class Signature { }

A signature is a static description of the parameter list of a code object. That is, it describes what and how many arguments you need to pass to the code or function in order to call it.

Passing arguments to a signature binds the arguments, contained in a Capture, to the signature.

Signature literals §

Signatures appear inside parentheses after subroutine and method names, on blocks after a -> or <-> arrow, as the input to variable declarators like my, or as a separate term starting with a colon.

sub f($x) { }
#    ^^^^ Signature of sub f 
my method x() { }
#          ^^ Signature of a method 
my $s = sub (*@a) { }
#           ^^^^^ Signature of an anonymous function 
 
for <a b c> -> $x { }
#              ^^   Signature of a Block 
 
my ($a, @b) = 5, (6, 7, 8);
#  ^^^^^^^^ Signature of a variable declarator 
 
my $sig = :($a, $b);
#          ^^^^^^^^ Standalone Signature object

Signature literals can be used to define the signature of a callback or a closure.

sub f(&c:(Int)) { }
sub will-work(Int) { }
sub won't-work(Str) { }
f(&will-work);
 
f(&won't-work);
CATCH { default { put .^name, ': ', .Str } };
# OUTPUT: «X::TypeCheck::Binding::Parameter: Constraint type check failed in binding to parameter '&c'␤» 
 
f(-> Int { 'this works too' } );

You can use any kind of literal, including numeric ones, as part of a signature; this is generally used in conjunction with multis

proto sub stuff(|) {*}
multi sub stuff(33) { 58 }
multi sub stuff(⅓) { 43 }
multi sub stuff(Int)  { 3 }
multi sub stuff(Complex)  { 66 }
say stuff($_) for (33, ⅓, i, 48); # OUTPUT: «58␤43␤66␤3␤» 

However, you can't use True or False as literals in signatures since they will always succeed (or fail). A warning will be issued if you do so:

sub foo(True) {};
my $sig =  :( True );

They will both warn "Literal values in signatures are smartmatched against and smartmatch with True will always succeed. Use the where clause instead.". Use of False will produce a similar warning.

Smartmatching signatures against a List is supported.

my $sig = :(Int $i, Str $s);
say (10, 'answer') ~~ $sig;
# OUTPUT: «True␤» 
my $sub = sub ( Str $s, Int $i ) { return $s xx $i };
say $sub.signature ~~ :( Str, Int );
# OUTPUT: «True␤» 
given $sig {
    when :(Str, Int) { say 'mismatch' }
    when :($, $)     { say 'match' }
    default          { say 'no match' }
}
# OUTPUT: «match␤»

It matches the second when clause since :($, $) represents a Signature with two scalar, anonymous, arguments, which is a more general version of $sig.

When smartmatching against a Hash, the signature is assumed to consist of the keys of the Hash.

my %h = left => 1, right => 2;
say %h ~~ :(:$left, :$right);
# OUTPUT: «True␤»

Signature literals can contain string/numeric literals

my $sig = :('Þor', Str, Int);
say <Þor Hammer 1> ~~ $sig; # OUTPUT: «True␤»

And they can also contain the invocant marker

class Foo {
    method bar( $self: ){ "baz" }
};
say Foo.^methods.first(*.name eq 'bar').signature ~~ :($: *%) ;
# OUTPUT: «True␤»

Parameter separators §

A signature consists of zero or more parameters, separated by commas.

my $sig = :($a, @b, %c);
sub add($a, $b) { $a + $b };

As an exception the first parameter may be followed by a colon instead of a comma to mark the invocant of a method. This is done in order to distinguish it from what would then be a regular positional parameter. The invocant is the object that was used to call the method, which is usually bound to self. By specifying it in the signature, you can change the variable name it is bound to.

method ($a: @b, %c) {};       # first argument is the invocant 
 
class Foo {
    method whoami($me:) {
        "Well I'm class $me.^name(), of course!"
    }
}
say Foo.whoami; # OUTPUT: «Well I'm class Foo, of course!␤»

Type constraints §

Parameters can optionally have a type constraint (the default is Any). These can be used to restrict the allowed input to a function.

my $sig = :(Int $a, Str $b);

Type constraints can have any compile-time defined value

subset Positive-integer of Int where * > 0;
sub divisors(Positive-integer $n) { $_ if $n %% $_ for 1..$n };
CATCH { default { put .^name, ': ', .Str; .resume } };
 
divisors 2.5;
# OUTPUT: «X::TypeCheck::Binding::Parameter: Type check failed in binding to parameter '$n'; expected Positive-integer but got Rat (2.5)␤» 
 
divisors -3;
# OUTPUT: «X::TypeCheck::Binding::Parameter: Constraint type check failed in binding to parameter '$n'; expected Positive-integer but got Int (-3)␤» 

Please note that in the code above type constraints are enforced at two different levels: the first level checks if it belongs to the type in which the subset is based, in this case Int. If it fails, a Type check error is produced. Once that filter is cleared, the constraint that defined the subset is checked, producing a Constraint type check error if it fails.

