Default functionality

You can also disable "Function::Parameters" within a block:

 {
     no Function::Parameters;  # disable all keywords
     ...
 }

Or explicitly list the keywords you want to disable:

 {
     no Function::Parameters qw(method);
     # 'method' is a normal identifier here
     ...
 }

You can also explicitly list the keywords you want to enable:

 use Function::Parameters qw(fun);  # provides 'fun' but not 'method'
 use Function::Parameters qw(method);  # provides 'method' but not 'fun'

Simple parameter lists

By default you get two keywords, "fun" and "method" (but see "Customizing and extending" below). "fun" is very similar to "sub". You can use it to define both named and anonymous functions:

 fun left_pad($str, $n) {
     return sprintf '%*s', $n, $str;
 }

 print left_pad("hello", 10), "\n";

 my $twice = fun ($x) { $x * 2 };
 print $twice->(21), "\n";

In the simplest case the parameter list is just a comma-separated list of zero or more scalar variables (enclosed in parentheses, following the function name, if any).

"Function::Parameters" automatically validates the arguments your function is called with. If the number of arguments doesn't match the parameter list, an exception is thrown.

Apart from that, the parameter variables are defined and initialized as if by:

 sub left_pad {
     sub left_pad;
     my ($str, $n) = @_;
     ...
 }

In particular, @_ is still available in functions defined by "fun" and holds the original argument list.

The inner "sub left_pad;" declaration is intended to illustrate that the name of the function being defined is in scope in its own body, meaning you can call it recursively without having to use parentheses:

 fun fac($n) {
     return 1 if $n < 2;
     return $n * fac $n - 1;
 }

In a normal "sub" the last line would have had to be written "return $n * fac($n - 1);".

"method" is almost the same as "fun" but automatically creates a $self variable as the first parameter (which is removed from @_):

 method foo($x, $y) {
    ...
 }

 # works like:
 sub foo :method {
    my $self = shift;
    my ($x, $y) = @_;
    ...
 }

As you can see, the ":method" attribute is also added automatically (see "method" in attributes for details).

In some cases (e.g. class methods) $self is not the best name for the invocant of the method. You can override it on a case-by-case basis by putting a variable name followed by a ":" (colon) as the first thing in the parameter list:

 method new($class: $x, $y) {
     return bless { x => $x, y => $y }, $class;
 }

Here the invocant is named $class, not $self. It looks a bit weird but still works the same way if the remaining parameter list is empty:

 method from_env($class:) {
     return $class->new($ENV{x}, $ENV{y});
 }

Default arguments

(Most of the following examples use "fun" only. Unless specified otherwise everything applies to "method" as well.)

You can make some arguments optional by giving them default values.

 fun passthrough($x, $y = 42, $z = []) {
     return ($x, $y, $z);
 }

In this example the first parameter $x is required but $y and $z are optional.

 passthrough('a', 'b', 'c', 'd')   # error: Too many arguments
 passthrough('a', 'b', 'c')        # returns ('a', 'b', 'c')
 passthrough('a', 'b')             # returns ('a', 'b', [])
 passthrough('a', undef)           # returns ('a', undef, [])
 passthrough('a')                  # returns ('a', 42, [])
 passthrough()                     # error: Too few arguments

Default arguments are evaluated whenever a corresponding real argument is not passed in by the caller. "undef" counts as a real argument; you can't use the default value for parameter N and still pass a value for parameter N+1. "$z = []" means each call that doesn't pass a third argument gets a new array reference (they're not shared between calls).

Default arguments are evaluated as part of the function body, allowing for silliness such as:

 fun weird($name = return "nope") {
     print "Hello, $name!\n";
     return $name;
 }

 weird("Larry");  # prints "Hello, Larry!" and returns "Larry"
 weird();         # returns "nope" immediately; function body doesn't run

Preceding parameters are in scope for default arguments:

 fun dynamic_default($x, $y = length $x) {
    return "$x/$y";
 }

 dynamic_default("hello", 0)  # returns "hello/0"
 dynamic_default("hello")     # returns "hello/5"
 dynamic_default("abc")       # returns "abc/3"

If you just want to make a parameter optional without giving it a special value, write "$param = undef". There is a special shortcut syntax for this case: "$param = undef" can also be written "$param =" (with no following expression).

 fun foo($x = undef, $y = undef, $z = undef) {
     # three arguments, all optional
     ...
 }

 fun foo($x=, $y=, $z=) {
     # shorter syntax, same meaning
     ...
 }

Optional parameters must come at the end. It is not possible to have a required parameter after an optional one.

