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Manual Reference Pages  -  FUNCTION::PARAMETERS (3)

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Function::Parameters - subroutine definitions with parameter lists



 use Function::Parameters qw(:strict);

 # simple function
 fun foo($bar, $baz) {
   return $bar + $baz;

 # function with prototype
 fun mymap($fun, @args)
   my @res;
   for (@args) {
     push @res, $fun->($_);

 print "$_\n" for mymap { $_ * 2 } 1 .. 4;

 # method with implicit $self
 method set_name($name) {
   $self->{name} = $name;

 # method with explicit invocant
 method new($class: %init) {
   return bless { %init }, $class;

 # function with optional parameters
 fun search($haystack, $needle = qr/^(?!)/, $offset = 0) {

 # method with named parameters
 method resize(:$width, :$height) {
   $self->{width}  = $width;
   $self->{height} = $height;

 $obj->resize(height => 4, width => 5);

 # function with named optional parameters
 fun search($haystack, :$needle = qr/^(?!)/, :$offset = 0) {

 my $results = search $text, offset => 200;


This module extends Perl with keywords that let you define functions with parameter lists. It uses Perl’s keyword plugin API, so it works reliably and doesn’t require a source filter.


The anatomy of a function (as recognized by this module):
1. The keyword introducing the function.
2. The function name (optional).
3. The parameter list (optional).
4. The prototype (optional).
5. The attribute list (optional).
6. The function body.

  # (1)   (2) (3)      (4)   (5)     (6)
    fun   foo ($x, $y) :($$) :lvalue { ... }

  #         (1) (6)
    my $f = fun { ... };

In the following section I’m going to describe all parts in order from simplest to most complex.


This is just a normal block of statements, as with sub. No surprises here, except the function name (along with its prototype, if any) is already in scope in its body. This is not the case with sub:

  sub foo ($$) {
  # foo 1, 2;  # a syntax error: foo is not known to be a function yet
    foo(42);   # OK: calls foo with one argument despite prototype

On the other hand:

  use Function::Parameters;
  fun foo :prototype($$) {
    foo 1, 2;  # OK: foo and its prototype are in scope
  # foo(42);   # error: Not enough arguments for main::foo at X line Y, near "42)"


If present, it specifies the name of the function being defined. As with sub, if a name is present, by default the whole declaration is syntactically a statement and its effects are performed at compile time (i.e. at runtime you can call functions whose definitions only occur later in the file - but see the runtime flag below). If no name is present, the declaration is an expression that evaluates to a reference to the function in question.


Attributes are relatively unusual in Perl code, but if you want them, they work exactly the same as with sub.


As with sub, a prototype, if present, contains hints as to how the compiler should parse calls to this function. This means prototypes have no effect if the function call is compiled before the function declaration has been seen by the compiler or if the function to call is only determined at runtime (e.g. because it’s called as a method or through a reference).

With sub, a prototype comes directly after the function name (if any). Function::Parameters reserves this spot for the parameter list. To specify a prototype, put it as the first attribute (e.g. fun foo :(&$$)). This is syntactically unambiguous because normal attributes need a name after the colon.

You can also use an attribute named prototype (e.g. fun foo :prototype(&$$)). In that case it does not have to be the first attribute. This syntax is also compatible with sub in perl 5.20 and newer.

Parameter list

The parameter list is a list of variables enclosed in parentheses, except it’s actually a bit more complicated than that.

Instead of a full variable name (such as $foo or @bar) you can write just the sigil ($, @, or %). This has the effect of creating an unnamed parameter, which is useful in functions that are called with a certain number of arguments but want to ignore one or more of them. This trick works for invocants, slurpies, and positional parameters (see below). You can’t have unnamed named parameters for what I hope are obvious reasons.

