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Method::Signatures - method and function declarations with signatures and no source filter


    package Foo;

    use Method::Signatures;

    method new (%args) {
        return bless {%args}, $self;

    method get ($key) {
        return $self->{$key};

    method set ($key, $val) {
        return $self->{$key} = $val;

    func hello($greeting, $place) {
        print "$greeting, $place!\n";


Provides two new keywords, func and method so you can write subroutines with signatures instead of having to spell out my $self = shift; my($thing) = @_

func is like sub but takes a signature where the prototype would normally go. This takes the place of my($foo, $bar) = @_ and does a whole lot more.

method is like func but specificly for making methods. It will automatically provide the invocant as $self. No more my $self = shift.

Also allows signatures, very similar to Perl 6 signatures.

And it does all this with no source filters.

Signature syntax

    func echo($message) {
        print "$message\n";

is equivalent to:

    sub echo {
        my($message) = @_;
        print "$message\n";

except the original line numbering is preserved and the arguments are checked to make sure they match the signature.


    method foo($bar, $baz) {
        $self->wibble($bar, $baz);

is equivalent to:

    sub foo {
        my $self = shift;
        my($bar, $baz) = @_;
        $self->wibble($bar, $baz);


Other than removing $self, @_ is left intact. You are free to use @_ alongside the arguments provided by Method::Signatures.

Named parameters

Parameters can be passed in named, as a hash, using the :$arg syntax.

    method foo(:$arg) {

    Class->foo( arg => 42 );

Named parameters by default are optional.

Required positional parameters and named parameters can be mixed, but the named params must come last.

    method foo( $a, $b, :$c )   # legal

Named parameters are passed in as a hash after all positional arguments.

    method display( $text, :$justify = 'left', :$enchef = 0 ) {

    # $text = "Some stuff", $justify = "right", $enchef = 0
    $obj->display( "Some stuff", justify => "right" );

You cannot mix optional positional params with named params as that leads to ambiguities.

    method foo( $a, $b?, :$c )  # illegal

    # Is this $a = 'c', $b = 42 or $c = 42?
    $obj->foo( c => 42 );

Aliased references

A signature of \@arg will take an array reference but allow it to be used as @arg inside the method. @arg is an alias to the original reference. Any changes to @arg will effect the original reference.

    package Stuff;
    method add_one(\@foo) {
        $_++ for @foo;

    my @bar = (1,2,3);
    Stuff->add_one(\@bar);  # @bar is now (2,3,4)

This feature requires Data::Alias to be installed. Method::Signatures does not depend on it because it does not currently work after 5.10.

Invocant parameter

The method invocant (ie. $self) can be changed as the first parameter. Put a colon after it instead of a comma.

    method foo($class:) {

    method stuff($class: $arg, $another) {
        $class->things($arg, $another);

Signatures have an implied default of $self:.


Each parameter can be given a default with the $arg = EXPR syntax. For example,

    method add($this = 23, $that = 42) {
        return $this + $that;

Almost any expression can be used as a default.

    method silly(
        $num    = 42,
        $string = q[Hello, world!],
        $hash   = { this => 42, that => 23 },
        $code   = sub { $num + 4 },
        @nums   = (1,2,3),

Defaults will only be used if the argument is not passed in at all. Passing in undef will override the default. That means...

    Class->add();            # $this = 23, $that = 42
    Class->add(99);          # $this = 99, $that = 42
    Class->add(99, undef);   # $this = 99, $that = undef

Earlier parameters may be used in later defaults.

    method copy_cat($this, $that = $this) {
        return $that;

All variables with defaults are considered optional.

Parameter traits

Each parameter can be assigned a trait with the $arg is TRAIT syntax.

    method stuff($this is ro) {

Any unknown trait is ignored.

Most parameters have a default traits of is rw is copy.


Read-only. Assigning or modifying the parameter is an error.


Read-write. It's ok to read or write the parameter.

This is a default trait.


The parameter will be a copy of the argument (just like <my $arg = shift>).

This is a default trait except for the \@foo parameter.


The parameter will be an alias of the argument. Any changes to the parameter will be reflected in the caller.

This is a default trait for the \@foo parameter.

Traits and defaults

To have a parameter which has both a trait and a default, set the trait first and the default second.

    method echo($message is ro = "what?") {
        return $message

Think of it as $message is ro being the left-hand side of the assignment.

Optional parameters

To declare a parameter optional, use the $arg? syntax.

Currently nothing is done with this. It's for forward compatibility.

Required parameters

To declare a parameter as required, use the $arg! syntax.

All parameters without defaults are required by default.

The @_ signature

The @_ signature is a special case which only shifts $self. It leaves the rest of @_ alone. This way you can get $self but do the rest of the argument handling manually.

Anonymous Methods

An anonymous method can be declared just like an anonymous sub.

    my $method = method ($arg) {
        return $self->foo($arg);


Differences from Perl 6

Method::Signatures is mostly a straight subset of Perl 6 signatures. The important differences...

Restrictions on named parameters

As noted above, there are more restrictions on named parameters than in Perl 6.

Named parameters are just hashes

Perl 5 lacks all the fancy named parameter syntax for the caller.

Parameters are copies.