Type constraints can define multiple allowable types

sub abbrev($arg where Str|List|Hash) {...} # throws if $arg is not one of those types 

Anonymous arguments are fine too, if you don't actually need to refer to a parameter by name, for instance to distinguish between different signatures in a multi or to check the signature of a Callable.

my $sig = :($, @, %a);          # two anonymous and a "normal" parameter 
$sig = :(Int, Positional);      # just a type is also fine (two parameters) 
sub baz(Str) { "Got passed a Str" }

Type constraints may also be type captures.

In addition to those nominal types, additional constraints can be placed on parameters in the form of code blocks which must return a true value to pass the type check

sub f(Real $x where { $x > 0 }, Real $y where { $y >= $x }) { }

The code in where clauses has some limitations: anything that produces side-effects (e.g., printing output, pulling from an iterator, or increasing a state variable) is not supported and may produce surprising results if used. Also, the code of the where clause may run more than once for a single typecheck in some implementations.

The where clause doesn't need to be a code block, anything on the right of the where-clause will be used to smartmatch the argument against it. So you can also write:

multi factorial(Int $ where 0) { 1 }
multi factorial(Int $x)        { $x * factorial($x - 1) }

The first of those can be shortened to

multi factorial(0) { 1 }

i.e., you can use a literal directly as a type and value constraint on an anonymous parameter.

Tip: pay attention to not accidentally leave off a block when you, say, have several conditions:

-> $y where   .so && .name    {}( sub one   {} ); # WRONG!! 
-> $y where { .so && .name }  {}( sub two   {} ); # OK! 
-> $y where   .so &  .name.so {}( sub three {} ); # Also good

The first version is wrong and will issue a warning about a sub object coerced to string. The reason is the expression is equivalent to ($y ~~ ($y.so && $y.name)); that is "call .so, and if that is True, call .name; if that is also True use its value for smartmatching…". It's the result of (.so && .name) it will be smartmatched against, but we want to check that both .so and .name are truthy values. That is why an explicit Block or a Junction is the right version.

All previous arguments that are not part of a sub-signature in a Signature are accessible in a where-clause that follows an argument. Therefore, the where-clause of the last argument has access to all arguments of a signature that are not part of a sub-signature. For a sub-signature place the where-clause inside the sub-signature.

sub foo($a, $b where * == $a ** 2) { say "$b is a square of $a" }
foo 2, 4; # OUTPUT: «4 is a square of 2␤»» 
# foo 2, 3; 
# OUTPUT: «Constraint type check failed in binding to parameter '$b'…»

Constraining optional arguments §

Optional arguments can have constraints, too. Any where clause on any parameter will be executed, even if it's optional and not provided by the caller. In that case you may have to guard against undefined values within the where clause.

sub f(Int $a, UInt $i? where { !$i.defined or $i > 5 }) { ... }

Constraining slurpy arguments §

Slurpy arguments can not have type constraints. A where-clause in conjunction with a Junction can be used to that effect.

sub f(*@a where {$_.all ~~ Int}) { say @a };
f(42);
f(<a>);
CATCH { default { say .^name, ' ==> ', .Str }  }
# OUTPUT: «[42]␤Constraint type check failed in binding to parameter '@a' ...» 

Constraining named arguments §

Constraints against named arguments apply to the value part of the colon-pair.

sub f(Int :$i){};
f :i<forty-two>;
CATCH { default { say .^name, ' ==> ', .Str }  }
# OUTPUT: «X::TypeCheck::Binding::Parameter ==> Type check failed in 
# binding to parameter '$i'; expected Int but got Str ("forty-two")␤»

Constraining argument definiteness §

Normally, a type constraint only checks whether the value of the parameter is of the correct type. Crucially, both object instances and type objects will satisfy such a constraint as illustrated below:

say  42.^name;    # OUTPUT: «Int␤» 
say  42 ~~ Int;   # OUTPUT: «True␤» 
say Int ~~ Int;   # OUTPUT: «True␤»

Note how both 42 and Int satisfy the match.

