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 construct 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. 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_error
Object
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_error
Proxy
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