>'s values are parsed as ordinary expressions.  By default a
C<List> is lazy.  This interacts oddly with the fact that a C<List>
is immutable in the abstract.  Once all of a C<List>'s arguments are
fully evaluated (which happens at compile time when all the arguments
are constants), the C<List> functions as an immutable tuple type.
Before that moment, the eventual value may well be unknown.  All we
know is that is that we have the promise to make the bits of it
immutable as they become known.  C<List> objects may contain multiple
unresolved iterators such as pipes or slices.  How these are resolved
depends on what they are eventually bound to.  Some bindings are
sensitive to multiple dimensions while others are not.

=item *

A signature object may be created with coloned parens:

    my ::MySig = :(Int,Num,Complex, Status :mice)

A signature's values are parsed as declarations rather than ordinary
expressions.  You may not put arbitrary expressions, but you may, for
instance stack multiple types that all must match:

    :(Any Num Dog|Cat $numdog)

Such a signature may be used within another signature to apply
additional type constraints.  When applied to a tuple argument, the
signature allows you to specify the types of parameters that would
otherwise be untyped:

    :(Any Num Dog|Cat $numdog, MySig *$a ($i,$j,$k,$mousestatus))

=item *

Unlike in Perl 5, the notation C<&foo> merely creates a reference
to function "C<foo>" without calling it.  Any function reference may
be dereferenced and called using parens (which may, of course,
contain arguments).  Whitespace is not allowed before the parens,
but there is a corresponding C<.()> operator, which allows you to
insert optional whitespace before the dot.

=item *

With multiple dispatch, C<&foo> may not be sufficient to uniquely name a
specific function.  In that case, the type may be refined by using a
signature literal as a postfix operator:

    &foo:(Int,Num)

It still just returns a function reference.  A call may also be partially
applied by using a tuple literal as a postfix operator:

    &foo\(1,2,3,:mice<blind>)

This is really just a shorthand for

    &foo.assuming(1,2,3,:mice<blind>)


=item *

Slicing syntax is covered in S09.  Multidimensional
slices will be done with semicolons between individual slice subscripts.
Each such slice is evaluated lazily.

=item *

To make a slice subscript return something other than values, append an
appropriate adverb to the subscript.

    @array = <A B>;
    @array[0,1,2];	# returns 'A', 'B', undef
    @array[0,1,2]:p;	# returns 0 => 'A', 1 => 'B'
    @array[0,1,2]:kv;	# returns 0, 'A', 1, 'B'
    @array[0,1,2]:k;	# returns 0, 1
    @array[0,1,2]:v;	# returns 'A', 'B'

    %hash = (:a<A>, :b<B>);
    %hash<a b c>;	# returns 'A', 'B', undef
    %hash<a b c>:p;	# returns a => 'A', b => 'B'
    %hash<a b c>:kv;	# returns 'a', 'A', 'b', 'B'
    %hash<a b c>:k;	# returns 'a', 'b'
    %hash<a b c>:v;	# returns 'A', 'B'

The adverbial forms all weed out non-existing entries.

=item *

A hash reference in numeric context returns the number of pairs
contained in the hash.  A hash reference in a boolean context returns
true if there are any pairs in the hash.  In either case, any intrinsic
iterator would be reset.  (If hashes do carry an intrinsic iterator
(as they do in Perl 5), there will be a C<.reset> method on the hash
object to reset the iterator explicitly.)

=item *

Sorting a list of pairs should sort on their keys by default, then
on their values.  Sorting a list of lists should sort on the first
elements, then the second elements, etc.  For more on C<sort> see S29.

=item *

Many of the special variables of Perl 5 are going away.  Those that
apply to some object such as a filehandle will instead be attributes
of the appropriate object.  Those that are truly global will have
global alphabetic names, such as C<$*PID> or C<@*ARGS>. 

=item *

Any remaining special variables will be lexically scoped.
This includes C<$_> and C<@_>, as well as the new C<$/>, which
is the return value of the last regex match.  C<$0>, C<$1>, C<$2>, etc.,
are aliases into the C<$/> object.

=item *

The C<$#foo> notation is dead.  Use C<@foo.end> or C<[-1]> instead.
(Or C<@foo.shape[$dimension]> for multidimensional arrays.)

=back

=head1 Names

=over 4

=item *

Ordinary package-qualified names look like in Perl 5:

    $Foo::Bar::baz	# the $baz variable in package Foo::bar

Sometimes it's clearer to keep the sigil with the variable name, so an
alternate way to write this is:

    Foo::Bar::<$baz>

This is resolved at compile time because the variable name is a constant.

=item *

The following pseudo-package names are reserved in the first position:

    MY
    OUR
    GLOBAL
    OUTER
    CALLER
    ENV
    SUPER
    COMPILING

Other all-caps names are semi-reserved.  We may add more of them in
the future, so you can protect yourself from future collisions by using
mixed case on your top-level packages.  (We promise not to break
any existing top-level CPAN package, of course.  Except maybe ACME,
and then only for coyotes.)