Slurpy/rest parameters

The last parameter of a function or method can be an array. This lets you slurp up any number of arguments the caller passes (0 or more).

 fun scale($factor, @values) {
     return map { $_ * $factor } @values;
 }

 scale(10, 1 .. 4)  # returns (10, 20, 30, 40)
 scale(10)          # returns ()

You can also use a hash, but then the number of arguments has to be even.

Named parameters

As soon as your functions take more than three arguments, it gets harder to keep track of what argument means what:

 foo($handle, $w, $h * 2 + 15, 1, 24, 'icon');
 # what do these arguments mean?

"Function::Parameters" offers an alternative for these kinds of situations in the form of named parameters. Unlike the parameters described previously, which are identified by position, these parameters are identified by name:

 fun create_point(:$x, :$y, :$color) {
     ...
 }

 # Case 1
 create_point(
     x     => 50,
     y     => 50,
     color => 0xff_00_00,
 );

To create a named parameter, put a ":" (colon) in front of it in the parameter list. When the function is called, the arguments have to be supplied in the form of a hash initializer (a list of alternating keys/values). As with a hash, the order of key/value pairs doesn't matter (except in the case of duplicate keys, where the last occurrence wins):

 # Case 2
 create_point(
     color => 0xff_00_00,
     x     => 50,
     y     => 50,
 );

 # Case 3
 create_point(
     x     => 200,
     color => 0x12_34_56,
     color => 0xff_00_00,
     x     => 50,
     y     => 50,
 );

Case 1, Case 2, and Case 3 all mean the same thing.

As with positional parameters, you can make named parameters optional by supplying a default argument:

 fun create_point(:$x, :$y, :$color = 0x00_00_00) {
     ...
 }

 create_point(x => 0, y => 64)  # color => 0x00_00_00 is implicit

If you want to accept any key/value pairs, you can add a rest parameter (hashes are particularly useful):

 fun accept_all_keys(:$name, :$age, %rest) {
     ...
 }

 accept_all_keys(
     age     => 42,
     gender  => 2,
     name    => "Jamie",
     marbles => [],
 );
 # $name = "Jamie";
 # $age = 42;
 # %rest = (
 #     gender  => 2,
 #     marbles => [],
 # );

You can combine positional and named parameters but all positional parameters have to come first:

 method output(
    $data,
    :$handle       = $self->output_handle,
    :$separator    = $self->separator,
    :$quote_fields = 0,
 ) {
     ...
 }

 $obj->output(["greetings", "from", "space"]);
 $obj->output(
    ["a", "random", "example"],
    quote_fields => 1,
    separator    => ";",
 );

Unnamed parameters

If your function doesn't use a particular parameter at all, you can omit its name and just write a sigil in the parameter list:

 register_callback('click', fun ($target, $) {
     ...
 });

Here we're calling a hypothetical "register_callback" function that registers our coderef to be called in response to a "click" event. It will pass two arguments to the click handler, but the coderef only cares about the first one ($target). The second parameter doesn't even get a name (just a sigil, "$"). This marks it as unused.

This case typically occurs when your functions have to conform to an externally imposed interface, e.g. because they're called by someone else. It can happen with callbacks or methods that don't need all of the arguments they get.

You can use unnamed slurpy parameters to accept and ignore all following arguments. In particular, "fun foo(@)" is a lot like "sub foo" in that it accepts and ignores any number of arguments (apart from leaving them in @_).

Type constraints

It is possible to automatically check the types of arguments passed to your function. There are two ways to do this.