Sometimes we need to distinguish between these object instances (42) and type objects (Int). Consider the following code:

sub limit-lines(Str $s, Int $limit) {
    my @lines = $s.lines;
    @lines[0 .. min @lines.elems, $limit].join("\n")
}
say (limit-lines "a \n b \n c \n d \n", 3).raku; # "a \n b \n c \n d " 
say limit-lines Str, 3;
CATCH { default { put .^name, ': ', .Str } };
# OUTPUT: «X::Multi::NoMatch: Cannot resolve caller lines(Str: ); 
# none of these signatures match: 
#     (Str:D $: :$count!, *%_) 
#     (Str:D $: $limit, *%_) 
#     (Str:D $: *%_)» 
say limit-lines "a \n b", Int; # Always returns the max number of lines

Here we really only want to deal with string instances, not type objects. To do this, we can use the :D type constraint. This constraint checks that the value passed is an object instance, in a similar fashion to calling its DEFINITE (meta)method.

To warm up, let's apply :D to the right-hand side of our humble Int example:

say  42 ~~ Int:D;  # OUTPUT: «True␤» 
say Int ~~ Int:D;  # OUTPUT: «False␤»

Note how only 42 matches Int:D in the above.

Returning to limit-lines, we can now amend its signature to catch the error early:

sub limit-lines(Str:D $s, Int $limit) { };
say limit-lines Str, 3;
CATCH { default { put .^name ~ '--' ~ .Str } };
# OUTPUT: «Parameter '$s' of routine 'limit-lines' must be an object instance of type 'Str', 
#          not a type object of type 'Str'.  Did you forget a '.new'?»

This is much better than the way the program failed before, since here the reason for failure is clearer.

It's also possible that type objects are the only ones that make sense for a routine to accept. This can be done with the :U type constraint, which checks whether the value passed is a type object rather than an object instance. Here's our Int example again, this time with :U applied:

say  42 ~~ Int:U;  # OUTPUT: «False␤» 
say Int ~~ Int:U;  # OUTPUT: «True␤»

Now 42 fails to match Int:U while Int succeeds.

Here's a more practical example:

sub can-turn-into(Str $string, Any:U $type) {
   return so $string.$type;
}
say can-turn-into("3", Int);        # OUTPUT: «True␤» 
say can-turn-into("6.5", Int);      # OUTPUT: «True␤» 
say can-turn-into("6.5", Num);      # OUTPUT: «True␤» 
say can-turn-into("a string", Num); # OUTPUT: «False␤»

Calling can-turn-into with an object instance as its second parameter will yield a constraint violation as intended:

say can-turn-into("a string", 123);
# OUTPUT: «Parameter '$type' of routine 'can-turn-into' must be a type object 
# of type 'Any', not an object instance of type 'Int'...» 

For explicitly indicating the normal behavior, that is, not constraining whether the argument will be an instance or a type object, :_ can be used but this is unnecessary since this is the default constraint (of this kind) on arguments. Thus, :(Num:_ $) is the same as :(Num $).

To recap, here is a quick illustration of these type constraints, also known collectively as type smileys:

# Checking a type object 
say Int ~~ Any:D;    # OUTPUT: «False␤» 
say Int ~~ Any:U;    # OUTPUT: «True␤» 
say Int ~~ Any:_;    # OUTPUT: «True␤» 
 
# Checking a subset 
subset Even of Int where * // 2;
say 3 ~~ Even:D;     # OUTPUT: «True␤» 
say 3 ~~ Even:U;     # OUTPUT: «False␤» 
say Int ~~ Even:U;   # OUTPUT: «True␤» 
 
# Checking an object instance 
say 42 ~~ Any:D;     # OUTPUT: «True␤» 
say 42 ~~ Any:U;     # OUTPUT: «False␤» 
say 42 ~~ Any:_;     # OUTPUT: «True␤» 
 
# Checking a user-supplied class 
class Foo {};
say Foo ~~ Any:D;    # OUTPUT: «False␤» 
say Foo ~~ Any:U;    # OUTPUT: «True␤» 
say Foo ~~ Any:_;    # OUTPUT: «True␤» 
 
# Checking an instance of a class 
my $f = Foo.new;
say $f  ~~ Any:D;    # OUTPUT: «True␤» 
say $f  ~~ Any:U;    # OUTPUT: «False␤» 
say $f  ~~ Any:_;    # OUTPUT: «True␤»

The Classes and Objects document further elaborates on the concepts of instances and type objects and discovering them with the .DEFINITE method.