=item *

You may interpolate a string into a package or variable name using
C<::($expr)> where you'd ordinarily put a package or variable name.
The string is allowed to contain additional instances of C<::>, which
will be interpreted as package nesting.  You may only interpolate
entire names, since the construct starts with C<::>, and either ends
immediately or is continued with another C<::> outside the curlies.
Most symbolic references are done with this notation:

    $foo = "Foo";
    $foobar = "Foo::Bar";
    $::($foo)		# package-scoped $Foo
    $::("MY::$foo")	# lexically-scoped $Foo
    $::("*::$foo")	# global $Foo
    $::($foobar)	# $Foo::Bar
    $::($foobar)::baz	# $Foo::Bar::baz
    $::($foo)::Bar::baz	# $Foo::Bar::baz
    $::($foobar)baz	# ILLEGAL at compile time (no operator baz)

Note that unlike in Perl 5, initial C<::> doesn't imply global.
Package names are searched for from inner lexical scopes to outer,
then from inner packages to outer.  Variable names are searched
for from inner lexical scopes to outer, but unlike package names
are looked for in only the current package and the global package.

The global namespace is the last place it looks in either case.
You must use the C<*> (or C<GLOBAL>) package on the front of the
string argument to force the search to start in the global namespace.

Use the C<MY> pseudopackage to limit the lookup to the current lexical
scope, and C<OUR> to limit the scopes to the current package scope.

=item *

When "strict" is in effect (which is the default except for one-liners),
non-qualified variables (such as C<$x> and C<@y>) are only looked up from
lexical scopes, but never from package scopes.

To bind package variables into a lexical scope, simply say C<our ($x, @y)>.
To bind global variables into a lexical scope, predeclare them with C<use>:

    use GLOBAL <$IN $OUT>;

Or just refer to them as C<$*IN> and C<$*OUT>.

=item *

To do direct lookup in a package's symbol table without scanning, treat
the package name as a hash:

    Foo::Bar::{'&baz'}	# same as &Foo::Bar::baz
    GLOBAL::<$IN>	# Same as $*IN
    Foo::<::Bar><::Baz>	# same as Foo::Bar::Baz

Unlike C<::()> symbolic references, this does not parse the argument
for C<::>, nor does it initiate a namespace scan from that initial
point.  In addition, for constant subscripts, it is guaranteed to
resolve the symbol at compile time.

The null pseudo-package is reserved to mean the same search list as an ordinary
name search.  That is, the following are all identical in meaning:

    $foo
    $::{'foo'}
    ::{'$foo'}
    $::<foo>
    ::<$foo>

That is, each of them scans lexical scopes outward, and then the current package scope
(though the package scope is then disallowed when "strict" is in effect).

As a result of these rules, you can write any arbitrary variable name as either of:

    $::{'!@#$#@'}
    ::{'$!@#$#@'}

You can also use the C<< ::<> >> form as long as there are no spaces in the name.

=item *

The current lexical symbol table may now be referenced through the
pseudo-package C<MY>.  The current package symbol table is visible as
pseudo-package C<OUR>.  The C<OUTER> name refers to the C<MY> symbol table
immediately surrounding the current C<MY>, and C<OUTER::OUTER> is the one
surrounding that one.

    our $foo = 41;
    say $::foo;		# prints 41, :: is no-op
    {
	my $foo = 42;
	say MY::<$foo>;		# prints "42"
	say $MY::foo;		# same thing
	say $::foo;		# same thing, :: is no-op here

	say OUR::<$foo>;	# prints "41"
	say $OUR::foo;		# same thing

	say OUTER::<$foo>;	# prints "41" (our $foo is also lexical)
	say $OUTER::foo;	# same thing
    }

You may not use any lexically scoped symbol table, either by name or
by reference, to add symbols to a lexical scope that is done compiling.
(We reserve the right to relax this if it turns out to be useful though.)

=item *

The C<CALLER> package refers to the lexical scope of the (dynamically
scoped) caller.  The caller's lexical scope is allowed to hide any
variable except C<$_> from you.  In fact, that's the default, and a
lexical variable must be declared using "C<env>" rather than C<my> to be
visible via C<CALLER>.  (C<$_>, C<$!> and C<$/> are always
environmental.) If the variable is not visible in the caller, it returns
failure.

An explicit C<env> declaration is implicitly readonly.  You may add
C<is rw> to allow subroutines from modifying your value.  C<$_> is
C<rw> by default.  In any event, your lexical scope can access the
variable as if it were an ordinary C<my>; the restriction on writing
applies only to subroutines.

=item *

The C<ENV> pseudo-package is just like C<CALLER> except that it scans
outward through all dynamic scopes until it finds an environmental
variable of that name in that caller's lexical scope.  (Use of C<$+FOO>
is equivalent to ENV::<$FOO> or $ENV::FOO.)  If after scanning all
the lexical scopes of each dynamic scope, there is no variable of
that name, it looks in the C<*> package.  If there is no variable in
the C<*> package, it looks in C<%*ENV> for the name, that is, in the
environment variables passed to program.  If the value is not found
there, it returns failure.  Note that C<$+_> is always the same as
CALLER::<$_> since all callers have a C<$_> that is automatically
considered environmental.  Note also that C<ENV> and C<$+> always
skip the current scope, since you can always name the variable
directly without the C<ENV> or C<+> if it's been declared C<env>
in the current lexical scope.