1.

 use Types::Standard qw(Str Int ArrayRef);

 fun foo(Str $label, ArrayRef[Int] $counts) {
     ...
 }
    

In this variant you simply put the name of a type in front of a parameter. The way this works is that "Function::Parameters" parses the type using very simple rules:

2.

 my ($my_type, $some_other_type);
 BEGIN {
     $my_type = Some::Constraint::Class->new;
     $some_other_type = Some::Other::Class->new;
 }

 fun foo(($my_type) $label, ($some_other_type) $counts) {
     ...
 }
    

In this variant you enclose an arbitrary Perl expression in "(" ")" (parentheses) and put it in front of a parameter. This expression is evaluated at compile time and must return a type constraint object as described above. (If you use variables here, make sure they're defined at compile time.)

Method modifiers

"Function::Parameters" has support for method modifiers as provided by "Moo" or "Moose". They're not exported by default, so you have to say

 use Function::Parameters qw(:modifiers);

to get them. This line gives you method modifiers only; "fun" and "method" are not defined. To get both the standard keywords and method modifiers, you can either write two "use" lines:

 use Function::Parameters;
 use Function::Parameters qw(:modifiers);

or explicitly list the keywords you want:

 use Function::Parameters qw(fun method :modifiers);

or add the ":std" import tag (which gives you the default import behavior):

 use Function::Parameters qw(:std :modifiers);

This defines the following additional keywords: "before", "after", "around", "augment", "override". These work mostly like "method", but they don't install the function into your package themselves. Instead they invoke whatever "before", "after", "around", "augment", or "override" function (respectively) is in scope to do the job.

 before foo($x, $y, $z) {
     ...
 }

works like

 &before('foo', method ($x, $y, $z) {
     ...
 });

"after", "augment", and "override" work the same way.

"around" is slightly different: Instead of shifting off the first element of @_ into $self (as "method" does), it shifts off two values:

 around foo($x, $y, $z) {
     ...
 }

works like

 &around('foo', sub :method {
     my $orig = shift;
     my $self = shift;
     my ($x, $y, $z) = @_;
     ...
 });

(except you also get the usual "Function::Parameters" features such as checking the number of arguments, etc).

$orig and $self both count as invocants and you can override their names like this:

 around foo($original, $object: $x, $y, $z) {
     # $original is a reference to the wrapped method;
     # $object is the object we're being called on
     ...
 }

If you use ":" to pick your own invocant names in the parameter list of "around", you must specify exactly two variables.

These modifiers also differ from "fun" and "method" (and "sub") in that they require a function name (there are no anonymous method modifiers) and they take effect at runtime, not compile time. When you say "fun foo() {}", the "foo" function is defined right after the closing "}" of the function body is parsed. But with e.g. "before foo() {}", the declaration becomes a normal function call (to the "before" function in the current package), which is performed at runtime.

Prototypes and attributes

You can specify attributes (see "Subroutine Attributes" in perlsub) for your functions using the usual syntax:

 fun deref($x) :lvalue {
    ${$x}
 }

 my $silly;
 deref(\$silly) = 42;

To specify a prototype (see "Prototypes" in perlsub), use the "prototype" attribute:

 fun mypush($aref, @values) :prototype(\@@) {
     push @{$aref}, @values;
 }

Introspection

The function "Function::Parameters::info" lets you introspect parameter lists at runtime. It 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 an object that describes the parameter list of the given function. See "Function::Parameters::Info" for details.

Customizing and extending

Due to its nature as a lexical pragma, importing from "Function::Parameters" always affects the scope that is currently being compiled. If you want to write a wrapper module that enables "Function::Parameters" automatically, just call "Function::Parameters->import" from your own "import" method (and "Function::Parameters->unimport" from your "unimport", as required).

Gory details of importing

At the lowest layer "use Function::Parameters ..." takes a list of one or more hash references. Each key is a keyword to be defined as specified by the corresponding value, which must be another hash reference containing configuration options.

 use Function::Parameters
     {
         keyword_1 => { ... },
         keyword_2 => { ... },
     },
     {
         keyword_3 => { ... },
     };

If you don't specify a particular option, its default value is used. The available configuration options are:

"attributes"

(string) The attributes that every function declared with this keyword should have (in the form of source code, with a leading ":").

