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.


This is ALPHA SOFTWARE. Use at your own risk. Features may change.


Provides a proper method keyword, like "sub" but specificly 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.

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 "affe"
    # $baz is named, required, must be an integer, defaults to 42 and needs
    #      to be even and greater than 10


Non-scalar parameters

Currently parameters that aren't scalars are unsupported. This is going to change soon.

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).


This totally breaks the debugger. Will have to wait on Devel::Declare fixes.

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 surprisnigly stable.

What about regular subroutines?

Devel::Declare cannot yet change the way sub behaves.

What about the return value?

Currently there is no support for types or declaring the type of the return value.

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.









Copyright (c) 2008 Florian Ragwitz

Code based on the tests for Devel::Declare.

Documentation based on MooseX::Method and Method::Signatures.

Licensed under the same terms as Perl itself.


Florian Ragwitz <>