Subprocesses are passed only the global C<%*ENV> values.  They do not
see any lexical variables or their values.  The C<ENV> package is only
for internal overriding of environmental parameters.  Change C<%*ENV>
to change what subprocesses see.  [Conjecture: This might be suboptimal
in the abstract, but it would be difficult to track the current set of
environment variable names unless we actually passed around a list.
The alternative seems to be to walk the entire dynamic scope and
reconstruct %*ENV for each subprogram call, and then we only slow
down subprogram calls.]

=item *

There is no longer any special package hash such as C<%Foo::>.  Just
subscript the package object itself as a hash object, the key of which
is the variable name, including any sigil.  The package object can
be derived from a type name by use of the C<::> postfix operator:

    MyType .:: .{'$foo'}
    MyType::<$foo>		# same thing

(Directly subscripting the type with either square brackets or curlies
is reserved for various generic type-theoretic operations.  In most other
matters type names and package names are interchangeable.)

Typeglobs are gone.  Use binding (C<:=> or C<::=>) to do aliasing.
Individual variable objects are still accessible through the
hash representing each symbol table, but you have to include the
sigil in the variable name now: C<MyPackage::{'$foo'}> or the
equivalent C<< MyPackage::<$foo> >>.

=item *

Truly global variables live in the C<*> package: C<$*UID>, C<%*ENV>.
(The C<*> may generally be omitted if there is no inner declaration
hiding the global name.)  C<$*foo> is short for C<$*::foo>, suggesting
that the variable is "wild carded" into every package.

=item *

Standard input is C<$*IN>, standard output is C<$*OUT>, and standard error
is C<$*ERR>.  The magic command-line input handle is C<$*ARGS>.

=item *

Magical file-scoped values live in variables with a C<=> secondary
sigil.  C<$=DATA> is the name of your C<DATA> filehandle, for instance.
All pod structures are available through C<%=POD> (or some such).
As with C<*>, the C<=> may also be used as a package name: C<$=::DATA>.

=item *

Magical lexically scoped values live in variables with a C<?> secondary
sigil.  These are all values that are known to the compiler, and may
in fact be dynamically scoped within the compiler itself, and only
appear to be lexically scoped because dynamic scopes of the compiler
resolve to lexical scopes of the program.  All C<$?> variables are considered
constants, and may not be modified after being compiled in, except insofar
as the compiler arranges in advance for such variables to be rebound (as
is the case with C<$?SELF>).

C<$?FILE> and C<$?LINE> are your current file and line number, for
instance.  C<?> is not a shortcut for a package name like C<*> is.
Instead of C<$?OUTER::SUB> you probably want to write C<< OUTER::<$?SUB>
>>.

Here are some possibilities:

    $?OS        Which os am I compiled for?
    $?OSVER     Which os version am I compiled for?
    $?PERLVER   Which Perl version am I compiled for?
    $?FILE      Which file am I in?
    $?LINE      Which line am I at?
    $?PACKAGE   Which package am I in?
    @?PACKAGE   Which packages am I in?
    $?MODULE    Which module am I in?
    @?MODULE    Which modules am I in?
    ::?CLASS	Which class am I in? (as package name)
    $?CLASS     Which class am I in? (as variable)
    @?CLASS     Which classes am I in?
    ::?ROLE     Which role am I in? (as package name)
    $?ROLE      Which role am I in? (as variable)
    @?ROLE      Which roles am I in?
    $?GRAMMAR   Which grammar am I in?
    @?GRAMMAR   Which grammars am I in?
    $?PARSER    Which Perl grammar was used to parse this statement?
    &?SUB       Which sub am I in?
    @?SUB       Which subs am I in?
    $?SUBNAME   Which sub name am I in?
    @?SUBNAME   Which sub names am I in?
    &?BLOCK     Which block am I in?
    @?BLOCK     Which blocks am I in?
    $?LABEL	Which block label am I in?
    @?LABEL	Which block labels am I in?

Note that some of these things have parallels in the C<*> space at run time:

    $*OS	Which OS I'm running under
    $*OSVER	Which OS version I'm running under
    $*PERLVER	Which Perl version I'm running under

You should not assume that these will have the same value as their
compile-time cousins.

=item *

While C<$?> variables are constant to the run time, the compiler
has to have a way of changing these values at compile time without
getting confused about its own C<$?> variables (which were frozen in
when the compile-time code was itself compiled).  The compiler can
talk about these compiler-dynamic values using the C<COMPILING> pseudopackage.