Default: nothing

"check_argument_count"

(boolean) Whether functions declared with this keyword should check how many arguments they are called with. If false, omitting a required argument sets it to "undef" and excess arguments are silently ignored. If true, an exception is thrown if too few or too many arguments are passed.

Default: 1

"check_argument_types"

(boolean) Whether functions declared with this keyword should check the types of the arguments they are called with. If false, type constraints are parsed but silently ignored. If true, an exception is thrown if an argument fails a type check.

Default: 1

"default_arguments"

(boolean) Whether functions declared with this keyword should allow default arguments in their parameter list. If false, default arguments are a compile-time error.

Default: 1

"install_sub"

(sub name or reference) If this is set, named functions declared with this keyword are not entered into the symbol table directly. Instead the subroutine specified here (by name or reference) is called with two arguments, the name of the function being declared and a reference to its body.

Default: nothing

"invocant"

(boolean) Whether functions declared with this keyword should allow explicitly specifying invocant(s) at the beginning of the parameter list (as in "($invocant: ...)" or "($invocant1, $invocant2, $invocant3: ...)").

Default: 0

"name"

(string) There are three possible values for this option. 'required' means functions declared with this keyword must have a name. 'prohibited' means specifying a name is not allowed. 'optional' means this keyword can be used for both named and anonymous functions.

Default: 'optional'

"named_parameters"

(boolean) Whether functions declared with this keyword should allow named parameters. If false, named parameters are a compile-time error.

Default: 1

"reify_type"

(coderef or 'auto' or 'moose') The code reference used to resolve type constraints in functions declared with this keyword. It is called once for each type constraint that doesn't use the "( EXPR )" syntax, with one argument, the text of the type in the parameter list (e.g. 'ArrayRef[Int]'). The package the function declaration is in is available through "caller".

The only requirement is that the returned value (here referred to as $tc, for "type constraint") is an object that provides "$tc->check($value)" and "$tc->get_message($value)" methods. "check" is called to determine whether a particular value is valid; it should return a true or false value. "get_message" is called on values that fail the "check" test; it should return a string that describes the error.

Instead of a code reference you can also specify one of two strings.

'auto' stands for a built-in type reifier that treats identifiers as subroutine names, "[" "]" as an array reference, and "|" as bitwise or. In other words, it parses and executes type constraints (mostly) as if they had been Perl source code.

'moose' stands for a built-in type reifier that loads "Moose::Util::TypeConstraints" and just forwards to "find_or_create_isa_type_constraint".

Default: 'auto'

"runtime"

(boolean) Whether functions declared with this keyword should be installed into the symbol table at runtime. If false, named functions are defined (or their "install_sub" is invoked if specified) immediately after their declaration is parsed (as with "sub"). If true, function declarations become normal statements that only take effect at runtime (similar to "*foo = sub { ... };" or "$install_sub->('foo', sub { ... });", respectively).

Default: 0

"shift"

(string or arrayref) In its simplest form, this is the name of a variable that acts as the default invocant (a required leading argument that is removed from @_) for all functions declared with this keyword (e.g. '$self' for methods). You can also set this to an array reference of strings, which lets you specify multiple default invocants, or even to an array reference of array references of the form "[ $name, $type ]" (where $name is the variable name and $type is a type constraint object), which lets you specify multiple default invocants with type constraints.

If you define any default invocants here and also allow individual declarations to override the default (with "invocant => 1"), the number of overridden invocants must match the default. For example, "method" has a default invocant of $self, so "method foo($x, $y: $z)" is invalid because it tries to define two invocants.

Default: "[]" (meaning no invocants)

"strict"

(boolean) Whether functions declared with this keyword should do "strict" checks on their arguments. Currently setting this simply sets "check_argument_count" to the same value with no other effects.

Default: nothing

"types"

(boolean) Whether functions declared with this keyword should allow type constraints in their parameter lists. If false, trying to use type constraints is a compile-time error.

