Logtalk reference manual
Control construct: ::/2
Contents > Reference Manual > Control Constructs > ::/2

## ::/2

#### Description

    Object::Message
    {Proxy}::Message

Sends a message to an object. The message argument must match a public predicate of the receiver object. When the message corresponds to a protected or private predicate, the call is only valid if the sender matches the predicate scope container. When the predicate is declared but not defined, the message simply fails (as per the closed-world assumption).

The `{Proxy}::Message` syntax allows simplified access to parametric object proxies. Its operational semantics is equivalent to the goal conjunction `(call(Proxy), Proxy::Message)`. I.e. `Proxy` is proved within the context of the pseudo-object `user` and, if successful, the goal term is used as a parametric object identifier. Exceptions thrown when proving `Proxy` are handled by the `::/2` control construct. This syntax construct supports backtracking over the `{Proxy}` goal.

The lookups for the message declaration and the corresponding method are performed using a depth-first strategy. Depending on the value of the `optimize` flag, these lookups are performed at compile time whenever sufficient information is available. When the lookups are performed at runtime, a caching mechanism is used to improve performance in subsequent messages.

#### Template and modes

    +object_identifier::+callable
    {+object_identifier}::+callable

#### Errors

Either Object or Message is a variable:
  `instantiation_error`
Object is not a valid object identifier:
  `type_error(object_identifier, Object)`
Message is neither a variable nor a callable term:
  `type_error(callable, Message)`
Message, with predicate indicator Functor/Arity, is declared private:
  `permission_error(access, private_predicate, Functor/Arity)`
Message, with predicate indicator Functor/Arity, is declared protected:
  `permission_error(access, protected_predicate, Functor/Arity)`
Message, with predicate indicator Functor/Arity, is not declared:
  `existence_error(predicate_declaration, Functor/Arity)`
Object does not exist:
  `existence_error(object, Object)`

Proxy is a variable:
  `instantiation_error`
Proxy is not a valid object identifier:
  `type_error(object_identifier, Proxy)`
The predicate Proxy does not exist in the user pseudo-object:
  `existence_error(procedure, ProxyFunctor/ProxyArity)`

#### Examples

    | ?- list::member(X, [1, 2, 3]).

    X = 1 ;
    X = 2 ;
    X = 3
    yes