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Michael G Schwern
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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;

    # Can also get type checking if you like:

    method set (Str $key, Int $val) {
        return $self->{$key} = $val;        # now you know $val is always an integer

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


Provides two new keywords, func and method, so that 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 specifically 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.

Also does type checking, understanding all the types that Moose (or Mouse) would understand.

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);

again with checks to make sure the arguments passed in match the signature.


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) {

    $object->foo( arg => 42 );

Named parameters are optional by default.

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 affect 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)

Invocant parameter

The method invocant (i.e. $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);

method has an implied default of $self:. func has no invocant.


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;

Any variable that has a default is considered optional.

Type Constraints

Parameters can also be given type constraints. If they are, the value passed in will be validated against the type constraint provided. Types are provided by Any::Moose which will load Mouse if Moose is not already loaded.

Type constraints can be a type, a role or a class. Each will be checked in turn until one of them passes.

    * First, is the $value of that type declared in Moose (or Mouse)?

    * Then, does the $value have that role?

    * Finally, is the $value an object of that class?

The set of default types that are understood can be found in Mouse::Util::TypeConstraints (or Moose::Util::TypeConstraints; they are generally the same, but there may be small differences).

    # avoid "argument isn't numeric" warnings
    method add(Int $this = 23, Int $that = 42) {
        return $this + $that;

Mouse and Moose also understand some parameterized types; see their documentation for more details.

    method add(Int $this = 23, Maybe[Int] $that) {
        # $this will definitely be defined
        # but $that might be undef
        return defined $that ? $this + $that : $this;

You may also use disjunctions, which means that you are willing to accept a value of either type.

    method add(Int $this = 23, Int|ArrayRef[Int] $that) {
        # $that could be a single number,
        # or a reference to an array of numbers
        use List::Util qw<sum>;
        my @ints = ($this);
        push @ints, ref $that ? @$that : $that;
        return sum(@ints);

If the value does not validate against the type, a run-time exception is thrown.

    # Error will be:
    # In call to Class::add : the 'this' parameter ("cow") is not of type Int
    Class->add('cow', 'boy'); # make a cowboy!

You cannot declare the type of the invocant.

    # this generates a compile-time error
    method new(ClassName $class:) {

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.

Slurpy parameters

A "slurpy" parameter is a list or hash parameter that "slurps up" all remaining arguments. Since any following parameters can't receive values, there can be only one slurpy parameter.

Slurpy parameters must come at the end of the signature and they must be positional.

Slurpy parameters are optional by default.

Required and optional parameters

Parameters declared using $arg! are explicitly required. Parameters declared using $arg? are explicitly optional. These declarations override all other considerations.

A parameter is implictly optional if it is a named parameter, has a default, or is slurpy. All other parameters are implicitly required.

    # $greeting is optional because it is named
    method hello(:$greeting) { ... }

    # $greeting is required because it is positional
    method hello($greeting) { ... }

    # $greeting is optional because it has a default
    method hello($greeting = "Gruezi") { ... }

    # $greeting is required because it is explicitly declared using !
    method hello(:$greeting!) { ... }

    # $greeting is required, even with the default, because it is
    # explicitly declared using !
    method hello(:$greeting! = "Gruezi") { ... }

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.

The empty signature

If a method is given the signature of () or no signature at all, it takes no arguments.

Anonymous Methods

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

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



Method::Signatures takes some options at `use` time of the form

    use Method::Signatures { option => "value", ... };


By default, named methods and funcs are evaluated at compile time, as if they were in a BEGIN block, just like normal Perl named subs. That means this will work:


    # This function is compiled first
    func echo($msg) { print $msg }

You can turn this off lexically by setting compile_at_BEGIN to a false value.

    use Method::Signatures { compile_at_BEGIN => 0 };

compile_at_BEGIN currently causes some issues when used with Perl 5.8. See "Earlier Perl versions".


