Type::Params

Synopsis

 use v5.36;
 use builtin qw( true false );
 
 package Horse {
   use Moo;
   use Types::Standard qw( Object );
   use Type::Params -sigs;
   use namespace::autoclean;
   
   ...;   # define attributes, etc
   
   signature_for add_child => (
     method     => true,
     positional => [ Object ],
   );
   
   sub add_child ( $self, $child ) {
     push $self->children->@*, $child;
     return $self;
   }
 }
 
 package main;
 
 my $boldruler = Horse->new;
 
 $boldruler->add_child( Horse->new );
 
 $boldruler->add_child( 123 );   # dies (123 is not an Object!)

Status

This module is covered by the Type-Tiny stability policy .

Description

This documents the details of the Type::Params package. Type::Tiny::Manual is a better starting place if you're new.

Type::Params uses Type::Tiny constraints to validate the parameters to a sub. It takes the slightly unorthodox approach of separating validation into two stages:

1. Compiling the parameter specification into a coderef; then
2. Using the coderef to validate parameters.

The first stage is slow (it might take a couple of milliseconds), but only needs to be done the first time the sub is called. The second stage is fast; according to my benchmarks faster even than the XS version of Params::Validate .

With the modern API, you rarely need to worry about the two stages being internally separate.

Note that most of the examples in this documentation use modern Perl features such as subroutine signatures, postfix dereferencing, and the true and false keywords from builtin . On Perl version 5.36+, you can enable all of these features using:

 use v5.36;
 use experimental 'builtin';
 use builtin 'true', 'false';

Type::Params does support older versions of Perl (as old as 5.8), but you may need to adjust the syntax for some examples.

Modern Api

The modern API can be exported using:

 use Type::Params -sigs;

Or:

 use Type::Params -v2;

Or by requesting functions by name:

 use Type::Params qw( signature signature_for );

Two optional shortcuts can be exported:

 use Type::Params qw( signature_for_func signature_for_method );

Or:

 use Type::Params -sigplus;

signature_for $function_name => ( %spec )

Wraps an existing function in additional code that implements all aspects of the subroutine's signature, including unpacking arguments from @_ , applying default values, coercing, and validating values.

signature_for( \@functions, %opts ) is a useful shortcut if you have multiple functions with the same signature.

 signature_for [ 'add_nums', 'subtract_nums' ] => (
   positional => [ Num, Num ],
 );

Although normally used in void context, signature_for does return a value.

 my $meta = signature_for add_nums => (
   positional => [ Num, Num ],
 );
 
 sub add_nums ( $x, $y ) {
   return $x + $y;
 }

Or when used with multiple functions:

 my @metas = signature_for [ 'add_nums', 'subtract_nums' ] => (...);

This is a blessed Type::Params::Signature object which provides some introspection possibilities. Inspecting $meta->coderef->code can be useful to see what the signature is doing internally.

Signature Specification Options

The signature specification is a hash which must contain either a positional , named , or multiple key indicating whether your function takes positional parameters, named parameters, or supports multiple calling conventions, but may also include other options.

positional ArrayRef

This is conceptually a list of type constraints, one for each positional parameter. For example, a signature for a function which accepts two integers:

 signature_for myfunc => ( positional => [ Int, Int ] );

However, each type constraint is optionally followed by a hashref of options which affect that parameter. For example:

 signature_for myfunc => ( positional => [
   Int, { default => 40 },
   Int, { default =>  2 },
 ] );

Type constraints can instead be given as strings, which will be looked up using dwim_type from Type::Utils .

 signature_for myfunc => ( positional => [
   'Int', { default => 40 },
   'Int', { default =>  2 },
 ] );

See the section below for more information on parameter options.

Optional parameters must follow required parameters, and can be specified using either the Optional parameterizable type constraint, the optional parameter option, or by providing a default.

 # All three parameters are effectively optional.
 signature_for myfunc => ( positional => [
   Optional[Int],
   Int, { optional => true },
   Int, { default  => 42 },
 ] );

A single slurpy parameter may be provided at the end, using the Slurpy parameterizable type constraint, or the slurpy parameter option:

 signature_for myfunc => ( positional => [
   Int,
   Slurpy[ ArrayRef[Int] ],
 ] );

 signature_for myfunc => ( positional => [
   Int,
   ArrayRef[Int], { slurpy => true },
 ] );

The positional option can also be abbreviated to pos .

So signature_for myfunc => ( pos => [...] ) can be used instead of the longer signature_for myfunc => ( positional => [...] ) .

 signature_for add_numbers => ( pos => [ Num, Num ] );
 
 sub add_numbers ( $num1, $num2 ) {
   return $num1 + $num2;
 }
 
 say add_numbers( 2, 3 );   # says 5

named ArrayRef

This is conceptually a list of pairs of names and type constraints, one name+type pair for each named parameter. For example, a signature for a function which accepts two integers:

 signature_for myfunc => ( named => [ foo => Int, bar => Int ] )

However, each type constraint is optionally followed by a hashref of options which affect that parameter. For example:

 signature_for myfunc => ( named => [
   foo => Int, { default => 40 },
   bar => Int, { default =>  2 },
 ] );

Type constraints can instead be given as strings, which will be looked up using dwim_type from Type::Utils .

 signature_for myfunc => ( named => [
   foo => 'Int', { default => 40 },
   bar => 'Int', { default =>  2 },
 ] );

Optional and slurpy parameters are allowed, but unlike positional parameters, they do not need to be at the end.

