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.

Similarly

    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.

The full signature syntax for each parameter is:

          Int|Str  \:$param!  where $SM_EXPR  is ro  = $AS_EXPR  when $SM_EXPR
          \_____/  ^^\____/^  \____________/  \___/  \________/  \___________/
             |     ||   |  |        |           |        |           |
       Type_/      ||   |  |        |           |        |           |
       Aliased?___/ |   |  |        |           |        |           |
       Named?______/    |  |        |           |        |           |
       Parameter var___/   |        |           |        |           |
       Required?__________/         |           |        |           |
       Parameter constraint(s)_____/            |        |           |
       Parameter trait(s)______________________/         |           |
       Default value____________________________________/            |
       When default value should be applied_________________________/

Every component except the parameter name (with sigil) is optional.

$SM_EXPR is any expression that is valid as the RHS of a smartmatch, or else a raw block of code. See "Value constraints".

$AS_EXPR is any expression that is valid as the RHS of an assignment operator. See "Defaults".

@_

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)

This feature requires Data::Alias to be installed.

Invocant parameter

The method invocant (i.e. $self) can be changed as the first parameter on a per-method basis. Put a colon after it instead of a comma:

    method foo($class:) {
        $class->bar;
    }

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

"method" has an implied default invocant of $self:, though that is configurable by setting the invocant parameter on the "use Method::Signatures" line.

"func" has no invocant, as it is intended for creating subs that will not be invoked on an object.

Defaults

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),
    )
    {
        ...
    }

Normally, 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

However, you can specify additional conditions under which a default is also to be used, using a trailing "when". For example:

    # Use default if no argument passed
    method get_results($how_many = 1) {...}

    # Use default if no argument passed OR argument is undef
    method get_results($how_many = 1 when undef) {...}

    # Use default if no argument passed OR argument is empty string
    method get_results($how_many = 1 when "") {...}

    # Use default if no argument passed OR argument is zero
    method get_results($how_many = 1 when 0) {...}

    # Use default if no argument passed OR argument is zero or less
    method get_results($how_many = 1 when sub{ $_[0] <= 0 }) {...}

    # Use default if no argument passed OR argument is invalid
    method get_results($how_many = 1 when sub{ !valid($_[0]) }) {...}

In other words, if you include a "when value" after the default, the default is still used if the argument is missing, but is also used if the argument is provided but smart-matches the specified value.

Note that the final two examples above use anonymous subroutines to conform their complex tests to the requirements of the smartmatch operator. Because this is useful, but syntactically clumsy, there is also a short-cut for this behaviour. If the test after "when" consists of a block, the block is executed as the defaulting test, with the actual argument value aliased to $_ (just like in a "grep" block). So the final two examples above could also be written:

    # Use default if no argument passed OR argument is zero or less
    method get_results($how_many = 1 when {$_ <= 0}) {...}

    # Use default if no argument passed OR argument is invalid
    method get_results($how_many = 1 when {!valid($_)}) } {...}

The most commonly used form of "when" modifier is almost certainly "when undef":

    # Use default if no argument passed OR argument is undef
    method get_results($how_many = 1 when undef) {...}

which covers the common case where an uninitialized variable is passed as an argument, or where supplying an explicit undefined value is intended to indicate: "use the default instead."

This usage is sufficiently common that a short-cut is provided: using the "//=" operator (instead of the regular assignment operator) to specify the default. Like so:

    # Use default if no argument passed OR argument is undef
    method get_results($how_many //= 1) {...}

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?
        $value->DOES($type);

    * Finally, is the $value an object of that class?
        $value->isa($type);

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:) {
        ...
    }

Value Constraints

In addition to a type, each parameter can also be specified with one or more additional constraints, using the "$arg where CONSTRAINT" syntax.

    method set_name($name where qr{\S+ \s+ \S+}x) {
        ...
    }

    method set_rank($rank where \%STD_RANKS) {
        ...
    }

    method set_age(Int $age where [17..75] ) {
        ...
    }

    method set_rating($rating where { $_ >= 0 } where { $_ <= 100 } ) {
        ...
    }

    method set_serial_num(Int $snum where {valid_checksum($snum)} ) {
        ...
    }

The "where" keyword must appear immediately after the parameter name and before any trait or default.

Each "where" constraint is smartmatched against the value of the corresponding parameter, and an exception is thrown if the value does not satisfy the constraint.

Any of the normal smartmatch arguments (numbers, strings, regexes, undefs, hashrefs, arrayrefs, coderefs) can be used as a constraint.

In addition, the constraint can be specified as a raw block. This block can then refer to the parameter variable directly by name (as in the definition of "set_serial_num()" above), or else as $_ (as in the definition of "set_rating()".

Unlike type constraints, value constraints are tested after any default values have been resolved, and in the same order as they were specified within the signature.

