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7.2  Prolog control constructs

GNU Prolog follows the ISO notion of control constructs.

7.2.1  true/0, fail/0, !/0

Templates

true
fail
!

Description

true always succeeds.

fail always fails (enforces backtracking).

! always succeeds and the for side-effect of removing all choice-points created since the invocation of the predicate activating it.

Errors

None.

Portability

ISO control constructs.

7.2.2  (’,’)/2 - conjunction, (;)/2 - disjunction, (->)/2 - if-then, (*->)/2 - soft-cut (soft if-then)

Templates

’,’(+callable_term, +callable_term)
;(+callable_term, +callable_term)
->(+callable_term, +callable_term)
*->(+callable_term, +callable_term)

Description

Goal1 , Goal2 executes Goal1 and, in case of success, executes Goal2.

Goal1 ; Goal2 first creates a choice-point and executes Goal1. On backtracking Goal2 is executed.

Goal1 -> Goal2 first executes Goal1 and, in case of success, removes all choice-points created by Goal1 and executes Goal2. This control construct acts like an if-then (Goal1 is the test part and Goal2 the then part). Note that if Goal1 fails ->/2 fails also. ->/2 is often combined with ;/2 to define an if-then-else as follows: Goal1 -> Goal2 ; Goal3. Note that Goal1 -> Goal2 is the first argument of the (;)/2 and Goal3 (the else part) is the second argument. Such an if-then-else control construct first creates a choice-point for the else-part (intuitively associated with ;/2) and then executes Goal1. In case of success, all choice-points created by Goal1 together with the choice-point for the else-part are removed and Goal2 is executed. If Goal1 fails then Goal3 is executed.

Goal1 *-> Goal2 ; Goal3 implements the so-called soft-cut. It acts as the above if-then-else except that if Goal1 succeeds only Goal3 is cut (the alternative solutions of Goal1 are preserved and can be found by backtracking). Note that Goal1 *-> Goal2 alone (i.e. without an else branch Goal3) is equivalent to (Goal1 , Goal2).

’,’, ;, -> and *-> are predefined infix operators (section 8.14.10).

Errors

Goal1 or Goal2 is a variable  instantiation_error
Goal1 is neither a variable nor a callable term  type_error(callable, Goal1)
Goal2 is neither a variable nor a callable term  type_error(callable, Goal2)
The predicate indicator Pred of Goal1 or Goal2 does not correspond to an existing procedure and the value of the unknown Prolog flag is error (section 8.22.1)  existence_error(procedure, Pred)

Portability

ISO control constructs except (*->)/2 which is GNU Prolog specific.

7.2.3  call/1

Templates

call(+callable_term)

Description

call(Goal) executes Goal. call/1 succeeds if Goal represents a goal which is true. When Goal contains a cut symbol ! (section 7.2.1) as a subgoal, the effect of ! does not extend outside Goal.

Errors

Goal is a variable  instantiation_error
Goal is neither a variable nor a callable term  type_error(callable, Goal)
The predicate indicator Pred of Goal does not correspond to an existing procedure and the value of the unknown Prolog flag is error (section 8.22.1)  existence_error(procedure, Pred)

Portability

ISO control construct.

7.2.4  catch/3, throw/1

Templates

catch(?callable_term, ?term, ?term)
throw(+nonvar)

Description

catch(Goal, Catcher, Recovery) is similar to call(Goal) (section 7.2.3). If this succeeds or fails, so does the call to catch/3. If however, during the execution of Goal, there is a call to throw(Ball), the current flow of control is interrupted, and control returns to a call of catch/3 that is being executed. This can happen in one of two ways:

throw(Ball) causes the normal flow of control to be transferred back to an existing call of catch/3. When a call to throw(Ball) happens, Ball is copied and the stack is unwound back to the call to catch/3, whereupon the copy of Ball is unified with Catcher. If this unification succeeds, then catch/3 executes the goal Recovery using call/1 (section 7.2.3) in order to determine the success or failure of catch/3. Otherwise, in case the unification fails, the stack keeps unwinding, looking for an earlier invocation of catch/3. Ball may be any non-variable term.

Errors

Ball is a variable  instantiation_error

If Ball does not unify with the Catcher argument of any call of catch/3, a system error message is displayed and throw/1 fails.

When catch/3 calls Goal or Recovery it uses call/1 (section 7.2.3), an instantiation_error, a type_error or an existence_error can then occur depending on Goal or Recovery.

Portability

ISO control constructs.


Copyright (C) 1999-2013 Daniel Diaz Verbatim copying and distribution of this entire article is permitted in any medium, provided this notice is preserved. More about the copyright
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