In Perl 6, parameters are aliases. This makes sense in Perl 6 because Perl 6 is an "everything is an object" language. In Perl 5 is not, so parameters are much more naturally passed as copies.

You can alias using the "alias" trait.

Can't use positional params as named params

Perl 6 allows you to use any parameter as a named parameter. Perl 5 lacks the named parameter disambiguating syntax so it is not allowed.

Addition of the \@foo reference alias prototype

Because in Perl 6 arrays and hashes don't get flattened, and their referencing syntax is much improved. Perl 5 has no such luxury, so Method::Signatures added a way to alias references to normal variables to make them easier to work with.

Addition of the @_ prototype

Method::Signatures lets you punt and use @_ like in regular Perl 5.


There is no run-time performance penalty for using this module above what it normally costs to do argument handling.


One of the best ways to figure out what Method::Signatures is doing is to run your code through B::Deparse (run the code with -MO=Deparse).


Here's an example of a method which displays some text and takes some extra options.

  use Method::Signatures;

  method display($text is ro, :$justify = "left", :$fh = \*STDOUT) {

  # $text = $stuff, $justify = "left" and $fh = \*STDOUT

  # $text = $stuff, $justify = "left" and $fh = \*STDERR
  $obj->display($stuff, fh => \*STDERR);

  # error, missing required $text argument

The display() method is equivalent to all this code.

  sub display {
      my $self = shift;

      croak('display() missing required argument $text') unless @_ > 0;
      Readonly my $text = $_[0];

      my(%args) = @_[1 .. $#_];
      my $justify = exists $args{justify} ? $args{justify} : 'left';
      my $fh      = exists $args{fh}      ? $args{'fh'}    : \*STDOUT;



If you want to experiment with the prototype syntax, replace Method::Signatures::make_proto_unwrap. It takes a method prototype and returns a string of Perl 5 code which will be placed at the beginning of that method.

This interface is experimental, unstable and will change between versions.


Please report bugs and leave feedback at <bug-Method-Signatures> at <>. Or use the web interface at Report early, report often.


You can see the Perl code Method::Signatures translates to by using B::Deparse.

One liners

If you want to write "use Method::Signatures" in a one-liner, do a -MMethod::Signatures first. This is due to a bug/limitation in Devel::Declare.

No source filter

While this module does rely on the black magic of Devel::Declare to access Perl's own parser, 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. It's being worked on and when it works I'll release another module unifying method and sub.

I might release something using func.

What about class methods?

Right now there's nothing special about class methods. Just use $class as your invocant like the normal Perl 5 convention.

There may be special syntax to separate class from object methods in the future.

What about types?

I would like to add some sort of types in the future or simply make the signature handler pluggable.

What about the return value?

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

How does this relate to Perl's built-in prototypes?

It doesn't. Perl prototypes are a rather different beastie from subroutine signatures. They don't work on methods anyway.

A syntax for function prototypes is being considered.

    func($foo, $bar?) is proto($;$)

Error checking

There currently is very little checking done on the prototype syntax. Here's some basic checks I would like to add, mostly to avoid ambiguous or non-sense situations.

* If one positional param is optional, everything to the right must be optional

    method foo($a, $b?, $c?)  # legal

    method bar($a, $b?, $c)   # illegal, ambiguous

Does <-bar(1,2)>> mean $a = 1 and $b = 2 or $a = 1, $c = 3?

* If you're have named parameters, all your positional params must be required.

    method foo($a, $b, :$c);    # legal
    method bar($a?, $b?, :$c);   # illegal, ambiguous

Does <-bar(c => 42)>> mean $a = 'c', $b = 42 or just $c = 42?

* Positionals are resolved before named params. They have precedence.

What about...

Method traits are in the pondering stage.

An API to query a method's signature is in the pondering stage.

Now that we have method signatures, multi-methods are a distinct possibility.

Applying traits to all parameters as a short-hand?

    # Equivalent?
    method foo($a is ro, $b is ro, $c is ro)
    method foo($a, $b, $c) is ro

A "go really fast" switch. Turn off all runtime checks that might bite into performance.

Method traits.

    method add($left, $right) is predictable   # declarative
    method add($left, $right) is cached        # procedural
                                               # (and Perl 6 compatible)


Most of this module is based on or copied from hard work done by many other people.

All the really scary parts are copied from or rely on Matt Trout's, Florian Ragwitz's and Rhesa Rozendaal's Devel::Declare work.

The prototype syntax is a slight adaptation of all the excellent work the Perl 6 folks have already done.

Also thanks to Matthijs van Duin for his awesome Data::Alias which makes the \@foo signature work perfectly and Sub::Name which makes the subroutine names come out right in caller().

And thanks to Florian Ragwitz for his parallel MooseX::Method::Signatures module from which I borrow ideas and code and Devel::BeginLift which lets the methods be declared at compile time.


The original code was taken from Matt S. Trout's tests for Devel::Declare.

Copyright 2007-2008 by Michael G Schwern <>.

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



MooseX::Method::Signatures for a method keyword that works well with Moose.

Perl6::Signature for a more complete implementation of Perl 6 signatures.

Method::Signatures::Simple for a more basic version of what Method::Signatures provides.

signatures for sub with signatures.

Perl 6 subroutine parameters and arguments -