See the section below for more information on parameter options.

If a signature uses named parameters, the values are supplied to the function as a single parameter object:

 signature_for add_numbers => ( named => [ num1 => Num, num2 => Num ] );
 
 sub add_numbers ( $arg ) {
   return $arg->num1 + $arg->num2;
 }
 
 say add_numbers(   num1 => 2, num2 => 3   );   # says 5
 say add_numbers( { num1 => 2, num2 => 3 } );   # also says 5

named_to_list ArrayRef|Bool

The named_to_list option is ignored for signatures using positional parameters, but for signatures using named parameters, allows them to be supplied to the function as a list of values instead of as a single object:

 signature_for add_numbers => (
   named         => [ num1 => Num, num2 => Num ],
   named_to_list => true,
 );
 
 sub add_numbers ( $num1, $num2 ) {
   return $num1 + $num2;
 }
 
 say add_numbers(   num1 => 2, num2 => 3   );   # says 5
 say add_numbers( { num1 => 2, num2 => 3 } );   # also says 5

You can think of add_numbers above as a function which takes named parameters from the outside, but receives positional parameters on the inside.

You can use an arrayref to control the order in which the parameters will be supplied. (By default they are returned in the order in which they were defined.)

 signature_for add_numbers => (
   named         => [ num1 => Num, num2 => Num ],
   named_to_list => [ qw( num2 num1 ) ],
 );
 
 sub add_numbers ( $num2, $num1 ) {
   return $num1 + $num2;
 }
 
 say add_numbers(   num1 => 2, num2 => 3   );   # says 5
 say add_numbers( { num1 => 2, num2 => 3 } );   # also says 5

list_to_named Bool

For a function that accepts named parameters, allows them to alternatively be supplied as a list in a hopefully do-what-you-mean manner.

 signature_for add_numbers => (
   named         => [ num1 => Num, num2 => Num ],
   list_to_named => true,
 );
 
 sub add_numbers ( $arg ) {
   return $arg->num1 + $arg->num2;
 }
 
 say add_numbers( num1 => 5, num2 => 10 );      # says 15
 say add_numbers( { num1 => 5, num2 => 10 } );  # also says 15
 say add_numbers( 5, num2 => 10 );              # says 15 yet again
 say add_numbers( 5, { num2 => 10 } );          # guess what? says 15
 say add_numbers( 10, num1 => 5 );              # 14. just kidding! 15
 say add_numbers( 10, { num1 => 5 } );          # another 15
 say add_numbers( 5, 10 );                      # surprise, it says 15
 
 # BAD: list_to_named argument cannot be at the end.
 say add_numbers( { num1 => 5 }, 10 );
 
 # BAD: list_to_named argument duplicated.
 say add_numbers( 5, 10, { num1 => 5 } );

Where a hash or hashref of named parameters are expected, any parameter which doesn't look like it fits that pattern will be treated as a "sneaky" positional parameter, and will be tried the first time a named parameter seems to be missing.

This feature is normally only applied to required parameters. It can be manually controlled on a per-parameter basis using the in_list option.

Type::Params attempts to be intelligent at figuring out what order the sneaky positional parameters were given in.

 signature_for add_to_ref => (
   named         => [ ref => ScalarRef[Num], add => Num ],
   list_to_named => true,
 );
 
 sub add_to_ref ( $arg ) {
   $arg->ref->$* += $arg->num;
 }
 
 my $sum = 0;
 add_to_ref( ref => \$sum, add => 1 );
 add_to_ref( \$sum, add => 2 );
 add_to_ref( \$sum, 3 );
 add_to_ref( 4, \$sum );
 add_to_ref( 5, sum => \$sum );
 add_to_ref( add => 5, sum => \$sum );
 say $sum; # 21

This approach is somewhat slower, but has the potential for very do-what-I-mean functions.

Note that list_to_named and named_to_list can both be used in the same signature as their meanings are not contradictory.

 signature_for add_to_ref => (
   named         => [ ref => ScalarRef[Num], add => Num ],
   list_to_named => true,
   named_to_list => true,
 );
 
 sub add_to_ref ( $ref, $num ) {
   $ref->$* += $num;
 }

head Int|ArrayRef

head provides an additional list of non-optional, positional parameters at the start of @_ . This is often used for method calls. For example, if you wish to define a signature for:

 $object->my_method( foo => 123, bar => 456 );

You could write it as this:

 signature_for my_method => (
   head    => [ Object ],
   named   => [ foo => Optional[Int], bar => Optional[Int] ],
 );
 
 sub my_method ( $self, $arg ) {
   ...;
 }

If head is set as a number instead of an arrayref, it is the number of additional arguments at the start:

 signature_for stash_foobar = (
   head    => 2,
   named   => [ foo => Optional[Int], bar => Optional[Int] ],
 );
 
 sub stash_foobar ( $self, $ctx, $arg ) {
   $ctx->stash->{foo} = $arg->foo if $arg->has_foo;
   $ctx->stash->{bar} = $arg->bar if $arg->has_bar;
   return $self;
 }
 