Placeholder parameters

A positional argument can be ignored by using a bare "$" sigil as its name.

    method foo( $a, $, $c ) {
        ...
    }

The argument's value doesn't get stored in a variable, but the caller must still supply it. Value and type constraints can be applied to placeholders.

    method bar( Int $ where { $_ < 10 } ) {
        ...
    }

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".

ro

Read-only. Assigning or modifying the parameter is an error. This trait requires Const::Fast to be installed.

rw

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

This is a default trait.

copy

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

This is a default trait except for the "\@foo" parameter (see "Aliased references").

alias

The parameter will be an alias of the argument. Any changes to the parameter will be reflected in the caller. This trait requires Data::Alias to be installed.

This is a default trait for the "\@foo" parameter (see "Aliased references").

Mixing value constraints, traits, and defaults

As explained in "Signature syntax", there is a defined order when including multiple trailing aspects of a parameter:

• Any value constraint must immediately follow the parameter name.
• Any trait must follow that.
• Any default must come last.

For instance, to have a parameter which has all three aspects:

    method echo($message where { length <= 80 } is ro = "what?") {
        return $message
    }

Think of "$message where { length <= 80 }" as being the left-hand side of the trait, and "$message where { length <= 80 } is ro" as being the left-hand side of the default 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.

The "yada yada" marker

The restriction that slurpy parameters must be positional, and must appear at the end of the signature, means that they cannot be used in conjunction with named parameters.

This is frustrating, because there are many situations (in particular: during object initialization, or when creating a callback) where it is extremely handy to be able to ignore extra named arguments that don't correspond to any named parameter.

While it would be theoretically possible to allow a slurpy parameter to come after named parameters, the current implementation does not support this (see "Slurpy parameter restrictions").

Instead, there is a special syntax (colloquially known as the "yada yada") that tells a method or function to simply ignore any extra arguments that are passed to it:

    # Expect name, age, gender, and simply ignore anything else
    method BUILD (:$name, :$age, :$gender, ...) {
        $self->{name}   = uc $name;
        $self->{age}    = min($age, 18);
        $self->{gender} = $gender // 'unspecified';
    }

    # Traverse tree with node-printing callback
    # (Callback only interested in nodes, ignores any other args passed to it)
    $tree->traverse( func($node, ...) { $node->print } );

The "..." may appear as a separate "pseudo-parameter" anywhere in the signature, but is normally placed at the very end. It has no other effect except to disable the usual "die if extra arguments" test that the module sets up within each method or function.

This means that a "yada yada" can also be used to ignore positional arguments (as the second example above indicates). So, instead of:

    method verify ($min, $max, @etc) {
        return $min <= $self->{val} && $self->{val} <= $max;
    }

you can just write:

    method verify ($min, $max, ...) {
        return $min <= $self->{val} && $self->{val} <= $max;
    }

This is also marginally more efficient, as it does not have to allocate, initialize, or deallocate the unused slurpy parameter @etc.

The bare "@" sigil is a synonym for "...". A bare "%" sigil is also a synonym for "...", but requires that there must be an even number of extra arguments, such as would be assigned to a hash.

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 implicitly 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.

Note that a signature of "(@_)" is exactly equivalent to a signature of "(...)". See "The yada yada marker".

The empty signature

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

Anonymous Methods

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

    $obj->$method(42);

Options

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

invocant

In some cases it is desirable for the invocant to be named something other than $self, and specifying it in the signature of every method is tedious and prone to human-error. When this option is set, methods that do not specify the invocant variable in their signatures will use the given variable name.

    use Method::Signatures { invocant => '$app' };

    method main { $app->config; $app->run; $app->cleanup; }

Note that the leading sigil must be provided, and the value must be a single token that would be valid as a perl variable. Currently only scalar invocant variables are supported (eg, the sigil must be a "$").

This option only affects the packages in which it is used. All others will continue to use $self as the default invocant variable.

compile_at_BEGIN

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:

    echo("something");

    # 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".

debug

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

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.

too_many_args_error, named_param_error, required_arg, type_error, where_error

signature_error

signature_error_handler

    package My::Method::Signatures;

    use Moose;
    extends 'Method::Signatures';

    sub signature_error_handler {
        my ($class, $msg) = @_;
        die bless { message => $msg }, 'My::ExceptionClass';
    };

type_check

inject_for_type_check

One liners

Close parends in quotes or comments

    func foo (
        $foo,       # $foo might contain )
        $bar
    )

is going to produce a syntax error, because the parend inside the comment is perceived as the end of the signature. On the other hand, this:

    func foo (
        $foo,       # (this is the $foo parend)
        $bar
    )

is fine, because the parends in the comments are balanced.

If you absolutely can't avoid an unbalanced close parend, such as in the following signature:

    func foo ( $foo, $bar = ")" )       # this won't parse correctly

you can always use a backslash to tell the parser that that close parend doesn't indicate the end of the signature:

    func foo ( $foo, $bar = "\)" )      # this is fine

This even works in single quotes:

    func foo ( $foo, $bar = '\)' )      # default is ')', *not* '\)'!

although we don't recomment that form, as it may be surprising to readers of your code.

No source filter

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

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

You can't use any feature that requires a smartmatch expression (i.e. conditional "Defaults" and "Value Constraints") in Perl 5.8.

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?

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

What about the return value?

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

A syntax for function prototypes is being considered.

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

Error checking

* 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

What about...

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)