control construct
(<<)/2
Description
Object<<Goal
{Proxy}<<Goal
Debugging control construct. Calls a goal within the context of the specified object. The goal is called with the following execution context:
The goal may need to be written between parenthesis to avoid parsing errors
due to operator conflicts. This control construct should only be used for
debugging or for writing unit tests. This control construct can only be used
for objects compiled with the
context_switching_calls compiler
flag set to allow
. Set this compiler flag to deny
to disable
this control construct and thus preventing using it to break encapsulation
when deploying applications.
The {Proxy}<<Goal
syntax allows simplified access to
parametric object proxies.
Its operational semantics is equivalent to the goal conjunction
(call(Proxy), Proxy<<Goal)
. 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.
Caveat: although the goal argument is fully compiled before calling, some necessary information for the second compiler pass may not be available at runtime.
Modes and number of proofs
+object_identifier<<+callable - zero_or_more
{+object_identifier}<<+callable - zero_or_more
Errors
Object
is a variable:instantiation_error
Object
is neither a variable nor a valid object identifier:type_error(object_identifier, Object)
Object
does not contain a local definition for the Goal predicate:existence_error(procedure, Goal)
Object
does not exist:existence_error(object, Object)
Object
was created/compiled with support for context switching calls turned off:permission_error(access, database, Goal)
Proxy
is a variable:instantiation_error
Proxy
is neither a variable nor an object identifier:type_error(object_identifier, Proxy)
Proxy
does not exist in the user
pseudo-object:existence_error(procedure, ProxyFunctor/ProxyArity)
Goal
is a variable:instantiation_error
Goal
is neither a variable nor a callable term:type_error(callable, Goal)
Examples
% call the member/2 predicate in the
% context of the "list" object:
test(member) :-
list << member(1, [1]).