References to C<COMPILING> variables are automatically hoisted into the
context currently being compiled.  Setting or temporizing a C<COMPILING>
variable sets or temporizes the incipient C<$?> variable in the
surrounding lexical context that is being compiled.  If nothing in
the context is being compiled, an exception is thrown.

    $?FOO // say "undefined";	# probably says undefined
    BEGIN { COMPILING::<$?FOO> = 42 }
    say $?FOO;			# prints 42
    {
	say $?FOO;		# prints 42
	BEGIN { temp COMPILING::<$?FOO> = 43 } # temporizes to *compiling* block
	say $?FOO;		# prints 43
	BEGIN { COMPILING::<$?FOO> = 44 }
	say $?FOO;		# prints 44
	BEGIN { say COMPILING::<$?FOO> }	# prints 44, but $?FOO probably undefined
    }
    say $?FOO;			# prints 42 (left scope of temp above)
    $?FOO = 45;			# always an error
    COMPILING::<$?FOO> = 45;	# an error unless we are compiling something

Note that C<< CALLER::<$?FOO> >> might discover the same variable
as C<COMPILING::<$?FOO>>, but only if the compiling context is the
immediate caller.  Likewise C<< OUTER::<$?FOO> >> might or might not
get you to the right place.  In the abstract, C<COMPILING::<$?FOO>>
goes outwards dynamically until it finds a compiling scope, and so is
guaranteed to find the "right" C<$?FOO>.  (In practice, the compiler
hopefully keeps track of its current compiling scope anyway, so no
scan is needed.)

Perceptive readers will note that this subsumes various "compiler hints"
proposals.  Crazy readers will wonder whether this means you could
set an initial value for other lexicals in the compiling scope.  The
answer is yes.  In fact, this mechanism is probably used by the
exporter to bind names into the importer's namespace.

=item *

The currently compiling Perl parser is switched by modifying
C<< COMPILING::<$?PARSER> >>.  Lexically scoped parser changes
should temporize the modification.  Changes from here to
end-of-compilation unit can just assign or bind it.  In general,
most parser changes involve deriving a new grammar and then pointing
C<< COMPILING::<$?PARSER> >> at that new grammar.  Alternately, the
tables driving the current parser can be modified without derivation,
but at least one level of anonymous derivation must intervene from
the standard Perl grammar, or you might be messing up someone else's
grammar.  Basically, the current grammar has to belong only to the
current compiling scope.  It may not be shared, at least not without
explicit consent of all parties.  No magical syntax at a distance.
Consent of the governed, and all that.

=back

=head1 Literals

=over 4

=item *

A single underscore is allowed only between any two digits in a
literal number, where the definition of digit depends on the radix.
Underscores are not allowed anywhere else in any numeric literal,
including next to the radix point or exponentiator, or at the beginning
or end.

=item *

Initial C<0> no longer indicates octal numbers by itself.  You must use
an explicit radix marker for that.  Pre-defined radix prefixes include:

    0b		base 2, digits 0..1
    0o		base 8, digits 0..7
    0d		base 10, digits 0..9
    0x		base 16, digits 0..9,a..f (case insensitive)

=item *

The general radix form of a number involves prefixing with the radix
in adverbial form:

    :10<42>		same as 0d42 or 42
    :16<dead_beef>	same as 0xdeadbeef
    :8<177777>		same as 0o177777 (65535)
    :2<1.1>		same as 0b1.1 (0d1.5)

Extra digits are assumed to be represented by 'a'..'z', so you can go up
to base 36.  (Use 'a' and 'b' for base twelve, not 't' and 'e'.)  Alternately
you can use a list of digits in decimal:

    :60[12,34,56]	# 12 * 3600 + 34 * 60 + 56
    :100[3,'.',14,16]	# pi

Any radix may include a fractional part.  A dot is never ambiguous
because you have to tell it where the number ends:

    :16<dead_beef.face>	# fraction
    :16<dead_beef>.face	# method call

=item *

Only base 10 (in any form) allows an additional exponentiator starting
with 'e' or 'E'.  All other radixes must either rely on the constant folding
properties of ordinary multiplication and exponentiation, or supply the
equivalent two numbers as part of the string, which will be interpreted
as they would outside the string, that is, as decimal numbers by default:

    :16<dead_beef> * 16**8
    :16<dead_beef*16**8>

It's true that only radixes that define C<e> as a digit are ambiguous that
way, but with any radix it's not clear whether the exponentiator should
be 10 or the radix, and this makes it explicit:

    0b1.1e10			illegal, could be read as any of:

    :2<1.1> * 2 ** 10		1536
    :2<1.1> * 10 ** 10		15,000,000,000
    :2<1.1> * :2<10> ** :2<10>	6

So we write those as

    :2<1.1*2**10>		1536
    :2<1.1*10**10>		15,000,000,000
    :2«1.1*:2<10>**:2<10>»	6

The generic string-to-number converter will recognize all of these
forms (including the * form, since constant folding is not available
to the run time).  Also allowed in strings are leading plus or minus,
and maybe a trailing Units type for an implied scaling.  Leading and
trailing whitespace is ignored.  Note also that leading C<0> by itself
I<never> implies octal in Perl 6.

