Control Predicates

The predicates of this section implement control structures. Normally these constructs are translated into virtual machine instructions by the compiler. It is still necessary to implement these constructs as true predicates to support meta-calls, as demonstrated in the example below. The predicate finds all currently defined atoms of 1 character long. Note that the cut has no effect when called via one of these predicates (see !/0).

fail0 Always fail. The predicate fail0 is translated into a single virtual machine instruction. true0 Always succeed. The predicate true0 is translated into a single virtual machine instruction. repeat0 Always succeed, provide an infinite number of choice points. 0 Cut. Discard choice points of parent frame and frames created after the parent frame. As of SWI-Prolog 3.3, the semantics of the cut are compliant with the ISO standard. This implies that the cut is transparent to 2, 2 and 2. Cuts appearing in the condition part of 2 and 2 as well as in 1 are local to the condition.

As an extension, a variable goal that is bound the the is handled as a true cut. This is the only difference between call1 and a variable appearing as subgoal.

t1 :- (a, !, fail ; b).	% cuts a/0 and t1/0
t2 :- (a -> b, ! ; c).	% cuts b/0 and t2/0
t3(G) :- a, G, fail.	% if `G = !' cuts a/0 and t1/1
t4(G) :- a, call(G), fail.	% if `G = !' cut has no effect
t5 :- call((a, !, fail ; b)).	% cuts a/0
t6 :- (a, !, fail ; b).	% cuts a/0

+Goal1+Goal2 Conjunction. Succeeds if both `Goal1' and `Goal2' can be proved. It is defined as (this definition does not lead to a loop as the second comma is handled by the compiler):

+Goal1+Goal2 The `or' predicate is defined as:

+Goal1+Goal2 Equivalent to 2. Retained for compatibility only. New code should use 2. Still nice though for grammar rules. +Condition+Action If-then and If-Then-Else. The 2 construct commits to the choices made at its left-hand side, destroying choice-points created inside the clause (by 2), or by goals called by this clause. Unlike 0, the choicepoint of the predicate as a whole (due to multiple clauses) is not destroyed. The combination 2 and 2 is defines as:

Note that the operator precedence relation between and ensure If -> Then ; Else is actually a term of the form ;(->(If, Then), Else). The first two clauses belong to the definition of 2), while only the last defines 2. +Condition+Action ; +Else This construct implements the so-called `soft-cut'. The control is defined as follows: If Condition succeeds at least once, the semantics is the same as (Condition, Action). If Condition does not succeed, the semantics is that of (Condition, Else). In other words, If Condition succeeds at least once, simply behave as the conjunction of Condition and Action, otherwise execute Else. +Goal Succeeds if `Goal' cannot be proven (mnemonic: refers to provable and the backslash () is normally used to indicate negation).