Signature.pod6

=begin pod

=TITLE class Signature

=SUBTITLE Parameter list pattern

    class Signature { }

A signature is a static description of the L<parameter|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 I<binds> the arguments, contained in
a L<Capture>, to the signature.

X<|signature literal (Signature)>
X<|:() (Signature)>
=head1 Signature Literals

Signatures appear inside parentheses after L<subroutine|/type/Sub> and
L<method|/type/Method> names, on blocks after a C<< -> >> or C<< <-> >> arrow,
as the input to L<variable
declarators|/language/variables#Variable_Declarators_and_Scope> like
L<C<my>|/syntax/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' } );

Smart matching signatures against a List is supported.

    my $sig = :(Int $i, Str $s);
    say (10, 'answer') ~~ $sig;
    # OUTPUT: «True␤»
    given ('answer', 10) {
        when :(Str, Int) { say 'match' }
        when $sig { say 'mismatch' }
    }
    # OUTPUT: «match␤»

When smart matching 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␤»

=head2 Parameter Separators

A signature consists of zero or more I<L<parameters|Parameter>>, 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.  The invocant is the
object that was used to call the method, which is usually bound to L<C<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!␤»

X<|type constraint (Signature)>
=head2 Type Constraints
X<|Constraint>

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

=for code
my $sig = :(Int $a, Str $b);

=begin code :skip-test
sub divisors(Int $n) { $_ if $n %% $_ for 1..$n };
divisors 2.5;
# ===SORRY!=== Error while compiling:
# Calling divisors(Rat) will never work with declared signature (Int $n)
=end code

X<|anonymous arguments (Signature)>
Anonymous arguments are fine too, if a parameter is only needed for
its type constraint.

    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 L<type captures|/type/Signature#Type_Captures>.

X<|where clause (Signature)>
In addition to those I<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 }) { }

In fact it doesn't need to be a code block, anything on the right of the
C<where>-clause will be used to L<smart-match|/language/operators#infix_~~> 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.

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

All previous arguments that are not part of a sub-signature in a C<Signature>
are accessible in a C<where>-clause that follows an argument. Therefore,
the C<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 C<where>-clause inside the sub-signature.

    sub one-of-them(:$a, :$b, :$c where { $a.defined ^^ $b.defined ^^ $c.defined }) {
        $a // $b // $c
    };
    say one-of-them(c=>42); # OUTPUT: «42␤»

Note: where-clauses that produce side-effects (e.g. printing output,
pulling from an iterator, or increasing a state variable) are not supported
and may produce surprising results if used.

=head3 Constraining Optional Arguments

L<Optional arguments|#Optional_and_Mandatory_Parameters> can have constraints,
too. Any C<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 C<where> clause.

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

=head3 Constraining Slurpy Arguments

L<Slurpy arguments|#Slurpy_(A.K.A._Variadic)_Parameters> can not have type
constraints. A C<where>-clause in conjunction with a L<Junction|/type/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' ...»

=head3 Constraining named Arguments

Constraints against L<Named arguments|#Positional_vs._Named> apply to the value
part of the L<colon-pair|/type/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")␤»

=head3 X<Constraining Defined and Undefined Values|
         type constraint,:D;type constraint,:U;type constraint,:_>

Normally, a type constraint only checks whether the value passed is of the
correct type.

    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).perl; # "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

In the code above, we really only want to deal with defined strings. To
do this, we use the C<:D> type constraint.  This constraint checks the
value passed is an I<object instance>, in a similar fashion to calling
the C<.DEFINITE> method on the value:

    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 undefined types are the only ones that make
sense for a routine to accept. This can be done with the C<:U> type
constraint, which checks the value passed if it is a I<type object>,
rather than an object instance. For example, we can turn the
C<&limit-lines> into a multi function to make use of the C<:U>
constraint:

=for code :allow<B L>
multi limit-lines(Str $s, Int:D $limit) { }
multi limit-lines(Str $s, Int:U $) { $s }
say limit-lines "a \n b \n c", Int; # OUTPUT: «"a \n b \n c"␤»

For explicitly indicating the normal behaviour, C<:_> can be used, but this is
unnecessary. C<:(Num:_ $)> is the same as C<:(Num $)>.