Any of the adverbial forms may be used as a function:

    :2($x)	# "bin2num"
    :8($x)	# "oct2num"
    :10($x)	# "dec2num"
    :16($x)	# "hex2num"

Think of these as setting the default radix, not forcing it.  Like Perl
5's old C<oct()> function, any of these will recognize a number starting
with a different radix marker and switch to the other radix.  However,
note that the C<:16()> converter function will interpret leading C<0b>
or C<0d> as hex digits, not radix switchers.

=item *

Characters indexed by hex, octal, and decimal can be interpolated
into strings using either C<"\x123"> (with C<\o> and C<\d> behaving
respectively) or using square brackets: C<"\x[123]">.  Multiple
characters may be put into any of these by separating the numbers
with comma: C<"\x[41,42,43]">.

=item *

The C<qw/foo bar/> quote operator now has a bracketed form: C<< <foo bar> >>.
When used as a subscript it performs a slice equivalent to C<{'foo','bar'}>.
Much like the relationship between single quotes and double quotes, single
angles do not interpolate while double angles do.  The double angles may
be written either with French quotes, C<«$foo @bar[]»>, or
with "Texas" quotes, C<<< <<$foo @bar[]>> >>>, as the ASCII workaround.
The implicit split is done after interpolation, but respects quotes
in a shell-like fashion, so that C<«'$foo' "@bar[]"»> is guaranteed to
produce a list of two "words" equivalent to C<< ('$foo', "@bar[]") >>.
C<Pair> notation is also recognized inside C<«...»> and such "words" are
returned as C<Pair> objects.

=item *

Generalized quotes may now take adverbs:

    Short	Long		Meaning
    =====	====		=======
    :x		:exec		Execute as command and return results
    :w		:words		Split result on words (no quote protection)
    :ww		:quotewords	Split result on words (with quote protection)
    :t		:to		Interpret result as heredoc terminator
    :n		:none		No escapes at all (unless otherwise adverbed)
    :q		:single		Interpolate \\, \q and \' (or whatever)
    :qq		:double		Interpolate all the following
    :s		:scalar		Interpolate $ vars
    :a		:array		Interpolate @ vars
    :h		:hash		Interpolate % vars
    :f		:function	Interpolate & calls
    :c		:closure	Interpolate {...} expressions
    :b		:backslash	Interpolate \n, \t, etc. (implies :q at least)

[Conjectural: Ordinarily the colon is required on adverbs, but the
"quote" declarator allows you to combine any of the existing adverbial
forms above without an intervening colon:

    quote qw;		# declare a P5-esque qw//
    quote qqx;		# equivalent to P5's qx//
    quote qn;		# completely raw quote qn//
    quote qnc;		# interpolate only closures
    quote qqxwto;	# qq:x:w:to//

]

If this is all too much of a hardship, you can define your own quote
adverbs and operators.  All the uppercase adverbs are reserved for
user-defined quotes.  All of Unicode above Latin-1 is reserved for
user-defined quotes.

=item *

A consequence of the previous item is that we can now say:

    %hash = qw:c/a b c d {@array} {%hash}/;

or

    %hash = qq:w/a b c d {@array} {%hash}/;

to interpolate items into a C<qw>.  Conveniently, arrays and hashes
interpolate with only whitespace separators by default, so the subsequent
split on whitespace still works out.  (But the built-in C<«...»> quoter
automatically does interpolation equivalent to C<qq:ww/.../>.  The
built-in C<< <...> >> is equivalent to C<q:w/.../>.)

=item *

Whitespace is allowed between the "q" and its adverb: C<q :w /.../>.

=item *

For these "q" forms the choice of delimiters has no influence on the
semantics.  That is, C<''>, C<"">, C<< <> >>, C<«»>, C<``>, C<()>,
C<[]>, and C<{}> have no special significance when used in place of
C<//> as delimiters.  There may be whitespace or a colon before the
opening delimiter. (Which is mandatory for parens because C<q()> is
a subroutine call and C<q:w(0)> is an adverb with arguments).  Other
brackets may also require a colon or space when they would be understood as
an argument to an adverb in something like C<< q:z<foo>// >>.
A colon may never be used as the delimiter since it will always be
taken to mean something else regardless of what's in front of it.

=item *

New quoting constructs may be declared as macros:

    macro quote:<qX> (*%adverbs) {...}

Note: macro adverbs are automatically evaluated at macro call
time if the adverbs are included in the parse.  If the adverbs are
to affect the parsing of the quoted text of the macro, then the text must
be parsed by the body of the macro rather than by an C<is parsed> rule.

=item *

You may interpolate double-quotish text into a single-quoted string
using the C<\qq[...]> construct.  Other "q" forms also work, including
user-defined ones, as long as they start with "q".  Otherwise you'll
just have to embed your construct inside a C<\qq[...]>.

