Note: information on this page refers to Ceylon 1.1, not to the current release.


Invocation is the act of calling something that is Callable.


Invocation using a positional argument list, in parentheses:

put(1, "one")

Invocation using a named argument lists, in braces:

put {


The thing that is being invoked is called the primary of the invocation.

The primary is followed by an argument list. There are two distinct kinds of argument list:

  • Positional argument lists are enclosed in parenthesis and separated with commas (see reference page for positional argument lists).
  • Named argument lists are enclosed in braces and separated with semicolons (see reference page for named argument lists).

It is useful to classify invocations as direct and indirect:

  • A direct invocation is one where the primary is a function, method, or class. The typechecker knows about the declaration of the primary, so invocation using named arguments is supported.
  • An indirect invocation is one where the primary is a reference with Callable type (such as a function reference). Since the primary isn't based on a declaration (or at least, there's no declaration known to the typechecker), invocation using named arguments is not supported.

For example:

print("hello world); // direct invocation
value p = print;
p("hello world); // indirect invocation

Function and method invocation

Function and method invocation is direct invocation, and supports both positional argument lists and named argument lists.

print("hello world);
    val="hello world";

Class invocation

Invoking a class (instantiating it) creates a new instance of the class.

Class invocation is a direct invocation, and therefore supports both positional argument lists and named argument lists.

value alice = Person("alice");
value bob = Person{

Indirect invocation

You can invoke a reference if it has a Callable type:

Callable<Anything, []> fn = // ...
Anything result = fn();

Because the Callable type does not retain any information about the parameter names, you cannot use a named argument invocation; only positional arguments.

Superclass initializer invocation

Technically, the extends clause of a class declatation is also an invocation: Of the superclass's initializer. For example:

class Example(String name) 
        extends Person(name) // invocation! 

Defaulted parameters

Both direct and indirect invocations allow you to omit arguments for some parameters if those parameters have a default value.

This works for indirect invocations because the Callable type can encode information about defaulted parameters, so the invocation need not specify arguments for parameters which are defaulted:

Callable<Anything, [String=]> defaulted = // ...
variable Anything result = defaulted();
result = defaulted("an argument");

Multiple argument lists

Because a Callable can itself have a Callable return type you sometimes see invocations with multiple parameter lists:

String(String)(Integer) higher = // ...
String result = higher(1)("");

Note that the type abbreviation for Callable means that the argument lists appear in reversed order because String(String)(Integer) is parsed as Callable<Callable<String, [String]>, [Integer]>

Named argument lists are only allowed as the first argument list in an invocation using multiple argument lists, because the second invocation is an indirect invocation.

Tuple and Iterable enumeration

Technically, tuple and iterable enumerations are also invocations: They instantiate new tuples and iterables respectively.

See also