Keep in mind all parameters have values; even optional ones have default values that are the type object of the constrained type for explicit type constraints. If no explicit type constraint exists, the default value is an Any type object for methods, submethods, and subroutines, and a Mu type object for blocks. This means that if you use the :D type smiley, you'd need to provide a default value or make the parameter required. Otherwise, the default value would be a type object, which would fail the definiteness constraint.

sub divide (Int:D :$a = 2, Int:D :$b!) { say $a/$b }
divide :1a, :2b; # OUTPUT: «0.5␤»

The default value will kick in when that particular parameter, either positional or named, gets no value at all.

sub f($a = 42){
  my $b is default('answer');
  say $a;
  $b = $a;
  say $b
};
f;     # OUTPUT: «42␤42␤» 
f Nil; # OUTPUT: «Nil␤answer␤»

$a has 42 as its default value. With no value, $a will be assigned the default value declared in the Signature. However, in the second case, it does receive a value, which happens to be Nil. Assigning Nil to any variable resets it to its default value, which has been declared as 'answer' by use of the default trait. That explains what happens the second time we call f. Routine parameters and variables deal differently with default value, which is in part clarified by the different way default values are declared in each case (using = for parameters, using the default trait for variables).

Note: in 6.c language, the default value of :U/:D constrained variables was a type object with such a constraint, which is not initializable, thus you cannot use the .= operator, for example.

use v6.c;
my Int:D $x .= new: 42;
# OUTPUT: You cannot create an instance of this type (Int:D) 
# in block <unit> at -e line 1 

In the 6.d language, the default default is the type object without the smiley constraint:

use v6.d;
my Int:D $x .= new: 42; # OUTPUT: «42␤» 

A closing remark on terminology: this section is about the use of the type smileys :D and :U to constrain the definiteness of arguments. Occasionally definedness is used as a synonym for definiteness; this may be confusing, since the terms have subtly different meanings.

As explained above, definiteness is concerned with the distinction between type objects and object instances. A type object is always indefinite, while an object instance is always definite. Whether an object is a type object/indefinite or an object instance/definite can be verified using the DEFINITE (meta)method.

Definiteness should be distinguished from definedness, which is concerned with the difference between defined and undefined objects. Whether an object is defined or undefined can be verified using the defined-method, which is implemented in class Mu. By default a type object is considered undefined, while an object instance is considered defined; that is: .defined returns False on a type object, and True otherwise. But this default behavior may be overridden by subclasses. An example of a subclass that overrides the default .defined behavior is Failure, so that even an instantiated Failure acts as an undefined value:

my $a = Failure;                # Initialize with type object 
my $b = Failure.new("foo");     # Initialize with object instance 
say $a.DEFINITE;                # Output: «False␤» : indefinite type object 
say $b.DEFINITE;                # Output: «True␤»  : definite object instance 
say $a.defined;                 # Output: «False␤» : default response 
say $b.defined;                 # Output: «False␤» : .defined override

Constraining signatures of Callables §

The signature of a Callable parameter can be constrained by specifying a Signature literal right after the parameter (no whitespace allowed):

sub apply(&l:(Int:D --> Int:D), Int:D \n) {
    l(n)
}
 
sub identity(Int:D \i --> Int:D) { i }
sub double(Int:D \x --> Int:D) { 2 * x }
 
say apply &identity, 10; # OUTPUT: «10␤» 
say apply &double, 10;   # OUTPUT: «20␤»

Typed lambdas also work with constrained callable parameters.

say apply -> Int:D \x --> Int:D { 2 * x }, 3;  # OUTPUT: «6␤» 
say apply -> Int:D \x --> Int:D { x ** 3 }, 3; # OUTPUT: «27␤» 

Note that this shorthand syntax is available only for parameters with the & sigil. For others, you need to use the long version:

sub play-with-tens($c where .signature ~~ :(Int, Str)) { say $c(10, 'ten') }
sub by-joining-them(Int $i, Str $s) { $s ~ $i }
play-with-tens &by-joining-them;                              # OUTPUT: «ten10␤» 
play-with-tens -> Int \i, Str \s { s x (1..10).roll mod i };  # OUTPUT: «tenten␤» 
 
sub g(Num $i, Str $s) { $s ~ $i }
# play-with-tens(&g); # Constraint type check failed

Constraining return types §

There are multiple ways to constrain return types on a Routine. All versions below are currently valid and will force a type check on successful execution of a routine.