=item *

Bare scalar variables always interpolate in double-quotish
strings.  Bare array, hash, and subroutine variables may I<never> be
interpolated.  However, any scalar, array, hash or subroutine variable may
start an interpolation if it is followed by a sequence of one or more bracketed
dereferencers: that is, any of:

=over 4

=item 1. An array subscript

=item 2. A hash subscript

=item 3. A set of parentheses indicating a function call

=item 4. Any of 1 through 3 in their B<dot> form

=item 5. A method call that includes argument parentheses

=item 6. A sequence of one or more unparenthesized method call, followed by any of 1 through 5

=back

In other words, this is legal:

    "Val = $a.ord.as('%x')\n"

and is equivalent to

    "Val = { $a.ord.as('%x') }\n"


=item *

In order to interpolate an entire array, it's necessary now to subscript
with empty brackets:

    print "The answers are @foo[]\n"

Note that this fixes the spurious "C<@>" problem in double-quoted email addresses.

As with Perl 5 array interpolation, the elements are separated by a space.
(Except that a space is not added if the element already ends in some kind
of whitespace.  In particular, a list of pairs will interpolate with a
tab between the key and value, and a newline after the pair.)

=item *

In order to interpolate an entire hash, it's necessary to subscript
with empty braces or angles:

    print "The associations are:\n%bar{}"
    print "The associations are:\n%bar<>"

Note that this avoids the spurious "C<%>" problem in double-quoted printf formats.

By default, keys and values are separated by tab characters, and pairs
are terminated by newlines.  (This is almost never what you want, but
if you want something polished, you can be more specific.)

=item *

In order to interpolate the result of a sub call, it's necessary to include
both the sigil and parentheses:

    print "The results are &baz().\n"

The function is called in scalar context.  (If it returns a list anyway,
that list is interpolated as if it were an array in string context.)

=item *

In order to interpolate the result of a method call without arguments,
it's necessary to include parentheses or extend the call with something
ending in brackets:

    print "The attribute is $obj.attr().\n"
    print "The attribute is $obj.attr<Jan>.\n"

The method is called in scalar context.  (If it returns a list,
that list is interpolated as if it were an array.)

It is allowed to have a cascade of argumentless methods as long as
the last one ends with parens:

    print "The attribute is %obj.keys.sort.reverse().\n"

(The cascade is basically counted as a single method call for the
end-bracket rule.)

=item *

Multiple dereferencers may be stacked as long as each one ends in
some kind of bracket:

    print "The attribute is @baz[3](1,2,3){$xyz}<blurfl>.attr().\n"

Note that the final period above is not taken as part of the expression since
it doesn't introduce a bracketed dereferencer.  Spaces are not allowed
between the dereferencers even when you use the dotted forms.

=item *

A bare closure also interpolates in double-quotish context.  It may
not be followed by any dereferencers, since you can always put them
inside the closure.  The expression inside is evaluated in scalar
(string) context.  You can force list context on the expression using
either the C<*> or C<list> operator if necessary.

The following means the same as the previous example.

    print "The attribute is { @baz[3](1,2,3){$xyz}<blurfl>.attr }.\n"

The final parens are unnecessary since we're providing "real" code in
the curlies.  If you need to have double quotes that don't interpolate
curlies, you can explicitly remove the capability:

    qq:c(0) "Here are { $two uninterpolated } curlies";

Alternately, you can build up capabilities from single quote to tell
it exactly what you I<do> want to interpolate:

    q:s 'Here are { $two uninterpolated } curlies';

=item *

Secondary sigils (twigils) have no influence over whether the primary sigil
interpolates.  That is, if C<$a> interpolates, so do C<$^a>, C<$*a>,
C<$=a>, C<$?a>, C<$.a>, etc.  It only depends on the C<$>.

=item *

No other expressions interpolate.  Use curlies.

=item *

A class method may not be directly interpolated.  Use curlies:

    print "The dog bark is {Dog.bark}.\n"

=item *

The old disambiguation syntax:

    ${foo[$bar]}
    ${foo}[$bar]

is dead.  Use closure curlies instead:

    {$foo[$bar]}
    {$foo}[$bar]

(You may be detecting a trend here...)

=item *

To interpolate a topical method, use curlies: C<"{.bark}">.

=item *

To interpolate a function call without a sigil, use curlies: C<"{abs $var}">.

=item *

And so on.

=item *

Backslash sequences still interpolate, but there's no longer any C<\v>
to mean I<vertical tab>, whatever that is...  (C<\v> now match vertical
whitespace in a rule.)

=item *

There's also no longer any C<\L>, C<\U>, C<\l>, C<\u>, or C<\Q>.
Use curlies with the appropriate function instead: C<"{ucfirst $word}">.

