bionic (3) Devel::Declare.3pm.gz
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