Control¶

Control predicates manage execution flow — forcing determinism, measuring performance, and controlling search. For delayed execution, see Coroutining.

Determinism¶

once/1¶

once(Goal) — call Goal and commit to the first solution. Prevents backtracking into the goal.

test("first only") <- (
    once(in_(X, [1, 2, 3])),
    X == 1
)

once is useful when you know a predicate has multiple solutions but you only want the first:

any_member(ELEM, LIST) <- once(in_(ELEM, LIST))

test("any") <- (any_member(X, [10, 20, 30]), X == 10)

Timing¶

time_goal/1¶

time_goal(Goal) — execute Goal and print the solution count, wall-clock and CPU time to stderr. The goal's solutions pass through unchanged; the line is printed once Goal is exhausted, so a caller that stops at the first solution (once, an if -- seam) prints nothing.

test("time") <- time_goal(append([1, 2], [3, 4], _))

Output (to stderr):

# 1 solution(s), 0.000123s wall, 0.000098s CPU

time_goal/2¶

time_goal(Goal, Elapsed) — execute Goal and unify Elapsed with the wall-clock time in seconds (as a float). Useful for programmatic benchmarking.

test("measure") <- (
    time_goal(length(LIST, 1000), SECS),
    SECS < 1.0
)

Patterns & Recipes¶

Benchmark two approaches¶

compare_approaches(GOAL_A, GOAL_B) <- (
    time_goal(GOAL_A, TIME_A),
    time_goal(GOAL_B, TIME_B),
    FASTER == (TIME_A < TIME_B),
    writeln_text(f"A: {TIME_A}s, B: {TIME_B}s")
)

Guard with once¶

Prevent a test predicate from generating multiple successes:

test("exactly one solution") <- once(
    permutation([1, 2, 3], [3, 2, 1])
)

Gotchas¶

once commits to the first solution — it discards every remaining alternative of Goal, not just the next one. Side effects from the first solution will have happened. (Clausal Prolog has no cut; once/1 is the ISO construct for committing.)
time_goal/1 prints to stderr — not stdout. It won't interfere with write/writeln output.
time_goal/2 measures wall time — on a loaded system, wall time may be higher than CPU time. Use /1 to see both.

See also: If-Then-Else — if_/3 and the reified conditionals, Tabling — automatic memoization for performance, Exception Handling — catch/throw for error control flow.