 ...;
 
 $app->stash_foobar( $context, foo => 123 );

In this case, no type checking is performed on those additional arguments; it is just checked that they exist.

tail Int|ArrayRef

A tail is like a head except that it is for arguments at the end of @_ .

 signature_for my_method => (
   head    => [ Object ],
   named   => [ foo => Optional[Int], bar => Optional[Int] ],
   tail    => [ CodeRef ],
 );
 
 sub my_method ( $self, $arg, $callback ) {
   ...;
 }
 
 $object->my_method( foo => 123, bar => 456, sub { ... } );

method Bool|TypeTiny

While head can be used for method signatures, a more declarative way is to set method => true .

If you wish to be specific that this is an object method, intended to be called on blessed objects only, then you may use method => Object , using the Object type from Types::Standard . If you wish to specify that it's a class method, then use method => Str , using the Str type from Types::Standard . ( method => ClassName is perhaps clearer, but it's a slower check.)

 signature_for my_method => (
   method  => true,
   named   => [ foo => Optional[Int], bar => Optional[Int] ],
 );
 
 sub my_method ( $self, $arg ) {
   ...;
 }

The method option has some other subtle differences from head . Any parameter defaults which are coderefs will be called as methods on the invocant instead of being called with no arguments. The package option will be interpreted slightly differently.

It is possible to use both method and head in the same signature. The invocant is interpreted as being before the head .

A shortcut is provided for method => true , though it also enables a couple of other options.

 use Type::Params qw( signature_for_method );
 
 signature_for_method my_method => (
   named => [ foo => Optional[Int], bar => Optional[Int] ],
 );
 
 sub my_method ( $self, $arg ) {
   ...;
 }

description Str

This is the description of the coderef that will show up in stack traces. It defaults to "parameter validation for X" where X is the sub name. Usually the default will be fine.

package Str

This allows you to add signatures to functions in other packages:

 signature_for foo => ( package "Some::Package", ... );

If method is true and Some::Package doesn't contain a sub called "foo", then Type::Params will traverse the inheritance heirarchy, looking for "foo".

If any type constraints are specified as strings, Type::Params will look for types imported by this package.

 # Expects the MyInt type to be known by Some::Package.
 signature_for foo => ( package "Some::Package", pos => [ 'MyInt' ] );

This is also supported:

 signature_for "Some::Package::foo" => ( ... );

fallback CodeRef|Bool

If the sub being wrapped cannot be found, then signature_for will usually throw an error. If you want it to "still work" in this situation, use the fallback option. fallback => \&alternative_coderef_to_wrap will instead wrap a different coderef if the original cannot be found. fallback => true is a shortcut for fallback => sub {} . An example where this might be useful is if you're adding signatures to methods which are inherited from a parent class, but you are not 100% confident will exist (perhaps dependent on the version of the parent class).

 signature_for add_nums => (
   positional => [ Num, Num ],
   fallback   => sub { $_[0] + $_[1] },
 );

on_die Maybe[CodeRef]

Usually when the signature check hits an error, it will throw an exception, which is a blessed Error::TypeTiny object.

If you provide an on_die coderef, then instead the Error::TypeTiny object will be passed to it.

 signature_for add_numbers => (
   positional => [ Num, Num ],
   on_die     => sub {
     my $error = shift;
     print "Existential crisis: $error\n";
     exit( 1 );
   },
 );
 
 sub add_numbers ( $num1, $num2 ) {
   return $num1 + $num2;
 }
 
 say add_numbers();   # has an existential crisis

If your on_die coderef doesn't exit or throw an exception, it can instead return a list which will be used as parameters for your function.

 signature_for add_numbers => (
   positional => [ Num, Num ],
   on_die     => sub { return ( 40, 2 ) },
 );
 
 sub add_numbers ( $num1, $num2 ) {
   return $num1 + $num2;
 }
 
 say add_numbers();   # 42

This is probably not very useful.

strictness Bool|Str

If you set strictness to false, then certain signature checks will simply never be done. The initial check that there's the correct number of parameters, plus type checks on parameters which don't coerce can be skipped.

If you set it to true or do not set it at all, then these checks will always be done.

Alternatively, it may be set to the quoted fully-qualified name of a Perl global variable or a constant, and that will be compiled into the coderef as a condition to enable strict checks.

 signature_for my_func => (
   strictness => '$::CHECK_TYPES',
   positional => [ Int, ArrayRef ],
 );
 
 sub my_func ( $int, $aref ) {
   ...;
 }
 
 # Type checks are skipped
 {
   local $::CHECK_TYPES = false;
   my ( $number, $list ) = my_func( {}, {} );
 }
 
 # Type checks are performed
 {
   local $::CHECK_TYPES = true;
   my ( $number, $list ) = my_func( {}, {} );
 }

A recommended use of strictness is with Devel::StrictMode .

