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Devel::Declare(3)     User Contributed Perl Documentation    Devel::Declare(3)

NAME
       Devel::Declare - Adding keywords to perl, in perl

SYNOPSIS
	 use Method::Signatures;
	 # or ...
	 use MooseX::Declare;
	 # etc.

	 # Use some new and exciting syntax like:
	 method hello (Str :$who, Int :$age where { $_ > 0 }) {
	   $self->say("Hello ${who}, I am ${age} years old!");
	 }

DESCRIPTION
       Devel::Declare can install subroutines called declarators which locally
       take over Perl's parser, allowing the creation of new syntax.

       This document describes how to create a simple declarator.

USAGE
       We'll demonstrate the usage of "Devel::Declare" with a motivating
       example: a new "method" keyword, which acts like the builtin "sub", but
       automatically unpacks $self and the other arguments.

	 package My::Methods;
	 use Devel::Declare;

   Creating a declarator with "setup_for"
       You will typically create

	 sub import {
	   my $class = shift;
	   my $caller = caller;

	   Devel::Declare->setup_for(
	       $caller,
	       { method => { const => \&parser } }
	   );
	   no strict 'refs';
	   *{$caller.'::method'} = sub (&) {};
	 }

       Starting from the end of this import routine, you'll see that we're
       creating a subroutine called "method" in the caller's namespace.	 Yes,
       that's just a normal subroutine, and it does nothing at all (yet!)
       Note the prototype "(&)" which means that the caller would call it like
       so:

	   method {
	       my ($self, $arg1, $arg2) = @_;
	       ...
	   }

       However we want to be able to call it like this

	   method foo ($arg1, $arg2) {
	       ...
	   }

       That's why we call "setup_for" above, to register the declarator
       'method' with a custom parser, as per the next section.	It acts on an
       optype, usually 'const' as above.  (Other valid values are 'check' and
       'rv2cv').

       For a simpler way to install new methods, see also
       Devel::Declare::MethodInstaller::Simple

   Writing a parser subroutine
       This subroutine is called at compilation time, and allows you to read
       the custom syntaxes that we want (in a syntax that may or may not be
       valid core Perl 5) and munge it so that the result will be parsed by
       the "perl" compiler.

       For this example, we're defining some globals for convenience:

	   our ($Declarator, $Offset);

       Then we define a parser subroutine to handle our declarator.  We'll
       look at this in a few chunks.

	   sub parser {
	     local ($Declarator, $Offset) = @_;

       "Devel::Declare" provides some very low level utility methods to parse
       character strings.  We'll define some useful higher level routines
       below for convenience, and we can use these to parse the various
       elements in our new syntax.

       Notice how our parser subroutine is invoked at compile time, when the
       "perl" parser is pointed just before the declarator name.

	     skip_declarator;	       # step past 'method'
	     my $name = strip_name;    # strip out the name 'foo', if present
	     my $proto = strip_proto;  # strip out the prototype '($arg1, $arg2)', if present

       Now we can prepare some code to 'inject' into the new subroutine.  For
       example we might want the method as above to have "my ($self, $arg1,
       $arg2) = @_" injected at the beginning of it.  We also do some clever
       stuff with scopes that we'll look at shortly.

	     my $inject = make_proto_unwrap($proto);
	     if (defined $name) {
	       $inject = scope_injector_call().$inject;
	     }
	     inject_if_block($inject);

       We've now managed to change "method ($arg1, $arg2) { ... }" into
       "method { injected_code; ... }".	 This will compile...  but we've lost
       the name of the method!

       In a cute (or horrifying, depending on your perspective) trick, we
       temporarily change the definition of the subroutine "method" itself, to
       specialise it with the $name we stripped, so that it assigns the code
       block to that name.

       Even though the next time "method" is compiled, it will be redefined
       again, "perl" caches these definitions in its parse tree, so we'll
       always get the right one!

       Note that we also handle the case where there was no name, allowing an
       anonymous method analogous to an anonymous subroutine.

	     if (defined $name) {
	       $name = join('::', Devel::Declare::get_curstash_name(), $name)
		 unless ($name =~ /::/);
	       shadow(sub (&) { no strict 'refs'; *{$name} = shift; });
	     } else {
	       shadow(sub (&) { shift });
	     }
	   }

   Parser utilities in detail
       For simplicity, we're using global variables like $Offset in these
       examples.  You may prefer to look at Devel::Declare::Context::Simple,
       which encapsulates the context much more cleanly.