When true, turns on debugging messages about compiling methods and funcs. See DEBUGGING. The flag is currently global, but this may change.

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. 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

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.

There is also no run-time penalty for type-checking if you do not declare types. The run-time penalty if you do declare types should be very similar to using Mouse::Util::TypeConstraints (or Moose::Util::TypeConstraints) directly, and should be faster than using a module such as MooseX::Params::Validate. The magic of Any::Moose is used to give you the lightweight Mouse if you have not yet loaded Moose, or the full-bodied Moose if you have.

Type-checking modules are not loaded until run-time, so this is fine:

    use Method::Signatures;
    use Moose;
    # you will still get Moose type checking
    # (assuming you declare one or more methods with types)


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).

Setting the METHOD_SIGNATURES_DEBUG environment variable will cause Method::Signatures to display debugging information when it is compiling signatures.


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;
      const 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, start with Method::Signatures::parse_func. It takes a method prototype and returns a string of Perl 5 code which will be placed at the beginning of that method.

If you would like to try to provide your own type checking, subclass Method::Signatures and either override type_check or inject_for_type_check. See "EXTENDING", below.

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


If you wish to subclass Method::Signatures, the following methods are good places to start.

too_many_args_error, named_param_error, required_arg, type_error

These are class methods which report the various run-time errors (extra parameters, unknown named parameter, required parameter missing, and parameter fails type check, respectively). Note that each one calls signature_error, which your versions should do as well.


This is a class method which calls die and reports the error as being from the caller's perspective. Most likely you will not need to override this. If you'd like to have Method::Signatures errors give full stack traces (similar to $Carp::Verbose), have a look at Carp::Always.


This is a class method which is called to verify that parameters have the proper type. If you want to change the way that Method::Signatures does its type checking, this is most likely what you want to override. It calls type_error (see above).


This is the object method that actually inserts the call to "type_check" into your Perl code. Most likely you will not need to override this, but if you wanted different parameters passed into type_check, this would be the place to do it.


Please report bugs and leave feedback at <bug-Method-Signatures> at <rt.cpan.org>. Or use the web interface at http://rt.cpan.org. Report early, report often.

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 affects compilation. After that, it's a normal subroutine. As such, for all that hairy magic, this module is surprisingly stable.

Earlier Perl versions

In Perl 5.8.x, parsing of methods at compile-time has intermittent issues, at least for versions of Devel::BeginLift 0.001003 and before. It's possible it will be fixed in future versions of Devel::BeginLift.

The most noticable is if an error occurs at compile time, such as a strict error, perl might not notice until it tries to compile something else via an eval or require at which point perl will appear to fail where there is no reason to fail.

We recommend you use the compile_at_BEGIN flag to turn off compile-time parsing.

Method::Signatures cannot be used with Perl versions prior to 5.8 because Devel::Declare does not work with those earlier versions.

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 the return value?

Currently there is no support for 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

Here's some additional 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?

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

Slurpy parameter restrictions

Slurpy parameters are currently more restricted than they need to be. It is possible to work out a slurpy parameter in the middle, or a named slurpy parameter. However, there's lots of edge cases and possible nonsense configurations. Until that's worked out, we've left it restricted.

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

Role::Basic roles are currently not recognized by the type system.

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.

The type checking and method modifier work was supplied by Buddy Burden (barefootcoder). Thanks to this, you can now use Method::Signatures (or, more properly, Method::Signatures::Modifiers) instead of MooseX::Method::Signatures, which fixes many of the problems commonly attributed to MooseX::Declare.

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 2007-2011 by Michael G Schwern <schwern@pobox.com>.

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

See http://www.perl.com/perl/misc/Artistic.html


MooseX::Method::Signatures for an alternative implementation.

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 - http://perlcabal.org/syn/S06.html#Parameters_and_arguments

Moose::Util::TypeConstraints or Mouse::Util::TypeConstraints for further details on how the type-checking works.