 use Devel::StrictMode qw( STRICT );
 
 state $signature = signature(
   strictness => STRICT,
   positional => [ Int, ArrayRef ],
 );

multiple ArrayRef

This option allows your signature to support multiple calling conventions. Each entry in the array is an alternative signature, as a hashref:

 signature_for my_func => (
   multiple => [
     {
       positional    => [ ArrayRef, Int ],
     },
     {
       named         => [ array => ArrayRef, index => Int ],
       named_to_list => true,
     },
   ],
 );
 
 sub my_func ( $aref, $int ) {
   ...;
 }

That signature will allow your function to be called as:

 your_function( $arr, $ix );
 your_function( array => $arr, index => $ix );
 your_function( { array => $arr, index => $ix } );

Sometimes the alternatives will return the parameters in different orders:

 signature_for my_func => (
   multiple => [
     { positional => [ ArrayRef, Int ] },
     { positional => [ Int, ArrayRef ] },
   ],
 );

So how does your sub know how it's been called? One option is to use the ${^_TYPE_PARAMS_MULTISIG} global variable which will be set to the index of the signature which was used:

 sub my_func {
   my ( $arr, $ix ) = ${^_TYPE_PARAMS_MULTISIG} == 1 ? reverse( @_ ) : @_;
   ...;
 }

A neater solution is to use a next coderef to re-order alternative signature results into your preferred order:

 signature_for my_func => (
   multiple => [
     { positional => [ ArrayRef, Int ] },
     { positional => [ Int, ArrayRef ], next => sub { reverse @_ } },
   ],
 );
 
 sub my_func ( $arr, $ix ) {
   ...;
 }

While conceptally multiple is an arrayref of hashrefs, it is also possible to use arrayrefs in the arrayref.

 multiple => [
   [ ArrayRef, Int ],
   [ Int, ArrayRef ],
 ]

When an arrayref is used like that, it is a shortcut for a positional signature.

Coderefs may additionally be used:

 signature_for my_func => (
   multiple => [
     [ ArrayRef, Int ],
     { positional => [ Int, ArrayRef ], next => sub { reverse @_ } },
     sub { ... },
     sub { ... },
   ],
 );

The coderefs should be subs which return a list of parameters if they succeed and throw an exception if they fail.

The following signatures are equivalent:

 signature_for my_func => (
   multiple => [
     { method => true, positional => [ ArrayRef, Int ] },
     { method => true, positional => [ Int, ArrayRef ] },
   ],
 );
 
 signature_for my_func => (
   method   => true,
   multiple => [
     { positional => [ ArrayRef, Int ] },
     { positional => [ Int, ArrayRef ] },
   ],
 );

The multiple option can also be abbreviated to multi . So signature( multi => [...] ) can be used instead of the longer signature( multiple => [...] ) . Three whole keystrokes saved!

( Note: in older releases of Type::Params, ${^_TYPE_PARAMS_MULTISIG} was called ${^TYPE_PARAMS_MULTISIG} . The latter name is no longer supported.)

message Str

Only used by multiple signatures. The error message to throw when no signatures match.

bless Bool|ClassName , class ClassName|ArrayRef , and constructor Str

Named parameters are usually returned as a blessed object:

 signature_for add_numbers => ( named => [ num1 => Num, num2 => Num ] );
 
 sub add_numbers ( $arg ) {
   return $arg->num1 + $arg->num2;
 }

The class they are blessed into is one built on-the-fly by Type::Params. However, these three signature options allow you more control over that process.

Firstly, if you set bless => false and do not set class or constructor , then $arg will just be an unblessed hashref.

 signature_for add_numbers => (
   named        => [ num1 => Num, num2 => Num ],
   bless        => false,
 );
 
 sub add_numbers ( $arg ) {
   return $arg->{num1} + $arg->{num2};
 }

This is a good speed boost, but having proper methods for each named parameter is a helpful way to catch misspelled names.

If you wish to manually create a class instead of relying on Type::Params generating one on-the-fly, you can do this:

 package Params::For::AddNumbers {
   sub num1 ( $self ) {
     return $self->{num1};
   }
   sub num2 ( $self ) {
     return $self->{num2};
   }
   sub sum ( $self ) {
     return $self->num1 + $self->num2;
   }
 }
 
 signature_for add_numbers => (
   named        => [ num1 => Num, num2 => Num ],
   bless        => 'Params::For::AddNumbers',
 );
 
 sub add_numbers ( $arg ) {
   return $arg->sum;
 }

Note that Params::For::AddNumbers here doesn't include a new method because Type::Params will directly do bless( $arg, $opts{bless} ) .