       "skip_declarator"

       This simple parser just moves across a 'token'.	The common case is to
       skip the declarator, i.e.  to move to the end of the string 'method'
       and before the prototype and code block.

	   sub skip_declarator {
	     $Offset += Devel::Declare::toke_move_past_token($Offset);
	   }

       "toke_move_past_token"

       This builtin parser simply moves past a 'token' (matching
       "/[a-zA-Z_]\w*/") It takes an offset into the source document, and
       skips past the token.  It returns the number of characters skipped.

       "strip_name"

       This parser skips any whitespace, then scans the next word (again
       matching a 'token').  We can then analyse the current line, and
       manipulate it (using pure Perl).	 In this case we take the name of the
       method out, and return it.

	   sub strip_name {
	     skipspace;
	     if (my $len = Devel::Declare::toke_scan_word($Offset, 1)) {
	       my $linestr = Devel::Declare::get_linestr();
	       my $name = substr($linestr, $Offset, $len);
	       substr($linestr, $Offset, $len) = '';
	       Devel::Declare::set_linestr($linestr);
	       return $name;
	     }
	     return;
	   }

       "toke_scan_word"

       This builtin parser, given an offset into the source document, matches
       a 'token' as above but does not skip.  It returns the length of the
       token matched, if any.

       "get_linestr"

       This builtin returns the full text of the current line of the source
       document.

       "set_linestr"

       This builtin sets the full text of the current line of the source
       document.  Beware that injecting a newline into the middle of the line
       is likely to fail in surprising ways.  Generally, Perl's parser can
       rely on the `current line' actually being only a single line.  Use
       other kinds of whitespace instead, in the code that you inject.

       "skipspace"

       This parser skips whitsepace.

	   sub skipspace {
	     $Offset += Devel::Declare::toke_skipspace($Offset);
	   }

       "toke_skipspace"

       This builtin parser, given an offset into the source document, skips
       over any whitespace, and returns the number of characters skipped.

       "strip_proto"

       This is a more complex parser that checks if it's found something that
       starts with '(' and returns everything till the matching ')'.

	   sub strip_proto {
	     skipspace;

	     my $linestr = Devel::Declare::get_linestr();
	     if (substr($linestr, $Offset, 1) eq '(') {
	       my $length = Devel::Declare::toke_scan_str($Offset);
	       my $proto = Devel::Declare::get_lex_stuff();
	       Devel::Declare::clear_lex_stuff();
	       $linestr = Devel::Declare::get_linestr();
	       substr($linestr, $Offset, $length) = '';
	       Devel::Declare::set_linestr($linestr);
	       return $proto;
	     }
	     return;
	   }

       "toke_scan_str"

       This builtin parser uses Perl's own parsing routines to match a
       "stringlike" expression.	 Handily, this includes bracketed expressions
       (just think about things like "q(this is a quote)").

       Also it Does The Right Thing with nested delimiters (like "q(this (is
       (a) quote))").

       It returns the effective length of the expression matched.  Really,
       what it returns is the difference in position between where the string
       started, within the buffer, and where it finished.  If the string
       extended across multiple lines then the contents of the buffer may have
       been completely replaced by the new lines, so this position difference
       is not the same thing as the actual length of the expression matched.
       However, because moving backward in the buffer causes problems, the
       function arranges for the effective length to always be positive,
       padding the start of the buffer if necessary.

       Use "get_lex_stuff" to get the actual matched text, the content of the
       string.	Because of the behaviour around multiline strings, you can't
       reliably get this from the buffer.  In fact, after the function
       returns, you can't rely on any content of the buffer preceding the end
       of the string.

       If the string being scanned is not well formed (has no closing
       delimiter), "toke_scan_str" returns "undef".  In this case you cannot
       rely on the contents of the buffer.

       "get_lex_stuff"

       This builtin returns what was matched by "toke_scan_str".  To avoid
       segfaults, you should call "clear_lex_stuff" immediately afterwards.

   Munging the subroutine
       Let's look at what we need to do in detail.