=item *

You may interpolate any Unicode codepoint by name using C<\c> and
square brackets:

    "\c[NEGATED DOUBLE VERTICAL BAR DOUBLE RIGHT TURNSTILE]"

Multiple codepoints constituting a single character may be interpolated
with a single C<\c> by separating the names with comma:

    "\c[LATIN CAPITAL LETTER A, COMBINING RING ABOVE]"

Whether that is regarded as one character or two depends on the
Unicode support level of the current lexical scope.  It is also
possible to interpolate multiple codepoints that do not resolve to
a single character:

    "\c[LATIN CAPITAL LETTER A, LATIN CAPITAL LETTER B]"

[Note: none of the official Unicode character names contains comma.]

=item *

There are no barewords in Perl 6.  An undeclared bare identifier will
always be taken to mean a subroutine or method name.  (Class names
(and other type names) are predeclared, or prefixed with the C<::>
type sigil when you're declaring a new one.)  A consequence of this
is that there's no longer any "C<use strict 'subs'>".

=item *

There's also no
"C<use strict 'refs'>" because symbolic dereferences are now syntactically
distinguished from hard dereferences.  C<@{$arrayref}> must now be a
hard reference, while C<@::($string)> is explicitly a symbolic reference.
(Yes, this may give fits to the P5-to-P6 translator, but I think it's
worth it to separate the concepts.  Perhaps the symbolic ref form will
admit hard refs in a pinch.)

=item *

There is no hash subscript autoquoting in Perl 6.  Use C<< %x<foo> >>
for constant hash subscripts, or the old standby C<< %x{'foo'} >>.  (It
also works to say C<%x«foo»> as long as you realized it's subject to
interpolation.)

But C<< => >> still autoquotes any bare identifier to its immediate
left (horizontal whitespace allowed but not comments).  The identifier is not
subject to keyword or even macro interpretation.  If you say

    $x = do {
	call_something();
	if => 1;
    }

then C<$x> ends up containing the pair C<< ("if" => 1) >>.  Always.
(Unlike in Perl 5, where version numbers didn't autoquote.)

You can also use the :key($value) form to quote the keys of option
pairs.  To align values of option pairs, you may not use the
dot postfix forms:

    :longkey  .($value)
    :shortkey .<string>
    :fookey   .{ $^a <=> $^b }

These will be interpreted as

    :longkey(1)  .($value)
    :shortkey(1) .<string>
    :fookey(1)   .{ $^a <=> $^b }

You just have to put spaces inside the parenthesis form to align things.

=item *

The double-underscore forms are going away:

    Old                 New
    ---                 ---
    __LINE__            $?LINE
    __FILE__            $?FILE
    __PACKAGE__         $?PACKAGE
    __END__             =begin END
    __DATA__            =begin DATA

The C<=begin END> pod stream is special in that it assumes there's
no corresponding C<=end END> before end of file.  The C<DATA>
stream is no longer special--any POD stream in the current file
can be accessed via a filehandle, named as C<< %=POD{'DATA'} >> and such.
Alternately, you can treat a pod stream as a scalar via C<$=DATA>
or as an array via C<@=DATA>.  Presumably a module could read all
its COMMENT blocks from C<@=COMMENT>, for instance.  Each chunk of
pod comes as a separate array element.  You have to split it into lines
yourself.  Each chunk has a C<.linenum> property that indicates its
starting line within the source file.

There is also a new C<$?SUBNAME> variable containing the name of current
lexical sub.  The lexical sub itself is C<&?SUB>.  The current block
is C<&?BLOCK>.  If the block has a label, that shows up in C<$?BLOCKLABEL>.

=item *

Heredocs are no longer written with C<<< << >>>, but with an adverb on
any other quote construct:

    print qq:to/END/
	Give $amount to the man behind curtain number $curtain.
	END

Other adverbs are also allowed:

    print q:c:to/END/
	Give $100 to the man behind curtain number {$curtain}.
	END

=item *

Here docs allow optional whitespace both before and after terminating
delimiter.  Leading whitespace equivalent to the indentation of the
delimiter will be removed from all preceding lines.  If a line is
deemed to have less whitespace than the terminator, only whitespace
is removed, and a warning may be issued.  (Hard tabs will be assumed
to be 8 spaces, but as long as tabs and spaces are used consistently
that doesn't matter.)  A null terminating delimiter terminates on
the next line consisting only of whitespace, but such a terminator
will be assumed to have no indentation.  (That is, it's assumed to
match at the beginning of any whitespace.)

=back

=head1 Context

=over 4

=item *

Perl still has the three main contexts: void, scalar, and list.

=item *

In addition to undifferentiated scalars, we also have these scalar contexts:

    Context	Type	OOtype	Operator
    -------	----	------	--------
    boolean	bit	Bit	?
    integer	int	Int	int
    numeric	num	Num	+
    string	str	Str	~

There are also various reference contexts that require particular kinds of
container references.

=item *

Unlike in Perl 5, references are no longer always considered true.
It depends on the state of their C<.bit> property.  Classes get to decide
which of their values are true and which are false.  Individual objects
can override the class definition:

    return 0 but True;

=back

=head1 Lists

=over 4

=item *

List context in Perl 6 is by default lazy.  This means a list can
contain infinite generators without blowing up.  No flattening happens
to a lazy list until it is bound to the signature of a function or
method at call time (and maybe not even then).  We say that such
an argument list is "lazily flattened", meaning that we promise to
flatten the list on demand, but not before.