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

    # Checking a type object
    say Int ~~ Any:D;    # OUTPUT: «False␤»
    say Int ~~ Any:U;    # OUTPUT: «True␤»
    say Int ~~ Any:_;    # 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␤»
    my $f = Foo.new;
    say $f  ~~ Any:D;    # OUTPUT: «True␤»
    say $f  ~~ Any:U;    # OUTPUT: «False␤»
    say $f  ~~ Any:_;    # OUTPUT: «True␤»

The L<Classes and Objects|/language/classtut#Starting_with_class>
document further elaborates on the concepts of instances and type
objects and discovering them with the C<.DEFINITE> method.

=head3 X«Constraining signatures of Callables|Callable (constrain)»

A L<Callable> parameter can be constrained by its signature, by specifying
a L<Signature> literal right after the parameter (no whitespace allowed):

    sub f(&c:(Int, Str))  { say c(10, 'ten') };
    sub g(Int $i, Str $s) { $s ~ $i };
    f(&g);
    # OUTPUT: «ten10␤»

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

    sub f($c: where .signature ~~ :(Int, Str))  { say $c(10, 'ten') };
    sub g(Num $i, Str $s) { $s ~ $i };
    f(&g);
    # OUTPUT: «ten10␤»

=head3 X«Constraining Return Types|-->;returns»

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

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

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

Type captures are not supported.

=item C<-->>

This form is preferred for several reasons:
(1) it can handle constant values while the others can't;
(2) 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 C<,> before it.

    =begin code
    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
    =end code

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.

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

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

=item C<returns>

The keyword C<returns> following a signature declaration
has the same function as C<-->> with two caveats.

(1) This form is planned for future removal.
(2) This form does not work with constant values

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

    =for code :skip-test
    sub favorite-number returns 42 {        } # This will fail.

=item C<of>

C<of> is just the real name of the C<returns> keyword.

    =for code
    sub foo() of Int { 42 }; # Valid

    =for code :skip-test
    sub foo() of 42 {  };    # This will fail.

=item prefix(C-like) form

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

    =for code
    my Int sub bar { 1 };     # Valid
    =for code :skip-test
    my 42 sub bad-answer {};  # This will fail.

=head3 X<Coercion Type|coercion type (signature)>

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 C<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␤»

    use MONKEY;
    augment class Str { method Date() { Date.new(self) } };
    sub foo(Date(Str) $d) { say $d.^name; say $d };
    foo "2016-12-01";
    # OUTPUT: «Date␤2016-12-01␤»

=head2 X<Slurpy (A.K.A. Variadic) Parameters|parameter,*@;parameter,*%,slurpy argument (Signature)>

A function is X<variadic> if it can take a varying number of arguments; that is,
its arity is not fixed. Therefore, optional, named, and slurpy parameters are
variadic. An array or hash parameter can be marked as I<slurpy> by leading
asterisk (*) or two leading asterisks (**) or a leading plus (+). A slurpy
parameter can bind to an arbitrary number of arguments (zero or more).

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

    $ = :($a, @b);     # exactly two arguments, where the second one must be Positional
    $ = :($a, *@b);    # at least one argument, @b slurps up any beyond that
    $ = :(*%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␤»

Note that positional parameters aren't allowed after slurpy parameters.

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

Normally a slurpy parameter will create an L<Array|/type/Array>, create a new
L<Scalar|/type/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.

Slurpy parameters have special behaviors when combined with some
L<traits and modifiers|#Parameter_Traits_and_Modifiers>,
as described below.

=head2 Types of Slurpy Array Parameters

There are three variations to slurpy array parameters.

=item The single asterisk form flattens passed arguments.

=item The double asterisk form does not flatten arguments.

=item The plus form flattens according to the single argument rule.

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.

=head3 Flattened Slurpy

Slurpy parameters declared with one asterisk will flatten arguments by
dissolving one or more layers of bare L<Iterable|/type/Iterable>s.

=begin code
my @array = <a b c>;
my $list := <d e f>;
sub a(*@a)  { @a.perl.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]»
=end code

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

=head3 Unflattened Slurpy

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

=begin code
my @array = <a b c>;
my $list := <d e f>;
sub b(**@b) { @b.perl.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),]␤»
=end code

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

=head3 Single Argument Rule Slurpy
X<|+ (Single Argument Rule Slurpy)>

A slurpy parameter created using a plus engages the I<"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 L<Iterable|/type/Iterable>, that argument
is used to fill the slurpy parameter array. In any other case, C<+@> works like
C<**@>.