If you want Type::Params to use a proper constructor, you should use the class option instead:

 package Params::For::AddNumbers {
   use Moo;
   has [ 'num1', 'num2' ] => ( is => 'ro' );
   sub sum {
     my $self = shift;
     return $self->num1 + $self->num2;
   }
 }
 
 signature_for add_numbers => (
   named        => [ num1 => Num, num2 => Num ],
   class        => 'Params::For::AddNumbers',
 );
 
 sub add_numbers ( $arg ) {
   return $arg->sum;
 }

If you wish to use a constructor named something other than new , then use:

 signature_for add_numbers => (
   named        => [ num1 => Num, num2 => Num ],
   class        => 'Params::For::AddNumbers',
   constructor  => 'new_from_hashref',
 );

Or as a shortcut:

 signature_for add_numbers => (
   named        => [ num1 => Num, num2 => Num ],
   class        => [ 'Params::For::AddNumbers' => 'new_from_hashref' ],
 );

It is doubtful you want to use any of these options, except bless => false .

returns TypeTiny , returns_scalar TypeTiny , and returns_list TypeTiny

These can be used to specify the type returned by your function.

 signature_for round_number => (
   pos          => [ Num ],
   returns      => Int,
 );
 
 sub round_number ( $num ) {
   return int( $num );
 }

If your function returns different types in scalar and list context, you can use returns_scalar and returns_list to indicate separate return types in different contexts.

 signature_for my_func => (
   pos             => [ Int, Int ],
   returns_scalar  => Int,
   returns_list    => Tuple[ Int, Int, Int ],
 );

The returns_list constraint is defined using an ArrayRef -like or HashRef -like type constraint even though it's returning a list, not a single reference.

If your function is called in void context, then its return value is unimportant and should not be type checked.

allow_dash Bool

For any "word-like" named parameters or aliases, automatically creates an alias with a leading hyphen.

 signature_for withdraw_funds => (
   named      => [ amount => Num, account => Str ],
   allow_dash => true,
 );
 
 sub withdraw_funds ( $arg ) {
   ...;
 }
 
 withdraw_funds(  amount => 11.99,  account => 'ABC123' );
 withdraw_funds( -amount => 11.99,  account => 'ABC123' );
 withdraw_funds(  amount => 11.99, -account => 'ABC123' );
 withdraw_funds( -amount => 11.99, -account => 'ABC123' );

Has no effect on names that are not word-like. Word-like names are those matching /\A[^\W0-9]\w*\z/ ; essentially anything Perl allows as a normal unqualified variable name.

Parameter Options

In the parameter lists for the positional and named signature options, each parameter may be followed by a hashref of options specific to that parameter:

 signature_for my_func => (
   positional => [
     Int, \%options_for_first_parameter,
     Int, \%options_for_other_parameter,
   ],
   %more_options_for_signature,
 );

 signature_for my_func => (
   named => [
     foo => Int, \%options_for_foo,
     bar => Int, \%options_for_bar,
   ],
   %more_options_for_signature,
 );

The following options are supported for parameters.

optional Bool

An option called optional!

This makes a parameter optional:

 signature_for add_nums => (
   positional => [
     Int,
     Int,
     Bool, { optional => true },
   ],
 );
 
 sub add_nums ( $num1, $num2, $debug ) {
   my $sum = $num1 + $num2;
   warn "$sum = $num1 + $num2" if $debug;
   return $sum;
 }
 
 add_nums( 2, 3, 1 );   # prints warning
 add_nums( 2, 3, 0 );   # no warning
 add_nums( 2, 3    );   # no warning

Types::Standard also provides a Optional parameterizable type which may be a neater way to do this:

 signature_for add_nums => ( pos => [ Int, Int, Optional[Bool] ] );

In signatures with positional parameters, any optional parameters must be defined after non-optional parameters. The tail option provides a workaround for required parameters at the end of @_ .

In signatures with named parameters, the order of optional and non-optional parameters is unimportant.

slurpy Bool

A signature may contain a single slurpy parameter, which mops up any other arguments the caller provides your function.

In signatures with positional parameters, slurpy params must always have some kind of ArrayRef or HashRef type constraint, must always appear at the end of the list of positional parameters, and they work like this:

 signature_for add_nums => (
   positional => [
     Num,
     ArrayRef[Num], { slurpy => true },
   ],
 );
 
 sub add_nums ( $first_num, $other_nums ) {
   my $sum = $first_num;
   for my $other ( $other_nums->@* ) {
     $sum += $other;
   }
   return $sum;
 }
 
 say add_nums( 1 );            # says 1
 say add_nums( 1, 2 );         # says 3
 say add_nums( 1, 2, 3 );      # says 6
 say add_nums( 1, 2, 3, 4 );   # says 10

In signatures with named parameters, slurpy params must always have some kind of HashRef type constraint, and they work like this:

 use builtin qw( true false );
 
 signature_for process_data => (
   method => true,
   named  => [
     input   => FileHandle,
     output  => FileHandle,
     flags   => HashRef[Bool], { slurpy => true },
   ],
 );
 
 sub process_data ( $self, $arg ) {
   warn "Beginning data processing" if $arg->flags->{debug};
   ...;
 }
 
 $widget->process_data(
   input  => \*STDIN,
   output => \*STDOUT,
   debug  => true,
 );

The Slurpy type constraint from Types::Standard may be used as a shortcut to specify slurpy parameters:

 signature_for add_nums => (
   positional => [ Num, Slurpy[ ArrayRef[Num] ] ],
 )

The type Slurpy[Any] is handled specially and treated as a slurpy ArrayRef in signatures with positional parameters, and a slurpy HashRef in signatures with named parameters, but has some additional optimizations for speed.

default CodeRef|ScalarRef|Ref|Str|Undef

A default may be provided for a parameter.

 signature_for my_func => (
   positional => [
     Int,
     Int, { default => "666" },
     Int, { default => "999" },
   ],
 );

Supported defaults are any strings (including numerical ones), undef , and empty hashrefs and arrayrefs. Non-empty hashrefs and arrayrefs are not allowed as defaults .