Nil and Failure are always allowed as return types, regardless of any type constraint. This allows Failure to be returned and passed on down the call chain.

sub foo(--> Int) { Nil };
say foo.raku; # OUTPUT: «Nil␤»

Type captures are not supported.

Return type arrow: --> §

This form of indicating return types (or constants) in the signature is preferred, since it can handle constant values while the others can't. For consistency, it is the only form accepted on this site.

The return type arrow has to be placed at the end of the parameter list, with or without a , before it.

sub greeting1(Str $name  --> Str) { say "Hello, $name" } # Valid 
sub greeting2(Str $name, --> Str) { say "Hello, $name" } # Valid 
 
sub favorite-number1(--> 42) {        } # OUTPUT: 42 
sub favorite-number2(--> 42) { return } # OUTPUT: 42 

If the type constraint is a constant expression, it is used as the return value of the routine. Any return statement in that routine has to be argumentless.

sub foo(Str $word --> 123) { say $word; return; }
my $value = foo("hello"); # OUTPUT: hello 
say $value;               # OUTPUT: 123 

# The code below will not compile
sub foo(Str $word --> 123) { say $word; return $word; }
my $value = foo("hello");
say $value;

returns §

The keyword returns following a signature declaration has the same function as --> with the caveat that this form does not work with constant values. You cannot use it in a block either. That is why the pointy arrow form is always preferred.

sub greeting(Str $name) returns Str { say "Hello, $name" } # Valid 

sub favorite-number returns 42 {        } # This will fail.

of §

of is just the real name of the returns keyword.

sub foo() of Int { 42 }; # Valid 

sub foo() of 42 {  };    # This will fail.

prefix(C-like) form §

This is similar to placing type constraints on variables like my Type $var = 20;, except the $var is a definition for a routine.

my Int sub bar { 1 };     # Valid 

my 42 sub bad-answer {};  # This will fail.

Coercion type §

To accept one type but coerce it automatically to another, use the accepted type as an argument to the target type. If the accepted type is Any it can be omitted.

sub f(Int(Str) $want-int, Str() $want-str) {
    say $want-int.^name ~ ' ' ~ $want-str.^name
}
f '10', 10;
# OUTPUT: «Int Str␤» 
 
sub foo(Date(Str) $d) { say $d.^name; say $d };
foo "2016-12-01";
# OUTPUT: «Date␤2016-12-01␤»

The coercion is performed by calling the method with the name of the type to coerce to, if it exists. In this example, we're calling the builtin method Date on the Str class. The method is assumed to return the correct type—no additional checks on the result are currently performed.

Coercion can also be performed on return types:

sub square-str (Int $x --> Str(Int)) {
    $x²
}
 
for 2,4, *²  … 256 -> $a {
    say $a, "² is ", square-str( $a ).chars, " figures long";
}
 
# OUTPUT: «2² is 1 figures long␤ 
#          4² is 2 figures long␤ 
#          16² is 3 figures long␤ 
#          256² is 5 figures long␤» 

In this example, coercing the return type to Str allows us to directly apply string methods, such as the number of characters.

Slurpy parameters §

A function is variadic if it can take a varying number of arguments; that is, its arity is not fixed. Therefore, optional, named, and slurpy parameters make routines that use them variadic, and by extension are called variadic arguments. Here we will focus on slurpy parameters, or simply slurpies.

An array or hash parameter can be marked as slurpy by leading single (*) or double asterisk (**) or a leading plus (+). A slurpy parameter can bind to an arbitrary number of arguments (zero or more), and it will result in a type that is compatible with the sigil.