=begin code
my @array = <a b c>;
my $list := <d e f>;
sub c(+@b) { @b.perl.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]␤»
=end code

For additional discussion and examples, see L<Slurpy Conventions for Functions|/language/functions#Slurpy_Conventions>.

=head2 Type Captures
X<|Type Capture (signature)>

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 $closure = $p1 / $p2;
        return sub (T $p1) {
            $closure * $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

X<|positional argument (Signature),named argument (Signature)>
=head2 Positional vs. Named

A parameter can be I<positional> or I<named>. All parameters are positional,
except slurpy hash parameters and parameters marked with a leading colon C<:>.
The latter is called a L<colon-pair|/type/Pair>.

    $ = :($a);               # a positional parameter
    $ = :(:$a);              # a named parameter of name 'a'
    $ = :(*@a);              # a slurpy positional parameter
    $ = :(*%h);              # a slurpy named parameter

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

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

In the case of named arguments and parameters, only the name is used for
mapping arguments to parameters

=for code :allow<L>
sub named(:$x, :$y) { "x=$x y=$y" }
named( y => 5, x => 4);             # RESULT: «x=4 y=5»

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

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

Aliases are also possible that way:

    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

More use of aliases can be found in L<sub MAIN|/language/functions#sub_MAIN>

A function with named arguments can be called dynamically, dereferencing a
L<Pair|/type/Pair> with C<|> 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)␤»

The same can be used to convert a C<Hash> into named arguments.

    sub f(:$also-named) { note &?ROUTINE.signature };
    my %pairs = also-named => 4;
    f |%pairs;                              # OUTPUT: «(:$also-named)␤»

A C<Hash> that contains a list may prove problematic when slipped into named
arguments. To avoid the extra layer of containers coerce to L<Map|/type/Map>
before slipping.

    class C { has $.x; has $.y; has @.z };
    my %h = <x y z> Z=> (5, 20, [1,2]);
    say C.new(|%h.Map);
    # OUTPUT: «C.new(x => 5, y => 20, z => [1, 2])␤»

X<|optional argument (Signature)>
=head2 Optional and Mandatory Parameters

Positional parameters are mandatory by default, and can be made optional
with a default value or a trailing question mark:

    $ = :(Str $id);         # required parameter
    $ = :($base = 10);      # optional parameter, default value 10
    $ = :(Int $x?);         # optional parameter, default is the Int type object

X<|mandatory named argument (Signature)>
Named parameters are optional by default, and can be made mandatory with a
trailing exclamation mark:

    $ = :(:%config);        # optional parameter
    $ = :(:$debug = False); # optional parameter, defaults to False
    $ = :(:$name!);         # mandatory 'name' named parameter

Default values can depend on previous parameters, and are (at least
notionally) computed anew for each call

    $ = :($goal, $accuracy = $goal / 100);
    $ = :(:$excludes = ['.', '..']);        # a new Array for every call

=head2 Dynamic Variables

L<Dynamic variables|/language/variables#The_*_Twigil> are allowed in signatures
although they don't provide special behaviour because argument binding does
connect two scopes anyway.

X<|destructuring arguments (Signature)>
=head2 Destructuring Parameters

Parameters can be followed by a sub-signature in brackets, which will
destructure the argument given. The destructuring of a list is just its
elements:

    sub first(@array ($first, *@rest)) { $first }

or

    sub first([$f, *@]) { $f }

While the destructuring of a hash is its pairs:

    sub all-dimensions(% (:length(:$x), :width(:$y), :depth(:$z))) {
        $x andthen $y andthen $z andthen True
    }

In general, an object is destructured based on its attributes. A common idiom
is to unpack a L<C<Pair>>'s key and value in a for loop:

    for <Peter Paul Merry>.pairs -> (:key($index), :value($guest)) { }

However, this unpacking of objects as their attributes is only the default
behavior. To make an object get destructured differently, change its
L<C<Capture>|/routine/Capture> method.

=head2 Sub-signatures

To match against a compound parameter use a sub-signature following the argument
name in parentheses.

    sub foo(|c(Int, Str)){
       put "called with {c.perl}"
    };
    foo(42, "answer");
    # OUTPUT: «called with \(42, "answer")␤»

X<|Long Names>
=head2 Long Names

To exclude certain parameters from being considered in multiple dispatch,
separate them with a double semi-colon.

    multi sub f(Int $i, Str $s;; :$b) { say "$i, $s, {$b.perl}" };
    f(10, 'answer');
    # OUTPUT: «10, answer, Any␤»

=head2 X<Capture Parameters|parameter,|>

Prefixing a parameter with a vertical bar C<|> makes the parameter a
L<C<Capture>>, using up all the remaining positional and named
arguments.