       "make_proto_unwrap"

       We may have defined our method in different ways, which will result in
       a different value for our prototype, as parsed above.  For example:

	   method foo	      {	 # undefined
	   method foo ()      {	 # ''
	   method foo ($arg1) {	 # '$arg1'

       We deal with them as follows, and return the appropriate "my ($self,
       ...) = @_;" string.

	   sub make_proto_unwrap {
	     my ($proto) = @_;
	     my $inject = 'my ($self';
	     if (defined $proto) {
	       $inject .= ", $proto" if length($proto);
	       $inject .= ') = @_; ';
	     } else {
	       $inject .= ') = shift;';
	     }
	     return $inject;
	   }

       "inject_if_block"

       Now we need to inject it after the opening '{' of the method body.  We
       can do this with the building blocks we defined above like "skipspace"
       and "get_linestr".

	   sub inject_if_block {
	     my $inject = shift;
	     skipspace;
	     my $linestr = Devel::Declare::get_linestr;
	     if (substr($linestr, $Offset, 1) eq '{') {
	       substr($linestr, $Offset+1, 0) = $inject;
	       Devel::Declare::set_linestr($linestr);
	     }
	   }

       "scope_injector_call"

       We want to be able to handle both named and anonymous methods.  i.e.

	   method foo () { ... }
	   my $meth = method () { ... };

       These will then get rewritten as

	   method { ... }
	   my $meth = method { ... };

       where 'method' is a subroutine that takes a code block.	Spot the
       problem?	 The first one doesn't have a semicolon at the end of it!
       Unlike 'sub' which is a builtin, this is just a normal statement, so we
       need to terminate it.  Luckily, using "B::Hooks::EndOfScope", we can do
       this!

	 use B::Hooks::EndOfScope;

       We'll add this to what gets 'injected' at the beginning of the method
       source.

	 sub scope_injector_call {
	   return ' BEGIN { MethodHandlers::inject_scope }; ';
	 }

       So at the beginning of every method, we are passing a callback that
       will get invoked at the end of the method's compilation... i.e. exactly
       then the closing '}' is compiled.

	 sub inject_scope {
	   on_scope_end {
	     my $linestr = Devel::Declare::get_linestr;
	     my $offset = Devel::Declare::get_linestr_offset;
	     substr($linestr, $offset, 0) = ';';
	     Devel::Declare::set_linestr($linestr);
	   };
	 }

   Shadowing each method.
       "shadow"

       We override the current definition of 'method' using "shadow".

	   sub shadow {
	     my $pack = Devel::Declare::get_curstash_name;
	     Devel::Declare::shadow_sub("${pack}::${Declarator}", $_[0]);
	   }

       For a named method we invoked like this:

	   shadow(sub (&) { no strict 'refs'; *{$name} = shift; });

       So in the case of a "method foo { ... }", this call would redefine
       "method" to be a subroutine that exports 'sub foo' as the (munged)
       contents of "{...}".

       The case of an anonymous method is also cute:

	   shadow(sub (&) { shift });

       This means that

	   my $meth = method () { ... };

       is rewritten with "method" taking the codeblock, and returning it as is
       to become the value of $meth.

       "get_curstash_name"

       This returns the package name currently being compiled.

       "shadow_sub"

       Handles the details of redefining the subroutine.

SEE ALSO
       One of the best ways to learn "Devel::Declare" is still to look at
       modules that use it:

       http://cpants.perl.org/dist/used_by/Devel-Declare
       <http://cpants.perl.org/dist/used_by/Devel-Declare>.

AUTHORS
       Matt S Trout - <mst@shadowcat.co.uk> - original author

       Company: http://www.shadowcat.co.uk/ Blog:
       http://chainsawblues.vox.com/

       Florian Ragwitz <rafl@debian.org> - maintainer

       osfameron <osfameron@cpan.org> - first draft of documentation

COPYRIGHT AND LICENSE
       This library is free software under the same terms as perl itself

       Copyright (c) 2007, 2008, 2009  Matt S Trout

       Copyright (c) 2008, 2009	 Florian Ragwitz

       stolen_chunk_of_toke.c based on toke.c from the perl core, which is

       Copyright (C) 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
       2000, 2001, 2002, 2003, 2004, 2005, 2006, by Larry Wall and others

perl v5.16.2			  2012-02-22		     Devel::Declare(3)
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