A parameter list can include the following 6 parts, all of which are optional:
1. Invocant This is a scalar variable followed by a colon (:) and no comma. If an invocant is present in the parameter list, the first element of @_ is automatically shifted off and placed in this variable. This is intended for methods:

  method new($class: %init) {
    return bless { %init }, $class;

  method throw($self:) {
    die $self;

2. Required positional parameters The most common kind of parameter. This is simply a comma-separated list of scalars, which are filled from left to right with the arguments that the caller passed in:

  fun add($x, $y) {
    return $x + $y;
  say add(2, 3);  # "5"

3. Optional positional parameters Parameters can be marked as optional by putting an equals sign (=) and an (optional) expression (the default argument) after them. If no corresponding argument is passed in by the caller, the default argument will be used to initialize the parameter:

  fun scale($base, $factor = 2) {
    return $base * $factor;

  say scale(3, 5);  # "15"
  say scale(3);     # "6"

Using just a = with no expression after is equivalent to specifying = undef, i.e. the corresponding parameter is optional and has a default value of undef.

The default argument is not cached. Every time a function is called with some optional arguments missing, the corresponding default arguments are evaluated from left to right. This makes no difference for a value like 2 but it is important for expressions with side effects, such as reference constructors ([], {}) or function calls.

Default arguments see not only the surrounding lexical scope of their function but also any preceding parameters. This allows the creation of dynamic defaults based on previous arguments:

  method set_name($self: $nick = $self->default_nick, $real_name = $nick) {
    $self->{nick} = $nick;
    $self->{real_name} = $real_name;

  $obj->set_name("simplicio");  # same as: $obj->set_name("simplicio", "simplicio");

Because default arguments are actually evaluated as part of the function body, you can also do silly things like this:

  fun foo($n = return "nope") {
    "you gave me $n"

  say foo(2 + 2);  # "you gave me 4"
  say foo();       # "nope"

4. Required named parameters By putting a colon (:) in front of a parameter you can make it named instead of positional:

  fun rectangle(:$width, :$height) {

  rectangle(width => 2, height => 5);
  rectangle(height => 5, width => 2);  # same thing!

That is, the caller must specify a key name in addition to the value, but in exchange the order of the arguments doesn’t matter anymore. As with hash initialization, you can specify the same key multiple times and the last occurrence wins:

  rectangle(height => 1, width => 2, height => 2, height => 5);
  # same as: rectangle(width => 2, height => 5);

You can combine positional and named parameters as long as the positional parameters come first:

  fun named_rectangle($name, :$width, :$height) {

  named_rectangle("Avocado", width => 0.5, height => 1.2);

5. Optional named parameters As with positional parameters, you can make named parameters optional by specifying a default argument (or nothing, which is equivalent to undef) after an equals sign (=):

  fun rectangle(:$width, :$height, :$color = "chartreuse") {

  rectangle(height => 10, width => 5);
  # same as: rectangle(height => 10, width => 5, color => "chartreuse");

  fun get($url, :$cookie_jar = HTTP::Cookies->new(), :$referrer = $url) {

  my $data = get "", referrer => undef;  # overrides $referrer = $url

The above example shows that passing any value (even undef) will override the default argument.

6. Slurpy parameter Finally you can put an array or hash in the parameter list, which will gobble up the remaining arguments (if any):

  fun foo($x, $y, @rest) { ... }

  foo "a", "b";            # $x = "a", $y = "b", @rest = ()
  foo "a", "b", "c";       # $x = "a", $y = "b", @rest = ("c")
  foo "a", "b", "c", "d";  # $x = "a", $y = "b", @rest = ("c", "d")

If you combine this with named parameters, the slurpy parameter will end up containing all unrecognized keys:

  fun bar(:$size, @whatev) { ... }

  bar weight => 20, size => 2, location => [0, -3];
  # $size = 2, @whatev = (weight, 20, location, [0, -3])

