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

       Devel::Declare -	(DEPRECATED) Adding keywords to	perl, in perl

	 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!");

       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.

       Warning:	Devel::Declare is a giant bag of crack originally implemented
       by mst with the goal of upsetting the perl core developers so much by
       its very	existence that they implemented	proper keyword handling	in the

       As of perl5 version 14, this goal has been achieved, and	modules	such
       as Devel::CallParser, Function::Parameters, and Keyword::Simple provide
       mechanisms to mangle perl syntax	that don't require hallucinogenic
       drugs to	interpret the error messages they produce.

       If you are using	something that uses Devel::Declare, please for the
       love of kittens use something else:

       o   Instead of TryCatch,	use Try::Tiny

       o   Instead of Method::Signatures, use real subroutine signatures
	   (requires perl 5.22)	or Moops

       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;

	       { 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

	   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

       For a simpler way to install new	methods, see also

   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;

       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.


       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);


       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.


       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 {
	     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) = '';
	       return $name;


       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.


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


       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.


       This parser skips whitsepace.

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


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


       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 {

	     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();
	       $linestr	= Devel::Declare::get_linestr();
	       substr($linestr,	$Offset, $length) = '';
	       return $proto;


       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.


       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.


       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;


       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;
	     my	$linestr = Devel::Declare::get_linestr;
	     if	(substr($linestr, $Offset, 1) eq '{') {
	       substr($linestr,	$Offset+1, 0) =	$inject;


       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

	 use B::Hooks::EndOfScope;

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

	 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) = ';';

   Shadowing each method.

       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.


       This returns the	package	name currently being compiled.


       Handles the details of redefining the subroutine.

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


       Matt S Trout - <> - original author

       Company: Blog:

       Florian Ragwitz <> - maintainer

       osfameron <> -	first draft of documentation

       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 v5.32.0			  2020-04-26		     Devel::Declare(3)


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