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MooseX::Method::Signatures - Method declarations with type constraints and no source filter


package Foo;

use Moose;
use MooseX::Method::Signatures;

method morning (Str $name) {
    $self->say("Good morning ${name}!");

method hello (Str :$who, Int :$age where { $_ > 0 }) {
    $self->say("Hello ${who}, I am ${age} years old!");

method greet (Str $name, Bool :$excited = 0) {
    if ($excited) {
        $self->say("GREETINGS ${name}!");
    else {
        $self->say("Hi ${name}!");

$foo->morning('Resi');                          # This works.

$foo->hello(who => 'world', age => 42);         # This too.

$foo->greet('Resi', excited => 1);              # And this as well.

$foo->hello(who => 'world', age => 'fortytwo'); # This doesn't.

$foo->hello(who => 'world', age => -23);        # This neither.

$foo->morning;                                  # Won't work.

$foo->greet;                                    # Will fail.


Provides a proper method keyword, like "sub" but specifically for making methods and validating their arguments against Moose type constraints.


The signature syntax is heavily based on Perl 6. However not the full Perl 6 signature syntax is supported yet and some of it never will be.

Type Constraints

method foo (             $affe) # no type checking
method bar (Animal       $affe) # $affe->isa('Animal')
method baz (Animal|Human $affe) # $affe->isa('Animal') || $affe->isa('Human')

Positional vs. Named

method foo ( $a,  $b,  $c) # positional
method bar (:$a, :$b, :$c) # named
method baz ( $a,  $b, :$c) # combined

Required vs. Optional

method foo ($a , $b!, :$c!, :$d!) # required
method bar ($a?, $b?, :$c , :$d?) # optional


method foo ($a = 42) # defaults to 42


method foo ($foo where { $_ % 2 == 0 }) # only even


method foo (        $moo) # invocant is called $self and is required
method bar ($self:  $moo) # same, but explicit
method baz ($class: $moo) # invocant is called $class


method foo (:     $affe ) # called as $obj->foo(affe => $value)
method bar (:apan($affe)) # called as $obj->foo(apan => $value)


method foo (Affe $bar does trait)
method foo (Affe $bar is trait)

The only currently supported trait is coerce, which will attempt to coerce the value provided if it doesn't satisfy the requirements of the type constraint.


method foo ($bar, $, $baz)

Sometimes you don't care about some params you're being called with. Just put the bare sigil instead of a full variable name into the signature to avoid an extra lexical variable to be created.

Complex Example

method foo ( SomeClass $thing where { $_->can('stuff') }:
             Str  $bar  = "apan",
             Int :$baz! = 42 where { $_ % 2 == 0 } where { $_ > 10 } )

# the invocant is called $thing, must be an instance of SomeClass and
       has to implement a 'stuff' method
# $bar is positional, required, must be a string and defaults to "apan"
# $baz is named, required, must be an integer, defaults to 42 and needs
#      to be even and greater than 10


This module is as stable now, but this is not to say that it is entirely bug free. If you notice any odd behaviour (messages not being as good as they could for example) then please raise a bug.

Fancy signatures

Parse::Method::Signatures is used to parse the signatures. However, some signatures that can be parsed by it aren't supported by this module (yet).

No source filter

While this module does rely on the hairy black magic of Devel::Declare it does not depend on a source filter. As such, it doesn't try to parse and rewrite your source code and there should be no weird side effects.

Devel::Declare only effects compilation. After that, it's a normal subroutine. As such, for all that hairy magic, this module is surprisingly stable.

What about regular subroutines?

Devel::Declare cannot yet change the way sub behaves. However, the signatures module can. Right now it only provides very basic signatures, but it's extendable enough that plugging MooseX::Method::Signatures signatures into that should be quite possible.

What about the return value?

Type constraints for return values can be declared using

method foo (Int $x, Str $y) returns (Bool) { ... }

however, this feature only works with scalar return values and is still considered to be experimental.

Interaction with Moose::Role

Methods not seen by a role's requires

Because the processing of the MooseX::Method::Signatures method and the Moose with keywords are both done at runtime, it can happen that a role will require a method before it is declared (which will cause Moose to complain very loudly and abort the program).

For example, the following will not work:

# in file

package Canine;

use Moose;
use MooseX::Method::Signatures;

with 'Watchdog';

method bark { print "Woof!\n"; }


# in file

package Watchdog;

use Moose::Role;

requires 'bark';  # will assert! evaluated before 'method' is processed

sub warn_intruder {
    my $self = shift;
    my $intruder = shift;

    $self->bark until $intruder->gone;


A workaround for this problem is to use with only after the methods have been defined. To take our previous example, Canine could be reworked thus:

package Canine;

use Moose;
use MooseX::Method::Signatures;

method bark { print "Woof!\n"; }

with 'Watchdog';


A better solution is to use MooseX::Declare instead of plain MooseX::Method::Signatures. It defers application of roles until the end of the class definition. With it, our example would becomes:

# in file

use MooseX::Declare;

class Canine with Watchdog {
    method bark { print "Woof!\n"; }


# in file

use MooseX::Declare;

role Watchdog {
    requires 'bark';

    method warn_intruder ( $intruder ) {
        $self->bark until $intruder->gone;


Subroutine redefined warnings

When composing a Moose::Role into a class that uses MooseX::Method::Signatures, you may get a "Subroutine redefined" warning. This happens when both the role and the class define a method/subroutine of the same name. (The way roles work, the one defined in the class takes precedence.) To eliminate this warning, make sure that your with declaration happens after any method/subroutine declarations that may have the same name as a method/subroutine within a role.











This software is copyright (c) 2012 by Florian Ragwitz.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

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