Alternatively, you may provide a coderef to generate a default value:

 signature_for my_func => (
   positional => [
     Int,
     Int, { default => sub { 6 * 111 } },
     Int, { default => sub { 9 * 111 } },
   ]
 );

That coderef may generate any value, including non-empty arrayrefs and non-empty hashrefs. For undef, simple strings, numbers, and empty structures, avoiding using a coderef will make your parameter processing faster.

Instead of a coderef, you can use a reference to a string of Perl source code:

 signature_for my_func => (
   positional => [
     Int,
     Int, { default => \ '6 * 111' },
     Int, { default => \ '9 * 111' },
   ],
 );

Defaults will be validated against the type constraint, and potentially coerced.

Any parameter with a default will automatically be optional, as it makes no sense to provide a default for required paramaters.

Note that having any defaults in a signature (even if they never end up getting used) can slow it down, as Type::Params will need to build a new array instead of just returning @_ .

coerce Bool

Speaking of coercion, the coerce option allows you to indicate that a value should be coerced into the correct type:

 signature_for my_func => (
   positional => [
     Int,
     Int,
     Bool, { coerce => true },
   ],
 );

Setting coerce to false will disable coercion.

If coerce is not specified, so is neither true nor false, then coercion will be enabled if the type constraint has a coercion, and disabled otherwise.

Note that having any coercions in a signature (even if they never end up getting used) can slow it down, as Type::Params will need to build a new array instead of just returning @_ .

clone Bool

If this is set to true, it will deep clone incoming values via dclone from Storable (a core module since Perl 5.7.3).

In the below example, $arr is a reference to a clone of @numbers , so pushing additional numbers to it leaves @numbers unaffected.

 signature_for foo => (
   positional => [ ArrayRef, { clone => true } ],
 );
 
 sub foo ( $arr ) {
   push @$arr, 4, 5, 6;
 }
 
 my @numbers = ( 1, 2, 3 );
 foo( \@numbers );
 print "@numbers\n";  ## 1 2 3

Note that cloning will significantly slow down your signature.

name Str

This overrides the name of a named parameter. I don't know why you would want to do that.

The following signature has two parameters: foo and bar . The name fool is completely ignored.

 signature_for my_func => (
   named => [
     fool   => Int, { name => 'foo' },
     bar    => Int,
   ],
 );

You can, however, also name positional parameters, which don't usually have names.

 signature_for my_func => (
   positional => [
     Int, { name => 'foo' },
     Int, { name => 'bar' },
   ],
 );

The names of positional parameters are not really used for anything at the moment, but may be incorporated into error messages or similar in the future.

getter Str

For signatures with named parameters, specifies the method name used to retrieve this parameter's value from the $arg object.

 signature_for process_data => (
   method => true,
   named  => [
     input   => FileHandle,    { getter => 'in' },
     output  => FileHandle,    { getter => 'out' },
     flags   => HashRef[Bool], { slurpy => true },
   ],
 );
 
 sub process_data ( $self, $arg ) {
   warn "Beginning data processing" if $arg->flags->{debug};
   
   my ( $in, $out ) = ( $arg->in, $arg->out );
   ...;
 }
 
 $widget->process_data(
   input  => \*STDIN,
   output => \*STDOUT,
   debug  => true,
 );

Ignored by signatures with positional parameters.

predicate Str

The $arg object provided by signatures with named parameters will also include "has" methods for any optional arguments. For example:

 signature_for process_data => (
   method => true,
   named  => [
     input   => Optional[ FileHandle ],
     output  => Optional[ FileHandle ],
     flags   => Slurpy[ HashRef[Bool] ],
   ],
 );
 
 sub process_data ( $self, $arg ) {
   
   if ( $self->has_input and $self->has_output ) {
     ...;
   }
   
   ...;
 }

Setting a predicate option allows you to choose a different name for this method instead of "has_*".

It is also possible to set a predicate for non-optional parameters, which don't normally get a "has" method.

Ignored by signatures with positional parameters.

alias Str|ArrayRef[Str]

A list of alternative names for the parameter, or a single alternative name.

 signature_for add_numbers => (
   named => [
     first_number   => Int, { alias => [ 'x' ] },
     second_number  => Int, { alias =>   'y'   },
   ],
 );
 
 sub add_numbers ( $arg ) {
   return $arg->first_number + $arg->second_number;
 }
 
 say add_numbers( first_number => 40, second_number => 2 );  # 42
 say add_numbers( x            => 40, y             => 2 );  # 42
 say add_numbers( first_number => 40, y             => 2 );  # 42
 say add_numbers( first_number => 40, x => 1, y => 2 );      # dies!

Ignored by signatures with positional parameters.

in_list Bool

In conjunction with list_to_named , determines if this parameter can be provided as part of the list of "sneaky" positional parameters. If list_to_named isn't being used, in_list is ignored.