These are called "slurpy" because they slurp up any remaining arguments to a function, like someone slurping up noodles.

my $sig1 = :($a, @b);  # exactly two arguments, second must be Positional 
my $sig2 = :($a, *@b); # at least one argument, @b slurps up any beyond that 
my $sig3 = :(*%h);     # no positional arguments, but any number 
                       # of named arguments 
 
sub one-arg (@)  { }
sub slurpy  (*@) { }
one-arg (5, 6, 7); # ok, same as one-arg((5, 6, 7)) 
slurpy  (5, 6, 7); # ok 
slurpy   5, 6, 7 ; # ok 
# one-arg(5, 6, 7) ; # X::TypeCheck::Argument 
# one-arg  5, 6, 7 ; # X::TypeCheck::Argument 
 
sub named-names (*%named-args) { %named-args.keys };
say named-names :foo(42) :bar<baz>; # OUTPUT: «foo bar␤» 

Positional and named slurpies can be combined; named arguments (i.e., Pairs) are collected in the specified hash, positional arguments in the array:

sub combined-slurpy (*@a, *%h) { { array => @a, hash => %h } }
# or: sub combined-slurpy (*%h, *@a) { ... } 
 
say combined-slurpy(one => 1, two => 2);
# OUTPUT: «{array => [], hash => {one => 1, two => 2}}␤» 
say combined-slurpy(one => 1, two => 2, 3, 4);
# OUTPUT: «{array => [3 4], hash => {one => 1, two => 2}}␤» 
say combined-slurpy(one => 1, two => 2, 3, 4, five => 5);
# OUTPUT: «{array => [3 4], hash => {five => 5, one => 1, two => 2}}␤» 
say combined-slurpy(one => 1, two => 2, 3, 4, five => 5, 6);
# OUTPUT: «{array => [3 4 6], hash => {five => 5, one => 1, two => 2}}␤» 

Note that positional parameters aren't allowed after slurpy (or, in fact, after any type of variadic) parameters:

:(*@args, $last);
# ===SORRY!=== Error while compiling:
# Cannot put required parameter $last after variadic parameters

Normally a slurpy parameter will create an Array (or compatible type), create a new Scalar container for each argument, and assign the value from each argument to those Scalars. If the original argument also had an intermediary Scalar it is bypassed during this process, and is not available inside the called function.

Sigiled parameters will always impose a context on the collected arguments. Sigilless parameters can also be used slurpily, preceded by a + sign, to work with whatever initial type they started with:

sub zipi( +zape ) {
    zape.^name => zape
};
say zipi( "Hey "); # OUTPUT: «List => (Hey )␤» 
say zipi( 1...* ); # OUTPUT: «Seq => (...)␤» 

Slurpy parameters have special behaviors when combined with some traits and modifiers, as described in the section on slurpy array parameters.

Methods automatically get a *%_ slurpy named parameter added if they don't have another slurpy named parameter declared.

Types of slurpy array parameters §

There are three variations to slurpy array parameters.

Each will be described in detail in the next few sections. As the difference between each is a bit nuanced, examples are provided for each to demonstrate how each slurpy convention varies from the others.

Flattened slurpy §

Slurpy parameters declared with one asterisk will flatten arguments by dissolving one or more layers of bare Iterables.

my @array = <a b c>;
my $list := <d e f>;
sub a(*@a)  { @a.raku.say };
a(@array);                 # OUTPUT: «["a", "b", "c"]» 
a(1, $list, [2, 3]);       # OUTPUT: «[1, "d", "e", "f", 2, 3]» 
a([1, 2]);                 # OUTPUT: «[1, 2]» 
a(1, [1, 2], ([3, 4], 5)); # OUTPUT: «[1, 1, 2, 3, 4, 5]» 
a(($_ for 1, 2, 3));       # OUTPUT: «[1, 2, 3]» 

A single asterisk slurpy flattens all given iterables, effectively hoisting any object created with commas up to the top level.

Unflattened slurpy §

Slurpy parameters declared with two stars do not flatten any Iterable arguments within the list, but keep the arguments more or less as-is:

my @array = <a b c>;
my $list := <d e f>;
sub b(**@b) { @b.raku.say };
b(@array);                 # OUTPUT: «[["a", "b", "c"],]␤» 
b(1, $list, [2, 3]);       # OUTPUT: «[1, ("d", "e", "f"), [2, 3]]␤» 
b([1, 2]);                 # OUTPUT: «[[1, 2],]␤» 
b(1, [1, 2], ([3, 4], 5)); # OUTPUT: «[1, [1, 2], ([3, 4], 5)]␤» 
b(($_ for 1, 2, 3));       # OUTPUT: «[(1, 2, 3),]␤» 

The double asterisk slurpy hides the nested comma objects and leaves them as-is in the slurpy array.