Apart from the shift performed by the invocant, all of the above leave @_ unchanged; and if you don’t specify a parameter list at all, @_ is all you get.
The keywords provided by Function::Parameters are customizable. Since Function::Parameters is actually a pragma, the provided keywords have lexical scope. The following import variants can be used:
use Function::Parameters :strict Provides the keywords fun and method (described below) and enables argument checks so that calling a function and omitting a required argument (or passing too many arguments) will throw an error.
use Function::Parameters Provides the keywords fun and method (described below) and enables lax mode: Omitting a required argument sets it to undef while excess arguments are silently ignored.
use Function::Parameters { KEYWORD1 => TYPE1, KEYWORD2 => TYPE2, ... } Provides completely custom keywords as described by their types. A type is either a string (one of the predefined types function, method, classmethod, function_strict, method_strict, classmethod_strict, function_lax, method_lax, classmethod_lax) or a reference to a hash with the following keys:
defaults Valid values: One of the predefined types function, method, classmethod, function_strict, method_strict, classmethod_strict, function_lax, method_lax, classmethod_lax. This will set the defaults for all other keys from the specified type, which is useful if you only want to override some properties:

  use Function::Parameters { defmethod => { defaults => method, shift => $this } };

This example defines a keyword called defmethod that works like the standard method keyword, but the implicit object variable is called $this instead of $self.

Using the string types directly is equivalent to defaults with no further customization:

  use Function::Parameters {
      foo => function,         # like: foo => { defaults => function },
      bar => function_strict,  # like: bar => { defaults => function_strict },
      baz => method_strict,    # like: baz => { defaults => method_strict },

name Valid values: optional (default), required (all functions defined with this keyword must have a name), and prohibited (functions defined with this keyword must be anonymous).
runtime Valid values: booleans. If enabled, this keyword takes effect at runtime, not compile time:

  use Function::Parameters { fun => { defaults => function_strict, runtime => 1 } };
  say defined &foo ? "defined" : "not defined";  # not defined
  fun foo() {}
  say defined &foo ? "defined" : "not defined";  # defined

&foo is only defined after fun foo() {} has been reached at runtime.

<B>CAVEAT:B> A future version of this module may enable runtime => 1 by default for methods.

shift Valid values: strings that look like scalar variables. This lets you specify a default invocant, i.e. a function defined with this keyword that doesn’t have an explicit invocant in its parameter list will automatically shift its first argument into the variable specified here.
invocant Valid values: booleans. If you set this to a true value, the keyword will accept invocants in parameter lists; otherwise specifying an invocant in a function defined with this keyword is a syntax error.
attributes Valid values: strings containing (source code for) attributes. This causes any function defined with this keyword to have the specified attributes (in addition to any attributes specified in the function definition itself).
default_arguments Valid values: booleans. This property is on by default; use default_arguments => 0 to turn it off. This controls whether optional parameters are allowed. If it is turned off, using = in parameter lists is a syntax error.
check_argument_count Valid values: booleans. If turned on, functions defined with this keyword will automatically check that they have been passed all required arguments and no excess arguments. If this check fails, an exception will by thrown.
check_argument_types Valid values: booleans. This property is on by default; use check_argument_types => 0 to turn it off. This controls whether functions defined with this keyword automatically check that the arguments they are passed pass the declared type constraints (if any). See Experimental feature: Types below.
strict Valid values: booleans. This turns on both check_argument_count and check_argument_types.
reify_type Valid values: code references. The function specified here will be called to turn type annotations into constraint objects (see Experimental feature: Types below). It will receive two arguments: a string containing the type description, and the name of the current package.

The default type reifier is equivalent to:

 sub {
     require Moose::Util::TypeConstraints;

The predefined type function_lax is equivalent to:

   name              => optional,
   default_arguments => 1,
   strict            => 0,
   invocant          => 0,
   runtime           => 0,

These are all default values, so function_lax is also equivalent to {}.

method_lax is equivalent to:

   defaults          => function_lax,
   attributes        => :method,
   shift             => $self,
   invocant          => 1,
   # runtime         => 1,  ## possibly in a future version of this module

classmethod_lax is equivalent to:

   defaults          => method_lax,
   shift             => $class,

function_strict, method_strict, and classmethod_strict are like function_lax, method_lax, and classmethod_lax, respectively, but with strict => 1.

function, method and classmethod are equivalent to function_lax, method_lax and classmethod_lax, respectively.

use Function::Parameters qw(:strict) is equivalent to use Function::Parameters { fun => function_strict, method => method_strict }.

use Function::Parameters qw(:lax) is equivalent to use Function::Parameters { fun => function_lax, method => method_lax }.

