Provided by: libdevel-declare-perl_0.006019-1_amd64 
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.
WARNING
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 core.
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:
• Instead of TryCatch, use Try::Tiny
• Instead of Method::Signatures, use real subroutine signatures (requires perl 5.22) or
Moops
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>.
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