=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.

=head1 Signature Literals

Signatures appear in parenthesis after subroutine and 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>>, or as a separate term starting with a colon.

=begin code :allow<B L>
sub fB<($x)> { }
#    ^^^^ Signature of sub f
method xB<()> { }
#       ^^ Signature of a method
my $s = sub B<(*@a)> { }
#           ^^^^^ Signature of an anonymous function

for @list -> B<$x> { }
#            ^^   Signature of a L<block|Block>

my B<($a, @b)> = 5, (6,7,8);
#  ^^^^^^^^ Signature of a variable declarator

my $sig = B<:($a, $b)>;
#          ^^^^^^^^ Standalone Signature object
=end code

=head2 Parameter Separators

A signature consists of zero or more I<L<parameters|Parameter>>, separated by comma.

=for code :allow<B>
:($aB<,> @bB<,> %c)
sub add ($aB<,> $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
thing 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.

=begin code :allow<B L>
:(B<$a:> @b, %c)       # first argument is the invocant

L<class> Foo {
    method whoami (B<$me:>) {
        "Well I'm class B<$me>.^L<name>(), of course!"
    }
}
say Foo.whoami; # => Well I'm class Foo, of course!
=end code

=head2 Type Constraints

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.

=begin code :allow<B L>
:(B<L<Int>> $a, B<L<Str>> $b)

sub divisors (B<L<Int>> $n) { $_ if $n L<%%> $_ for 1..$n }
divisors 2.5; # !!! Calling 'divisors' will never work with argument types (L<Rat>)
=end code

Anonymous parameters are fine too, if a parameter is only needed for
its type constraint.

=for code :allow<B L>
:(B<$>, B<@>, %a)               # two anonymous and a "normal" parameter
:(B<L<Int>>, B<L<Positional>>)  # just a type is also fine (two parameters)
sub baz (B<L<Str>>) { "Got passed a Str" }

Type constraints may also be L<type captures|/type/Signature#Type_Captures>.

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>-block will be used to smart-match 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.

=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.

=for code :allow<B L>
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 "
say limit-lines Str,      3;  # Uh-oh. Dies with "Cannot call 'lines';"
say limit-lines "a \n b", Int # Always returns the max number of lines

In this case, we really only want to deal with defined strings. To do this, we
use the C<:D> type constraint.

=for code :allow<B L>
sub limit-lines (StrB<:D> $s, Int $limit) {
    ...
}
# Dies with "Parameter '$s' requires an instance, but a type object was passed
#   in sub limit-lines"
say limit-lines Str, 3;

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

It's also possible undefined types are the only ones that make sense for a
routine to accept. This can be constrained with the C<:U> type constraint. 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, IntB<:U> $) { $s }
say limit-lines "a \n b \n c", Int; # "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 $)>.

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

The token C<-->> followed by a type will force a type check on successful
execution of a routine. The return type arrow has to be placed at the end of
the parameter list. The keyword C<returns> following a signature declaration
has the same function. C<Nil> is considered as being defined for the type check.


	sub foo(--> Int) { 1 };
	sub foo() returns Int { 1 };
	sub does-not-work(--> Int) { "" }; # throws X::TypeCheck::Return

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(--> 123) { return }

C<Nil> and C<Failure> are always allowed as return types, reguardless of any
type contrain.

	sub foo(--> Int) { Nil };
	say foo.perl; # Nil

Type captures and coercion types are not supported.

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

An array or hash parameter can be marked as I<slurpy> by leading asterisk(s),
which means it can bind to an arbitrary amount of arguments (zero or more).

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

=for code :allow<B L>
:($a, @b)              # exactly two arguments, where the second one must be Positional
:($a, B<*@b>)          # at least one argument, @b slurps up any beyond that
:(B<*%h>)              # no positional arguments, but any number of named arguments

=for code :allow<B L>
sub one-arg (@)     { }
sub slurpy  (B<*@>) { }
one-arg(5, 6, 7) ; # !!! too many arguments
one-arg (5, 6, 7); # ok, same as one-arg((5,6,7))
slurpy  (5, 6, 7); # ok
one-arg  5, 6, 7 ; # !!! too many arguments
slurpy   5, 6, 7 ; # ok

=for code :allow<B L>
sub named-names (B<*%named-args>) { %named-args.L<keys> }
say named-names :foo(42) :bar<baz> # => foo bar

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

    :(*@args, $last) # !!! Cannot put required parameter after variadic parameters

Slurpy parameters declared with one asterisk will flatten arguments by
dissolving one or more layers of bare C<Iterables>.  Slurpy parameters declared
with two stars do not do so:

    sub a (*@a) { @a.join("|").say };
    a(1,[1,2],([3,4],5)); # 1|1|2|3|4|5
    sub b (**@b) { @b.join("|").say };
    b(1,[1,2],([3,4],5)); # 1|1 2|3 4 5

Normally a slurpy parameter will create an Array, 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.

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

=head2 Type Captures

Type Captures allow to defer the specification of a type constraint to the time
the function is called. They allow to refer 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

=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<:>.

    :($a)               # a positional parameter
    :(:$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);                          # 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 L«=>» 5, x => 4);             # 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 paint( :color(:colour($c)) ) { }    # 'color' and 'colour' are both OK
    sub paint( :color(:$colour) ) { }      # same API for the caller

=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

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 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:

    =for code :allow<B L>
    sub first (@array B<($first, *@rest)>) { $first }

or

    =for code :allow<B L>
    sub first (B<[$f, *@]>) { $f }

While the destructuring of a hash is its pairs:

    =for code :allow<B L>
    sub all-dimensions (% B<(:length(:$x), :width(:$y), :depth(:$z))>) {
        $x L<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 code :allow<B L>
    for @guest-list.L<pairs> -> B<(: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>> method.

=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.

=head2 X<Parameter Traits and Modifiers|trait,is copy;trait,is rw>

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

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;
    }

The C<is rw> trait makes the parameter only 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.

The L<C<is raw> trait|/type/Parameter#method_raw> is automatically applied to
parameters declared with a backslash 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.

=head1 Methods

=head2 method params

    method params(Signature:D:) returns Positional

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

=head2 method arity

    method arity(Signature:D:) returns Int:D

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

=head2 method count

    method count(Signature:D:) returns 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 # Int

=head2 method ACCEPTS

=for code :allow<B L>
multi method ACCEPTS(Signature:D: L<Capture> $topic)
multi method ACCEPTS(Signature:D: @topic)
multi method ACCEPTS(Signature:D: %topic)
multi method ACCEPTS(Signature:D: Signature $topic)

The first three see if the argument could be bound to the capture,
i.e., if a function with that C<Signature> would be able to be called
with the C<$topic>:

    (1,2, :foo) ~~ :($a, $b, :foo($bar))    # True
    <a b c d> ~~ :(Int $a)                  # False

The last returns C<True> if anything accepted by C<$topic> would also
be accepted by the C<Signature>.

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

=end pod