Plain use Function::Parameters is equivalent to use Function::Parameters { fun => function, method => method }.


You can ask a function at runtime what parameters it has. This functionality is available through the function Function::Parameters::info (which is not exported, so you have to call it by its full name). It takes a reference to a function, and returns either undef (if it knows nothing about the function) or a Function::Parameters::Info object describing the parameter list.

Note: This feature is implemented using Moo, so you’ll need to have Moo installed if you want to call Function::Parameters::info (alternatively, if Moose is already loaded by the time Function::Parameters::info is first called, it will use that instead).

See Function::Parameters::Info for examples.

Wrapping CWFunction::Parameters

If you want to write a wrapper around Function::Parameters, you only have to call its import method. Due to its pragma nature it always affects the file that is currently being compiled.

 package Some::Wrapper;
 use Function::Parameters ();
 sub import {
   # or Function::Parameters->import(@custom_import_args);

    Experimental feature: Types

An experimental feature is now available: You can annotate parameters with types. That is, before each parameter you can put a type specification consisting of identifiers (Foo), unions (... | ...), and parametric types (...[...]). Example:

  fun foo(Int $n, ArrayRef[Str | CodeRef] $cb) { ... }

If you do this, the type reification function corresponding to the keyword will be called to turn the type (a string) into a constraint object. The default type reifier simply loads Moose and forwards to Moose::Util::TypeConstraints::find_or_parse_type_constraint, which creates Moose types.

If you are in lax mode, nothing further happens and the types are ignored. If you are in strict mode, Function::Parameters generates code to make sure any values passed in conform to the type (via $constraint->check($value)).

In addition, these type constraints are inspectable through the Function::Parameters::Info object returned by Function::Parameters::info.

    Experimental experimental feature: Type expressions

An even more experimental feature is the ability to specify arbitrary expressions as types. The syntax for this is like the literal types described above, but with an expression wrapped in parentheses (( EXPR )). Example:

  fun foo((Int) $n, ($othertype) $x) { ... }

Every type expression must return either a string (which is resolved as for literal types), or a type constraint object (providing check and get_message methods).

Note that these expressions are evaluated (once) at parse time (similar to BEGIN blocks), so make sure that any variables you use are set and any functions you call are defined at parse time.

    How it works

The module is actually written in C and uses PL_keyword_plugin to generate opcodes directly. However, you can run perl -MO=Deparse ... on your code to see what happens under the hood. In the simplest case (no argument checks, possibly an invocant, required positional/slurpy parameters only), the generated code corresponds to:

  fun foo($x, $y, @z) { ... }
  # ... turns into ...
  sub foo { my ($x, $y, @z) = @_; sub foo; ... }

  method bar($x, $y, @z) { ... }
  # ... turns into ...
  sub bar :method { my $self = shift; my ($x, $y, @z) = @_; sub bar; ... }


A future version of this module may enable runtime => 1 by default for methods. If this would break your code, please send me a note or file a bug on RT.

A future version of this module may default to strict mode instead of lax.


After installing, you can find documentation for this module with the perldoc command.

    perldoc Function::Parameters

You can also look for information at:
MetaCPAN <>
RT, CPAN’s request tracker <>
AnnoCPAN, Annotated CPAN documentation <>
CPAN Ratings <>
Search CPAN <>




Lukas Mai, <l.mai at>


Copyright 2010-2014 Lukas Mai.

This program is free software; you can redistribute it and/or modify it under the terms of either: the GNU General Public License as published by the Free Software Foundation; or the Artistic License.

See for more information.

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perl v5.20.3 FUNCTION::PARAMETERS (3) 2016-02-13

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