This is often used in C<proto> definitions (like C<proto foo (|) {*}>) to
indicate that the routine's L<C<multi> definitions|multi> can have any L<type
constraints|#Type_Constraints>. See L<proto|/language/functions#proto> for an
example.

If bound to a variable arguments can be forwarded as a whole using the slip operator C<|>.

    sub a(Int $i, Str $s) { say $i.^name ~ ' ' ~ $s.^name }
    sub b(|c) { say c.^name; a(|c) }
    b(42, "answer");
    # OUTPUT: «Capture␤Int Str␤»

=head2 Parameter Traits and Modifiers

By default, parameters are bound to their argument and marked as
read-only. One can change that with traits on the parameter.

X<|trait,is copy>
The C<is copy> trait causes the argument to be copied, and allows it
to be modified inside the routine

    sub count-up($x is copy) {
        $x = Inf if $x ~~ Whatever;
        .say for 1..$x;
    }

X<|trait,is rw>
The C<is rw> trait, which stands for I<is read-write>,
makes the parameter bind to a variable (or other writable container). Assigning
to the parameter changes the value of the variable at the caller side.

    sub swap($x is rw, $y is rw) {
        ($x, $y) = ($y, $x);
    }

On slurpy parameters, C<is rw> is reserved for future use by language
designers.

X<|trait, is raw>
The L<C<is raw> trait|/type/Parameter#method_raw> is automatically applied to
parameters declared with a L<backslash|/language/variables#Sigilless_variables>
as a "sigil", and may also be used to make normally sigiled parameters behave
like these do. In the special case of slurpies, which normally produce an
C<Array> full of C<Scalar>s as described above, C<is raw> will instead cause
the parameter to produce a C<List>.  Each element of that list will be bound
directly as raw parameter.

X<|trait,is readonly>
To explicitly ask for a read-only parameter use the C<is readonly> trait.
Please note that this applies only to the container. The object inside can very
well have mutator methods and Perl 6 will not enforce immutability on the
attributes of the object.

Traits can be followed by the where clause:

    sub ip-expand-ipv6($ip is copy where m:i/^<[a..f\d\:]>**3..39$/) { }

=head1 Methods

=head2 method params

    method params(Signature:D: --> Positional)

Returns the list of L<C<Parameter>> objects that make up the signature.

=head2 method arity

    method arity(Signature:D: --> Int:D)

Returns the I<minimal> number of positional arguments required to satisfy
the signature.

=head2 method count

    method count(Signature:D: --> Real:D)

Returns the I<maximal> number of positional arguments which can be bound
to the signature. Returns C<Inf> if there is a slurpy positional parameter.

=head2 method returns

Whatever the Signature's return constraint is:

    :($a, $b --> Int).returns # RESULT: «(Int)»

=head2 method ACCEPTS

    multi method ACCEPTS(Signature:D: Signature $topic)
    multi method ACCEPTS(Signature:D: Capture $topic)
    multi method ACCEPTS(Signature:D: Mu \topic)

If C<$topic> is a L<Signature> returns C<True> if anything accepted by C<$topic>
would also be accepted by the invocant, otherwise returns C<False>:

    :($a, $b) ~~ :($foo, $bar, $baz?);   # RESULT: «True»
    :(Int $n) ~~ :(Str);                 # RESULT: «False»

The C<$topic> is a L<Capture>, returns C<True> if it can be bound to the
invocant, i.e., if a function with invocant's C<Signature> would be able to be
called with the C<$topic>:

    \(1, 2, :foo) ~~ :($a, $b, :foo($bar)); # RESULT: «True»
    \(1, :bar)    ~~ :($a);                 # RESULT: «False»

Lastly, the candidate with C<Mu> C<topic> converts C<topic> to L<Capture>
and follows the same semantics as L<Capture> C<$topic>:

    <a b c d>  ~~ :(Int $a);      # RESULT: «False»
    42         ~~ :(Int);         # RESULT: «False» (Int.Capture throws)
    set(<a b>) ~~ :(:$a, :$b);    # RESULT: «True»


=head2 method Capture

Defined as:

    method Capture()

Throws C<X::Cannot::Capture>.

=end pod

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