Default: 1

You can get the same effect as "use Function::Parameters;" by saying:

 use Function::Parameters {
     fun => {
         # 'fun' uses default settings only
     },
     method => {
         attributes => ':method',
         shift      => '$self',
         invocant   => 1,
         # the rest is defaults
     },
 };

Configuration bundles

Because specifying all these configuration options from scratch each time is a lot of writing, "Function::Parameters" offers configuration bundles in the form of special strings. These strings can be used to replace a configuration hash completely or as the value of the "defaults" pseudo-option within a configuration hash. The latter lets you use the configuration bundle behind the string to provide defaults and tweak them with your own settings.

The following bundles are available:

"function_strict"

Equivalent to "{}", i.e. all defaults.

"function_lax"

Equivalent to:

 {
     defaults => 'function_strict',
     strict   => 0,
 }
    

i.e. just like "function_strict" but with "strict" checks turned off.

"function"

Equivalent to "function_strict". This is what the default "fun" keyword actually uses. (In version 1 of this module, "function" was equivalent to "function_lax".)

"method_strict"

Equivalent to:

 {
     defaults   => 'function_strict',
     attributes => ':method',
     shift      => '$self',
     invocant   => 1,
 }
    

"method_lax"

Equivalent to:

 {
     defaults => 'method_strict',
     strict   => 0,
 }
    

i.e. just like "method_strict" but with "strict" checks turned off.

"method"

Equivalent to "method_strict". This is what the default "method" keyword actually uses. (In version 1 of this module, "method" was equivalent to "method_lax".)

"classmethod_strict"

Equivalent to:

 {
     defaults => 'method_strict',
     shift    => '$class',
 }
    

i.e. just like "method_strict" but the implicit first parameter is called $class, not $self.

"classmethod_lax"

Equivalent to:

 {
     defaults => 'classmethod_strict',
     strict   => 0,
 }
    

i.e. just like "classmethod_strict" but with "strict" checks turned off.

"classmethod"

Equivalent to "classmethod_strict". This is currently not used anywhere within "Function::Parameters".

"around"

Equivalent to:

 {
     defaults    => 'method',
     install_sub => 'around',
     shift       => ['$orig', '$self'],
     runtime     => 1,
     name        => 'required',
 }
    

i.e. just like "method" but with a custom installer ('around'), two implicit first parameters, only taking effect at runtime, and a method name is required.

"before"

Equivalent to:

 {
     defaults    => 'method',
     install_sub => 'before',
     runtime     => 1,
     name        => 'required',
 }
    

i.e. just like "method" but with a custom installer ('before'), only taking effect at runtime, and a method name is required.

"after"

Equivalent to:

 {
     defaults    => 'method',
     install_sub => 'after',
     runtime     => 1,
     name        => 'required',
 }
    

i.e. just like "method" but with a custom installer ('after'), only taking effect at runtime, and a method name is required.

"augment"

Equivalent to:

 {
     defaults    => 'method',
     install_sub => 'augment',
     runtime     => 1,
     name        => 'required',
 }
    

i.e. just like "method" but with a custom installer ('augment'), only taking effect at runtime, and a method name is required.

"override"

Equivalent to:

 {
     defaults    => 'method',
     install_sub => 'override',
     runtime     => 1,
     name        => 'required',
 }
    

i.e. just like "method" but with a custom installer ('override'), only taking effect at runtime, and a method name is required.

You can get the same effect as "use Function::Parameters;" by saying:

 use Function::Parameters {
     fun    => { defaults => 'function' },
     method => { defaults => 'method' },
 };

or:

 use Function::Parameters {
     fun    => 'function',
     method => 'method',
 };

Import tags

In addition to hash references you can also use special strings in your import list. The following import tags are available:

'fun'

Equivalent to "{ fun => 'function' }".

'method'

Equivalent to "{ method => 'method' }".

'classmethod'

Equivalent to "{ classmethod => 'classmethod' }".

'before'

Equivalent to "{ before => 'before' }".

