Call a function: Difference between revisions

=={{header|Perl 6}}==

Fundamentally, nearly everything you do in Perl 6 is a function call if you look hard enough.

At the lowest level, a function call merely requires a reference to any

kind of invokable object, and a call to its <tt>postcircumfix:&lt;( )&gt;</tt> method.

However, there are various forms of sugar and indirection that you

can use to express these function calls differently. In particular,

operators are all just sugar for function calls.

Calling a function that requires no arguments:

<lang perl6>foo # as list operator

foo() # as function

foo.() # as function, explicit postfix form

$ref() # as object invocation

$ref.() # as object invocation, explicit postfix

&foo() # as object invocation

&foo.() # as object invocation, explicit postfix

::($name)() # as symbolic ref</lang>

Calling a function with exactly one argument:

<lang perl6>foo 1 # as list operator

foo(1) # as named function

foo.(1) # as named function, explicit postfix

$ref(1) # as object invocation (must be hard ref)

$ref.(1) # as object invocation, explicit postfix

1.$foo # as pseudo-method meaning $foo(1) (hard ref only)

1.$foo() # as pseudo-method meaning $foo(1) (hard ref only)

1.&foo # as pseudo-method meaning &foo(1) (is hard foo)

1.&foo() # as pseudo-method meaning &foo(1) (is hard foo)

1.foo # as method via dispatcher

1.foo() # as method via dispatcher

1."$name"() # as method via dispatcher, symbolic

+1 # as operator to prefix:<+> function</lang>

Method calls are included here because they do eventually dispatch to a true

function via a dispatcher. However, the dispatcher in question is not going

to dispatch to the same set of functions that a function call of that name

would invoke. That's why there's a dispatcher, after all. Methods are declared

with a different keyword, <tt>method</tt>, in Perl 6, but all that does is

install the actual function into a metaclass. Once it's there, it's merely

a function that expects its first argument to be the invocant object. Hence we

feel justified in including method call syntax as a form of indirect function call.

Operators like <tt>+</tt> also go through a dispatcher, but in this case it is

multiply dispatched to all lexically scoped candidates for the function. Hence

the candidate list is bound early, and the function itself can be bound early

if the type is known. Perl 6 maintains a clear distinction between early-bound

linguistic constructs that force Perlish semantics, and late-bound OO dispatch

that puts the objects and/or classes in charge of semantics. (In any case, <tt>&foo</tt>,

though being a hard ref to the function named "foo", may actually be a ref to

a dispatcher to a list of candidates that, when called, makes all the candidates behave as a single unit.)

Calling a function with exactly two arguments:

<lang perl6>foo 1,2 # as list operator

foo(1,2) # as named function

foo.(1,2) # as named function, explicit postfix

$ref(1,2) # as object invocation (must be hard ref)

$ref.(1,2) # as object invocation, explicit postfix

1.$foo: 2 # as pseudo-method meaning $foo(1,2) (hard ref only)

1.$foo(2) # as pseudo-method meaning $foo(1,2) (hard ref only)

1.&foo: 2 # as pseudo-method meaning &foo(1,2) (is hard foo)

1.&foo(2) # as pseudo-method meaning &foo(1,2) (is hard foo)

1.foo: 2 # as method via dispatcher

1.foo(2) # as method via dispatcher

1."$name"(2) # as method via dispatcher, symbolic

1 + 2 # as operator to infix:<+> function</lang>

Calling a function with a variable number of arguments (varargs):

<lang perl6>foo @args # as list operator

foo(@args) # as named function

foo.(@args) # as named function, explicit postfix

$ref(@args) # as object invocation (must be hard ref)

$ref.(@args) # as object invocation, explicit postfix

1.$foo: @args # as pseudo-method meaning $foo(1,@args) (hard ref)

1.$foo(@args) # as pseudo-method meaning $foo(1,@args) (hard ref)

1.&foo: @args # as pseudo-method meaning &foo(1,@args)

1.&foo(@args) # as pseudo-method meaning &foo(1,@args)

1.foo: @args # as method via dispatcher

1.foo(@args) # as method via dispatcher

1."$name"(@args) # as method via dispatcher, symbolic

@args X @blargs # as list infix operator to infix:<X></lang>

Note: whether a function may actually be called with a variable number of arguments depends entirely

on whether a signature accepts a list at that position in the argument list, but

describing that is not the purpose of this task. Suffice to say that we assume here that the

foo function is declared with a signature of the form (*@params).

Calling a function with named arguments:

<lang perl6>foo :a, :b(4), :!c, d => "stuff"

foo(:a, :b(4), :!c, d => "stuff")</lang>

...and so on. Operators may also be called with named arguments, but only

colon adverbials are allowed:

<lang perl6>1 + 1 :a :b(4) :!c :d("stuff") # calls infix:<+>(1,1,:a, :b(4), :!c, d => "stuff")</lang>

Using a function in command context:

<lang perl6>foo(); bar(); # evaluate for side effects</lang>

Using a function in first class context within an expression:

<lang perl6>1 / find-a-func(1,2,3)(4,5,6) ** 2;</lang>

Obtaining the return value of a function:

<lang perl6>my $result = somefunc(1,2,3) + 2;</lang>

There is no difference between calling builtins and user-defined functions and operators (or

even control stuctures). This was a major design goal of Perl 6, and apart from a very few

low-level primitives, all of Perl 6 can be written in Perl 6.

There is no difference between calling subroutines and functions in Perl 6, other than that

calling a function in void context that has no side effects is likely to get you a "Useless use of..." warning.