control construct
(::)/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).
When the predicate used to answer the message is a meta-predicate, the calling context for the predicate meta-arguments is the object sending the message.
The {Proxy}::Message syntax allows simplified access to
parametric object proxies.
Its operational semantics is equivalent to the conjunction
(call(Proxy), Proxy::Message). I.e. Proxy is proved
within the context of the pseudo-object user and,
if successful, the Proxy term is used as an object identifier.
Exceptions thrown when proving Proxy are handled by the (::)/2
control construct. This control 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 for subsequent messages. See the User Manual section on performance for details.
Modes and number of proofs
+object_identifier::+callable - zero_or_more
{+object_identifier}::+callable - zero_or_more
Errors
Object or Message is a variable:instantiation_errorObject is neither a variable nor 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 Name/Arity, is declared private:permission_error(access, private_predicate, Name/Arity)Message, with predicate indicator Name/Arity, is declared protected:permission_error(access, protected_predicate, Name/Arity)Message, with predicate indicator Name/Arity, is not declared:existence_error(predicate_declaration, Name/Arity)Object does not exist:existence_error(object, Object)Proxy is a variable:instantiation_errorProxy is neither a variable nor a callable term:type_error(callable, Proxy)Proxy, with predicate indicator Name/Arity, does not exist in the user pseudo-object:existence_error(procedure, Name/Arity)Examples
| ?- list::member(X, [1, 2, 3]).
X = 1 ;
X = 2 ;
X = 3
yes