'after'

Equivalent to "{ after => 'after' }".

'around'

Equivalent to "{ around => 'around' }".

'augment'

Equivalent to "{ augment => 'augment' }".

'override'

Equivalent to "{ override => 'override' }".

':strict'

Equivalent to "{ fun => 'function_strict', method => 'method_strict' }" but that's just the default behavior anyway.

':lax'

Equivalent to "{ fun => 'function_lax', method => 'method_lax' }", i.e. it provides "fun" and "method" keywords that define functions that don't check their arguments.

':std'

Equivalent to 'fun', 'method'. This is what's used by default:

 use Function::Parameters;
    

is the same as:

 use Function::Parameters qw(:std);
    

':modifiers'

Equivalent to 'before', 'after', 'around', 'augment', 'override'.

For example, when you say

 use Function::Parameters qw(:modifiers);

":modifiers" is an import tag that expands to

 use Function::Parameters qw(before after around augment override);

Each of those is another import tag. Stepping through the first one:

 use Function::Parameters qw(before);

is equivalent to:

 use Function::Parameters { before => 'before' };

This says to define the keyword "before" according to the configuration bundle "before":

 use Function::Parameters {
     before => {
         defaults    => 'method',
         install_sub => 'before',
         runtime     => 1,
         name        => 'required',
     },
 };

The "defaults => 'method'" part pulls in the contents of the 'method' configuration bundle (which is the same as 'method_strict'):

 use Function::Parameters {
     before => {
         defaults    => 'function_strict',
         attributes  => ':method',
         shift       => '$self',
         invocant    => 1,
         install_sub => 'before',
         runtime     => 1,
         name        => 'required',
     },
 };

This in turn uses the 'function_strict' configuration bundle (which is empty because it consists of default values only):

 use Function::Parameters {
     before => {
         attributes  => ':method',
         shift       => '$self',
         invocant    => 1,
         install_sub => 'before',
         runtime     => 1,
         name        => 'required',
     },
 };

But if we wanted to be completely explicit, we could write this as:

 use Function::Parameters {
     before => {
         check_argument_count => 1,
         check_argument_types => 1,
         default_arguments    => 1,
         named_parameters     => 1,
         reify_type           => 'auto',
         types                => 1,

         attributes  => ':method',
         shift       => '$self',
         invocant    => 1,
         install_sub => 'before',
         runtime     => 1,
         name        => 'required',
     },
 };

Incompatibilites with version 1 of "Function::Parameters"

• Version 1 defaults to lax mode (no argument checks). To get the same behavior on both version 1 and version 2, explicitly write either "use Function::Parameters qw(:strict);" (the new default) or "use Function::Parameters qw(:lax);" (the old default). (Or write "use Function::Parameters 2;" to trigger an error if an older version of "Function::Parameters" is loaded.)
• Parameter lists used to be optional. The syntax "fun foo { ... }" would accept any number of arguments. This syntax has been removed; you now have to write "fun foo(@) { ... }" to accept (and ignore) all arguments. On the other hand, if you meant for the function to take no arguments, write "fun foo() { ... }".
• There used to be a shorthand syntax for prototypes: Using ":(...)" (i.e. an attribute with an empty name) as the first attribute was equivalent to ":prototype(...)". This syntax has been removed.
• The default type reifier used to be hardcoded to use "Moose" (as in "reify_type => 'moose'"). This has been changed to use whatever type functions are in scope ("reify_type => 'auto'").
• Type reifiers used to see the wrong package in "caller". As a workaround the correct calling package used to be passed as a second argument. This problem has been fixed and the second argument has been removed. (Technically this is a core perl bug (RT #129239 <https://rt.perl.org/Public/Bug/Display.html?id=129239>) that wasn't so much fixed as worked around in "Function::Parameters".)

  If you want your type reifier to be compatible with both versions, you can do this:

   sub my_reifier {
       my ($type, $package) = @_;
       $package //= caller;
       ...
   }
      

  Or using "Function::Parameters" itself:

   fun my_reifier($type, $package = caller) {
       ...
   }