Single argument rule slurpy §

A slurpy parameter created using a plus engages the "single argument rule", which decides how to handle the slurpy argument based upon context. Basically, if only a single argument is passed and that argument is Iterable, that argument is used to fill the slurpy parameter array. In any other case, +@ works like **@.

my @array = <a b c>;
my $list := <d e f>;
sub c(+@b) { @b.raku.say };
c(@array);                 # OUTPUT: «["a", "b", "c"]␤» 
c(1, $list, [2, 3]);       # OUTPUT: «[1, ("d", "e", "f"), [2, 3]]␤» 
c([1, 2]);                 # OUTPUT: «[1, 2]␤» 
c(1, [1, 2], ([3, 4], 5)); # OUTPUT: «[1, [1, 2], ([3, 4], 5)]␤» 
c(($_ for 1, 2, 3));       # OUTPUT: «[1, 2, 3]␤» 

For additional discussion and examples, see Slurpy Conventions for Functions.

Type captures §

Type captures allow deferring the specification of a type constraint to the time the function is called. They allow referring to a type both in the signature and the function body.

sub f(::T $p1, T $p2, ::C){
    # $p1 and $p2 are of the same type T, that we don't know yet 
    # C will hold a type we derive from a type object or value 
    my C $division = $p1 / $p2;
    return sub (T $p1) {
        $division * $p1;
    }
}
 
# The first parameter is Int and so must be the 2nd. 
# We derive the 3rd type from calling the operator that is used in &f. 
my &s = f(10, 2, Int.new / Int.new);
say s(2); # 10 / 2 * 2 == 10

Positional vs. named arguments §

An argument can be positional or named. By default, arguments are positional, except slurpy hash and arguments marked with a leading colon :. The latter is called a colon-pair. Check the following signatures and what they denote:

$sig1 = :($a);               # a positional argument 
$sig2 = :(:$a);              # a named argument of name 'a' 
$sig3 = :(*@a);              # a slurpy positional argument 
$sig4 = :(*%h);              # a slurpy named argument 

On the caller side, positional arguments are passed in the same order as the arguments are declared.

sub pos($x, $y) { "x=$x y=$y" }
pos(4, 5);                          # OUTPUT: «x=4 y=5»

In the case of named arguments and parameters, only the name is used for mapping arguments to parameters. If a fat arrow is used to construct a Pair only those with valid identifiers as keys are recognized as named arguments.

sub named(:$x, :$y) { "x=$x y=$y" }
named( y => 5, x => 4);             # OUTPUT: «x=4 y=5» 

You can invoke the routine using a variable with the same name as the named argument; in that case : will be used for the invocation so that the name of the variable is understood as the key of the argument.

sub named-shortcut( :$shortcut ) {
    say "Looks like $shortcut"
}
named-shortcut( shortcut => "to here"); # OUTPUT: «Looks like to here␤» 
my $shortcut = "Þor is mighty";
named-shortcut( :$shortcut );           # OUTPUT: «Looks like Þor is mighty␤»

It is possible to have a different name for a named argument than the variable name:

sub named(:official($private)) { "Official business!" if $private }
named :official;

Argument aliases §

The colon-pair syntax can be used to provide aliases for arguments:

sub alias-named(:color(:$colour), :type(:class($kind))) {
    say $colour ~ " " ~ $kind
}
alias-named(color => "red", type => "A");    # both names can be used 
alias-named(colour => "green", type => "B"); # more than two names are ok 
alias-named(color => "white", class => "C"); # every alias is independent

The presence of the colon : will decide whether we are creating a new named argument or not. :$colour will not only be the name of the aliased variable, but also a new named argument (used in the second invocation). However, $kind will just be the name of the aliased variable, that does not create a new named argument. More uses of aliases can be found in sub MAIN.

A function with named arguments can be called dynamically, dereferencing a Pair with | to turn it into a named argument.

multi f(:$named) { note &?ROUTINE.signature };
multi f(:$also-named) { note &?ROUTINE.signature };
for 'named', 'also-named' -> $n {
    f(|($n => rand))                # OUTPUT: «(:$named)␤(:$also-named)␤» 
}
 
my $pair = :named(1);
f |$pair;                           # OUTPUT: «(:$named)␤»