control construct

{}/1

Description

{Goal}
{Closure}
{Term}

This control construct allows the programmer to bypass the Logtalk compiler (including its linter but not its optimizer) in multiple contexts:

  • Calling a goal as-is (from within an object or category) in the context of the user pseudo-object.

  • Extending a closure as-is with the remaining arguments of a call/2-N call in order to construct a goal that will be called within the context of the user pseudo-object.

  • Wrapping a source file term (either a clause or a directive) or a source file goal to bypass the term-expansion mechanism.

  • Using it in place of an object identifier when sending a message. In this case, its argument is proved as a goal within the context of the user pseudo-object with the resulting term being used as an object identifier in the message-sending goal. This feature is mainly used with parametric objects when their identifiers correspond to predicates defined in user.

  • Using it as a message to an object. This is mainly useful when the message is a conjunction of messages where some of goals are calls to Prolog built-in predicates.

Note

This control construct is opaque to cuts when used to wrap a goal (thus ensuring the same semantics independently of the argument being bound at compile-time or at runtime).

Modes and number of proofs

{+callable} - zero_or_more

Errors

Goal is a variable:
instantiation_error
Goal is neither a variable nor a callable term:
type_error(callable, Goal)

Closure is a variable:
instantiation_error
Closure is neither a variable nor a callable term:
type_error(callable, Closure)

Term is a variable:
instantiation_error
Term is neither a variable nor a callable term:
type_error(callable, Term)

Examples

% overload the standard (<)/2 operator by
% calling its standard built-in definition:
N1/D1 < N2/D2 :-
    {N1*D2 < N2*D1}.

% call a closure in the context of "user":
call_in_user(F, X, Y, Z) :-
    call({F}, X, Y, Z).

% bypass the compiler for a proprietary backend directive:
{:- load_foreign_resource(file)}.

% use parametric object proxies:
| ?- {circle(Id, Radius, Color)}::area(Area).
...

% use Prolog built-in predicates as messages:
| ?- logtalk::{write('hello world!'), nl}.
hello world!
yes