Call a function

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 postcircumfix:<( )> 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:

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

Calling a function with exactly one argument:

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

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, method, 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 + 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, &foo, 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:

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

Optional arguments don't look any different from normal arguments. The optionality is all on the binding end.

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

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>

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). The calls above might be interpreted as having a single array argument if the signature indicates a normal parameter instead of a variadic one. What you cannot do in Perl 6 (unlike Perl 5) is pass an array as several fixed arguments. By default it must either represent a single argument, or be part of a variadic list. You can force the extra level of argument list interpolation using a prefix | however:

my @args = 1,2,3;
foo(|@args);  # equivalent to foo(1,2,3)

Calling a function with named arguments:

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

...and so on. Operators may also be called with named arguments, but only colon adverbials are allowed:

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

Using a function in statement context:

foo(); bar(); baz();    # evaluate for side effects

Using a function in first class context within an expression:

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

Obtaining the return value of a function:

my $result = somefunc(1,2,3) + 2;

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. And, of course, the fact that pure functions can participate in more optimizations such as constant folding.

By default, arguments are passed readonly, which allows the implementation to decide whether pass-by-reference or pass-by-value is more efficient on a case-by-case basis. Explicit lvalue, reference, or copy semantics may be requested on a parameter-by-parameter basis, and the entire argument list may be processed raw if that level of control is needed.