Defaults to false if the parameter is optional or has a default. Defaults to true if the parameter is required.

strictness Bool|Str

Overrides the signature option strictness on a per-parameter basis.

signature_for_func $function_name => ( %spec )

Like signature_for and defaults to method => false .

If the signature has named parameters, it will additionally default list_to_named and allow_dash to true.

 signature_for_func add_to_ref => (
   named         => [ ref => ScalarRef[Num], add => Num ],
   named_to_list => true,
 );
 
 sub add_to_ref ( $ref, $add ) {
   $ref->$* += $add;
 }
 
 my $sum = 0;
 add_to_ref( ref => \$sum, add => 1 );
 add_to_ref( \$sum, 2 );
 add_to_ref( 3, \$sum );
 add_to_ref( 4, { -ref => \$sum } );
 say $sum; # 10

The exact behaviour of signature_for_func is unstable and may change in future versions of Type::Params.

signature_for_method $function_name => ( %spec )

Like signature_for but will default method => true .

If the signature has named parameters, it will additionally default list_to_named and allow_dash to true.

 package Calculator {
   use Types::Standard qw( Num ScalarRef );
   use Type::Params qw( signature_for_method );
   
   ...;
   
   signature_for_method add_to_ref => (
     named         => [ ref => ScalarRef[Num], add => Num ],
     named_to_list => true,
   );
   
   sub add_to_ref ( $self, $ref, $add ) {
     $ref->$* += $add;
   }
 }
 
 my $calc = Calculator->new;
 my $sum = 0;
 $calc->add_to_ref( ref => \$sum, add => 1 );
 $calc->add_to_ref( \$sum, 2 );
 $calc->add_to_ref( 3, \$sum );
 $calc->add_to_ref( 4, { -ref => \$sum } );
 say $sum; # 10

The exact behaviour of signature_for_method is unstable and may change in future versions of Type::Params.

signature( %spec )

The signature function allows more fine-grained control over signatures. Instead of automatically wrapping your function, it returns a coderef that you can pass @_ to.

The following are roughly equivalent:

 signature_for add_nums => ( pos => [ Num, Num ] );
 
 sub add_nums ( $x, $y ) {
   return $x + $y;
 }

And:

 sub add_nums {
   state $signature = signature( pos => [ Num, Num ] );
   my ( $x, $y ) = $signature->( @_ );
   
   return $x + $y;
 }

Perl allows a slightly archaic way of calling coderefs without using parentheses, which may be slightly faster at the cost of being more obscure:

 sub add_nums {
   state $signature = signature( pos => [ Num, Num ] );
   my ( $x, $y ) = &$signature; # important: no parentheses!
   
   return $x + $y;
 }

If you need to support Perl 5.8, which didn't have the state keyword:

 my $__add_nums_sig;
 sub add_nums {
   $__add_nums_sig ||= signature( pos => [ Num, Num ] );
   my ( $x, $y ) = &$__add_nums_sig;
   
   ...;
 }

This gives you more control over how and when the signature is built and used, and what is done with the values it unpacks.

In particular, note that if your function is never called, the signature never even gets built, meaning that for functions you rarely use, there's less cost to having the signature.

As of 2025, you probably want to be using signature_for instead of signature in most cases.

Additional Signature Specification Options

There are certain options which make no sense for signature_for , and are only useful for signature . Others may behave slightly differently. These are noted here.

returns TypeTiny , returns_scalar TypeTiny , and returns_list TypeTiny

Because signature isn't capable of fully wrapping your function, the returns , returns_scalar , and returns_list options cannot do anything. You should consider them to be documentation only.

subname Str

The name of the sub whose parameters we're supposed to be checking. This is useful in stack traces, etc. Defaults to the caller.

package Str

Works the same as in signature_for , but it's worth mentioning it again as it ties in closely with subname .

caller_level Int

If you're wrapping signature so that you can check signatures on behalf of another package, then setting caller_level to 1 (or more, depending on the level of wrapping!) may be an alternative to manually setting the package and subname .

next Bool|CodeLike

This can be used for chaining coderefs. If you understand on_die , this acts like an "on_live".

 sub add_numbers {
   state $sig = signature(
     positional => [ Num, Num ],
     next => sub {
       my ( $num1, $num2 ) = @_;
       
       return $num1 + $num2;
     },
   );
   
   my $sum = $sig->( @_ );
   return $sum;
 }
 
 say add_numbers( 2, 3 );   # says 5

If set to true instead of a coderef, has a slightly different behaviour:

 sub add_numbers {
   state $sig = signature(
     positional => [ Num, Num ],
     next       => true,
   );
   
   my $sum = $sig->(
     sub { return $_[0] + $_[1] },
     @_,
   );
   return $sum;
 }
 
 say add_numbers( 2, 3 );   # says 5

This looks strange. Why would this be useful? Well, it works nicely with Moose's around keyword.

 sub add_numbers {
   return $_[1] + $_[2];
 }
 
 around add_numbers => signature(
   method     => true,
   positional => [ Num, Num ],
   next       => true,
   package    => __PACKAGE__,
   subname    => 'add_numbers',
 );
 
 say __PACKAGE__->add_numbers( 2, 3 );   # says 5

Note the way around works in Moose is that it expects a wrapper coderef as its final argument. That wrapper coderef then expects to be given a reference to the original function as its first parameter.

