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6.2  Prolog control constructs

6.2.1  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.

6.2.2  (',')/2 - conjunction, (;)/2 - disjunction, (->)/2 - if-then

Templates
','(+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.

',', ; and -> are predefined infix operators (section 7.14.10).

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 7.22.1)    existence_error(procedure, Pred)

Portability

ISO control constructs.

6.2.3  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 6.2.1) 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 7.22.1)    existence_error(procedure, Pred)

Portability

ISO control construct.

6.2.4  catch/3, throw/1

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

catch(Goal, Catcher, Recovery) is similar to call(Goal) (section 6.2.3). 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 6.2.3) 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
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 7.22.1)    existence_error(procedure, Pred)
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 Recovery it uses call/1 (section 6.2.3), an instantiation_error, a type_error or an existence_error can then occur depending on Recovery.

Portability

ISO control constructs.

Copyright (C) 1999-2007 Daniel Diaz

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