=item *

There is a "C<list>" operator which imposes a list context on
its arguments even if C<list> itself occurs in a scalar context.
In list context, it flattens lazily.  In a scalar context, it returns
a reference to the resulting list.  (So the C<list> operator really
does exactly the same thing as putting a list in parentheses.  But
it's more readable in some situations.)

=item *

The C<*> unary operator may be used to force list context on its
argument and I<also> defeat any scalar argument checking imposed by
subroutine signature declarations.  This list flattens lazily.
When applied to a scalar value containing an iterator, C<*> causes
the iterator's return values be interpolated into the list lazily.
Note that C<*> is destructive when applied to a scalar iterator,
but non-destructive when applied to an array, even if that array
represents an iterator.

There is an argumentless form of C<*> which may be used within a
multi-dimensional array or hash subscript to indicate all of the
current set of subscripts available for this dimension.  It actually
returns a type value of C<Any>, so it can be used in any selector
where you would use C<Any>.

=item *

To force non-lazy list flattening, use the C<**> unary operator.
Don't use it on an infinite generator unless you have a machine with
infinite memory, and are willing to wait a long time.  It may also
be applied to a scalar iterator to force immediate iteration to completion.

Argumentless C<**> in a multi-dimensional subscript indicates 0 or
more dimensions of C<*> where the number of dimension isn't necessarily
known: C<@foo[1;**;5]>.  It has a value of C<List of Any>, or something
like that.  The argumentless C<*> and C<**> forms are probably only
useful in "dimensional" list contexts.

=item *

Signatures on non-multi subs can be checked at compile time, whereas
multi sub and method call signatures can only be checked at run time
(in the absence of special instructions to the optimizer).
This is not a problem for arguments that are arrays or hashes,
since they don't have to care about their context, but just return
a reference in any event, which may or may not be lazily flattened.
However, function calls in the argument list can't know their eventual
context because the method hasn't been dispatched yet, so we don't
know which signature to check against.  As in Perl 5, list context
is assumed unless you explicitly qualify the argument with a scalar
context operator.

=item *

The C<< => >> operator now constructs C<Pair> objects rather than merely
functioning as a comma.  Both sides are in scalar context.

=item *

The C<< .. >> operator now constructs C<Range> objects rather than merely
functioning as an operator.  Both sides are in scalar context.

=item *

There is no such thing as a hash list context.  Assignment to a hash
produces an ordinary list context.  You may assign alternating keys
and values just as in Perl 5.  You may also assign lists of C<Pair> objects, in
which case each pair provides a key and a value.  You may, in fact,
mix the two forms, as long as the pairs come when a key is expected.
If you wish to supply a C<Pair> as a key, you must compose an outer C<Pair>
in which the key is the inner C<Pair>:

    %hash = (($keykey => $keyval) => $value);

=item *

The anonymous C<enum> function takes a list of keys or pairs, and adds
values to any keys that are not already part of a key.  The value added
is one more than the previous key or pair's value.  This works nicely with
the new C<qq:ww> form:

    %hash = enum <<:Mon(1) Tue Wed Thu Fri Sat Sun>>;
    %hash = enum « :Mon(1) Tue Wed Thu Fri Sat Sun »;

are the same as:

    %hash = ();
    %hash<Mon Tue Wed Thu Fri Sat Sun> = 1..7;

=item *

In contrast to assignment, binding to a hash requires a C<Hash> (or
C<Pair>) reference.  Binding to a "splat" hash requires a list of pairs
or hashes, and stops processing the argument list when it runs out
of pairs or hashes.  See S06 for much more about parameter binding.

=back

=head1 Files

=over 4

=item *

Filename globs are no longer done with angle brackets.  Use the C<glob>
function.

=item *

Input from a filehandle is no longer done with angle brackets.  Instead
of

    while (<HANDLE>) {...}

you now write

    for =$handle {...}

As a unary prefix operator, you may also apply adverbs to C<=>:

    for =$handle :prompt('$ ') { say $_ + 1 }

or

    for =($handle):prompt('$ ') { say $_ + 1 }

or you may even write it in its functional form, passing the adverbs
as ordinary named arguments.

    for prefix:<=>($handle, :prompt('$ ')) { say $_ + 1 }

=back

=head1 Properties

=over 4

=item *

Properties work as detailed in S12.  They're actually object
attributes provided by role mixins.  Compile-time properties applied
to containers and such still use the C<is> keyword, but are now called
"traits".  On the other hand, run-time properties are attached to
individual objects using the C<but> keyword instead, but are still
called "properties".

=item *

Properties are accessed just like attributes because they are in fact
attributes of some class or other, even if it's an anonymous singleton
class generated on the fly for that purpose.  Since "C<rw>" attributes
behave in all respects as variables, properties may therefore also
be temporized with C<temp>, or hypotheticalized with C<let>.

=back