This can allow, for example, a role to provide a signature wrapping a method defined in a class.

This is kind of complex, and you're unlikely to use it, but it's been proven useful for tools that integrate Type::Params with Moose-like method modifiers.

Note that next is the mechanism that signature_for internally uses to connect the signature with the wrapped sub, so using next with signature_for is a good recipe for headaches.

If using multiple signatures, next is useful for each "inner" signature to massage parameters into the correct order. This use of next is supported for signature_for .

The option goto_next is supported as a historical alias for next .

want_source Bool

Instead of returning a coderef, return Perl source code string. Handy for debugging.

want_details Bool

Instead of returning a coderef, return a hashref of stuff including the coderef. This is mostly for people extending Type::Params and I won't go into too many details about what else this hashref contains.

want_object Bool

Instead of returning a coderef, return a Type::Params::Signature object. This is the more modern version of want_details .

Legacy Api

The following functions were the API prior to Type::Params v2. They are still supported, but their use is now discouraged.

If you don't provide an import list at all, you will import compile and compile_named :

 use Type::Params;

This does the same:

  use Type::Params -v1;

The following exports compile , compile_named , and compile_named_oo :

 use Type::Params -compile;

The following exports wrap_subs and wrap_methods :

 use Type::Params -wrap;

compile( @pos_params )

Equivalent to signature( positional => \@pos_params ) .

compile( \%spec, @pos_params ) is equivalent to signature( %spec, positional => \@pos_params ) .

compile_named( @named_params )

Equivalent to signature( bless => 0, named => \@named_params ) .

compile_named( \%spec, @named_params ) is equivalent to signature( bless => false, %spec, named => \@named_params ) .

compile_named_oo( @named_params )

Equivalent to signature( bless => true, named => \@named_params ) .

compile_named_oo( \%spec, @named_params ) is equivalent to signature( bless => true, %spec, named => \@named_params ) .

validate( \@args, @pos_params )

Equivalent to signature( positional => \@pos_params )->( @args ) .

The validate function has never been recommended, and is not exported unless requested by name.

validate_named( \@args, @named_params )

Equivalent to signature( bless => false, named => \@named_params )->( @args ) .

The validate_named function has never been recommended, and is not exported unless requested by name.

wrap_subs( func1 => \@params1, func2 => \@params2, ... )

Equivalent to:

 signature_for func1 => ( positional => \@params1 );
 signature_for func2 => ( positional => \@params2 );

One slight difference is that instead of arrayrefs, you can provide the output of one of the compile functions:

 wrap_subs( func1 => compile_named( @params1 ) );

wrap_subs is not exported unless requested by name.

wrap_methods( func1 => \@params1, func2 => \@params2, ... )

Equivalent to:

 signature_for func1 => ( method => 1, positional => \@params1 );
 signature_for func2 => ( method => 1, positional => \@params2 );

One slight difference is that instead of arrayrefs, you can provide the output of one of the compile functions:

 wrap_methods( func1 => compile_named( @params1 ) );

wrap_methods is not exported unless requested by name.

multisig( @alternatives )

Equivalent to:

 signature( multiple => \@alternatives )

multisig( \%spec, @alternatives ) is equivalent to signature( %spec, multiple => \@alternatives ) .

Type Constraints

Although Type::Params is not a real type library, it exports two type constraints. Their use is no longer recommended.

Invocant

Type::Params exports a type Invocant on request. This gives you a type constraint which accepts classnames and blessed objects.

 use Type::Params qw( compile Invocant );
 
 signature_for my_method => (
   method     => Invocant,
   positional => [ ArrayRef, Int ],
 );
 
 sub my_method ($self_or_class, $arr, $ix) {
   return $arr->[ $ix ];
 }

Invocant is not exported unless requested by name.

Recommendation: use Defined from Types::Standard instead.

ArgsObject

Type::Params exports a parameterizable type constraint ArgsObject . It accepts the kinds of objects returned by signature checks for named parameters.

  use v5.36;
  
  package Foo {
    use Moo;
    use Type::Params 'ArgsObject';
    
    has args => (
      is  => 'ro',
      isa => ArgsObject['Bar::bar'],
    );
  }
  
  package Bar {
    use Types::Standard -types;
    use Type::Params 'signature_for';
    
    signature_for bar => ( named => [ xxx => Int, yyy => ArrayRef ] );
    
    sub bar ( $got ) {
      return 'Foo'->new( args => $got );
    }
  }
  
  Bar::bar( xxx => 42, yyy => [] );

The parameter "Bar::bar" refers to the caller when the check is compiled, rather than when the parameters are checked.

ArgsObject is not exported unless requested by name.

Recommendation: use Object from Types::Standard instead.

Environment

PERL_TYPE_PARAMS_XS

Affects the building of accessors for $arg objects. If set to true, will use Class::XSAccessor . If set to false, will use pure Perl. If this environment variable does not exist, will use Class::XSAccessor.

If Class::XSAccessor is not installed or is too old, pure Perl will always be used as a fallback.

See Also

The Type::Tiny homepage .

Type::Tiny , Type::Coercion , Types::Standard .