Type::Params(3pm)

NAME
Type::Params - sub signature validation using Type::Tiny type constraints and coercions

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

Since Type::Params 2.009_000 the $arg object has methods called "__TO_LIST__", "__TO_ARRAYREF__", and "__TO_HASHREF__".

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

 signature_for add_numbers => ( named => [ num1 => Num, num2 => Num ] );
 sub add_numbers ( $arg ) {
   my $nums = $arg->__TO_ARRAYREF__;
   return $nums->[0] + $nums->[1];
 }

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

Each of these can be given an optional arrayref indicating which fields to return.

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

The arrayref accepts aliases (see "alias") but methods may throw an exception if the arrayref contains unknown field names. (See "strictness" to control whether an exception is thrown.)

These methods start and end with double underscores to reduce the chance that they'll conflict with the name of a named parameter, however they are considered part of the public, supported API.

The object overloads "@{}" to call "__TO_ARRAYREF__".

"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|HashRef

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( @_ ) : @_;
   ...;
 }

If you'd prefer to use identifying names instead of a numeric index, you can specify these using "ID":

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

Or by using a hashref:

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

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 @_.

"default_on_undef" Bool

Normally defaults are only applied when a parameter is missing (think "exists" for hashes or the array being too short). Setting "default_on_undef" to true will also trigger the default if a parameter is provided but undefined.

If the caller might legitimately want to supply undef as a value, it is not recommended you uswe this.

"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.

CONSTANTS
The constants "SIGNATURE_SUBNAME_PREFIX" and "SIGNATURE_SUBNAME_SUFFIX" exist. They normally return the empty string, but can be influenced by environment variables. (See below.) They are not exportable.

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.

"PERL_TYPE_PARAMS_SUBNAME_PREFIX" and "PERL_TYPE_PARAMS_SUBNAME_SUFFIX"
    Each Perl subroutines has an idea of its fully-qualified subname, which is distinct from how it is actually called. For example:

      package Foo;
      *bar = sub { return "foobar" };

    The subroutine can be called as Foo::bar(), however the subroutine still "thinks" it is an anonymous coderef because that is how it was initially created. That is how it will show up in stack traces, profiling tools, etc. These names can be manipulated with Sub::Util.

    By default, if you use "signature_for" to define a signature for YourModule::yourfunc(), then the wrapper function it creates will also give itself the subname of YourModule::yourfunc(). This is normally helpful.

    However, if you are debugging or profiling and wish to give the wrapper subs a different subname in stack traces or profiling reports, these environment variables allow you to control that. They have will have a global effect on your application.

    "PERL_TYPE_PARAMS_SUBNAME_PREFIX" can be set to true to tell Type::Params to add "SIGNATURE_FOR::" to the start of the wrapper function's subname. Or it can be set to any string ending "::" to specify an alternative prefix.

    "PERL_TYPE_PARAMS_SUBNAME_SUFFIX" can be set to true to tell Type::Params to add "_SIGNATURE" to the end of the wrapper function's subname. Or it can be set to any string starting "_" to specify an alternative suffix.

    There's usually no reason to apply both a prefix and a suffix.

    Suffixes may play nicer with utilities like namespace::autoclean.

    Example usage:

      export PERL_TYPE_PARAMS_SUBNAME_SUFFIX=1
      perl -d:NYTProf some_perl.pl

HOOKS
You can install coderefs which will be called whenever "signature_for", "signature_for_func", or "signature_for_method" are used to wrap a function or method. They are ignored by "signature" and by the pre-v2 API.

  push @{ $Type::Params::POST_INSTALL{'My::Package'} ||= [] }, sub {
    my $signature = shift;
    ...;
  };

  push @{ $Type::Params::PRE_INSTALL{'My::Package'} ||= [] }, sub {
    my $signature = shift;
    ...;
  };

The $signature will be a blessed Type::Params::Signature object.

The intention for this is to allow for future extensions to expose signature data as metadata.

BUGS
Please report any bugs to <https://github.com/tobyink/p5-type-tiny/issues>.

SEE ALSO
The Type::Tiny homepage <https://typetiny.toby.ink/>.

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

AUTHOR
Toby Inkster <tobyink@cpan.org>.

COPYRIGHT AND LICENCE
This software is copyright (c) 2013-2014, 2017-2025 by Toby Inkster.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

DISCLAIMER OF WARRANTIES
THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.