text: 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, $fun->($_); } @res } 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 $text, offset => 200;
  This module extends Perl with keywords that let you define functions with parameter lists. It uses Perl's keyword plugin API, so it works reliably and doesn't require a source filter.
  The anatomy of a function (as recognized by this module):
  Example:
  # (1) (2) (3) (4) (5) (6) fun foo ($x, $y) :($$) :lvalue { ... } # (1) (6) my $f = fun { ... };
  In the following section I'm going to describe all parts in order from simplest to most complex.
  This is just a normal block of statements, as with
  sub. No surprises here.
  If present, it specifies the name of the function being defined. As with
  sub, if a name is present, the whole declaration is syntactically a statement and its effects are performed at compile time (i.e. at runtime you can call functions whose definitions only occur later in the file). If no name is present, the declaration is an expression that evaluates to a reference to the function in question. No surprises here either.
  Attributes are relatively unusual in Perl code, but if you want them, they work exactly the same as with
  sub.
  As with
  sub, a prototype, if present, contains hints as to how the compiler should parse calls to this function. This means prototypes have no effect if the function call is compiled before the function declaration has been seen by the compiler or if the function to call is only determined at runtime (e.g. because it's called as a method or through a reference).
  With
  sub, a prototype comes directly after the function name (if any).
  Function::Parameters reserves this spot for the parameter list. To specify a prototype, put it as the first attribute (e.g.
  fun foo :(&$$)). This is syntactically unambiguous because normal attributes need a name after the colon.
  The parameter list is a list of variables enclosed in parentheses, except it's actually a bit more complicated than that. A parameter list can include the following 6 parts, all of which are optional:
  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 $self; }
  The most common kind of parameter. This is simply a comma-separated list of scalars, which are filled from left to right with the arguments that the caller passed in:
  fun add($x, $y) { return $x + $y; } say add(2, 3); # "5"
  Parameters can be marked as optional by putting an equals sign (
  =) and an expression (the "default argument") after them. If no corresponding argument is passed in by the caller, the default argument will be used to initialize the parameter:
  fun scale($base, $factor = 2) { return $base * $factor; } say scale(3, 5); # "15" say scale(3); # "6"
  The default argument is not cached. Every time a function is called with some optional arguments missing, the corresponding default arguments are evaluated from left to right. This makes no difference for a value like
  2 but it is important for expressions with side effects, such as reference constructors (
  [],
  {}) or function calls.
  Default arguments see not only the surrounding lexical scope of their function but also any preceding parameters. This allows the creation of dynamic defaults based on previous arguments:
  method set_name($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 $n" } say foo(2 + 2); # "you gave me 4" say foo(); # "nope"
  By putting a colon (
  :) in front of a parameter you can make it named instead of positional:
  fun rectangle(:$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);
  As with positional parameters, you can make named parameters optional by specifying a default argument 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 = $url
  The above example shows that passing any value (even
  undef) will override the default argument.
  Finally you can put an array or hash in the parameter list, which will gobble up the remaining arguments (if any):
  fun foo($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) or a reference to a hash with the following keys:
  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).
  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 via
  Carp::croak.
  The predefined type
  function is equivalent to:
  { name => 'optional', invocant => 0, default_arguments => 1, check_argument_count => 0, }
  These are all default values, so
  function is also equivalent to
  {}.
  method is equivalent to:
  { name => 'optional', shift => '$self', invocant => 1, attributes => ':method', default_arguments => 1, check_argument_count => 0, }
  classmethod is equivalent to:
  { name => 'optional', shift => '$class', invocant => 1, attributes => ':method', default_arguments => 1, check_argument_count => 0, }
  function_strict,
  method_strict, and
  classmethod_strict are like
  function,
  method, and
  classmethod, respectively, but with
  check_argument_count => 1.
  Plain
  use Function::Parameters is equivalent to
  use Function::Parameters { fun => 'function', method => 'method' }.
  use Function::Parameters qw(:strict) is equivalent to
  use Function::Parameters { fun => 'function_strict', method => 'method_strict' }.
  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.
  Function::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 { Function::Parameters->import; # or Function::Parameters->import(@custom_import_args); }
  An experimental feature is now available: You can annotate parameters with Moose 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[String | CodeRef] $cb) { ... }
  If you do this, Moose will be loaded automatically (if that hasn't happened yet). These specifications are parsed and validated using
  Moose::Util::TypeConstraints::find_or_parse_type_constraint.
  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.
  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) $x) { ... }
  Every type expression must return either a string (which is resolved as for literal types), or a type constraint object (providing
  check and
  get_message methods).
  Note that these expressions are evaluated (once) at parse time (similar to
  BEGIN blocks), so make sure that any variables you use are set and any functions you call are defined at parse time.
  The module is actually written in C and uses
  PL_keyword_plugin to generate opcodes directly. However, you can run
  perl -MO=Deparse ... on your code to see what happens under the hood. In the simplest case (no argument checks, possibly an invocant, required positional/slurpy parameters only), the generated code corresponds to:
  fun foo($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; ... }
  After installing, you can find documentation for this module with the perldoc command.
  perldoc Function::Parameters
  You can also look for information at:
  Function::Parameters::Info.