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).

one_character_atoms(As) :-
	findall(A, (current_atom(A), atom_length(A, 1)), As).

fail

  Always fail. The predicate fail/0 is translated into a single virtual machine instruction.

true

  Always succeed. The predicate true/0 is translated into a single virtual machine instruction.

repeat

  Always succeed, provide an infinite number of choice points.

!

  Cut. Discard choice points of parent frame and frames created after the parent frame. Note that the control structures ;/2, |/2 ->/2 and /1 are normally handled by the compiler and do not create a frame, which implies the cut operates through these predicates. Some examples are given below. Note the difference between t3/1 and t4/1. Also note the effect of call/1 in t5/0. As the argument of call/1 is evaluated by predicates rather than the compiler the cut has no effect.

+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 :- Goal1, Goal2.

+Goal1 ; +Goal2

  The `or' predicate is defined as:

Goal1 ; _Goal2 :- Goal1.
_Goal1 ; Goal2 :- Goal2.

+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. Implemented as:

If -> Then; _Else :- If, !, Then.
If -> _Then; Else :- !, Else.
If -> Then :- If, !, Then.

\+ +Goal
  Succeeds if `Goal' cannot be proven (mnemnonic: + refers to provable and the backslash is normally used to indicate negation).