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(nafc g) is a three-line primitive: peek the goal's stream for one answer; if empty, yield (unit s); else mzero. Carries the standard miniKanren caveats — open-world unsound, diverges on infinite streams. 7 tests: failed-goal-succeeds, successful-goal-fails, double-negation, conde-all-fail-makes-nafc-succeed, conde-any-success-makes-nafc-fail, nafc as a guard accepting and blocking. 201/201 cumulative.
25 lines
859 B
Plaintext
25 lines
859 B
Plaintext
;; lib/minikanren/nafc.sx — Phase 5 piece C: negation as finite failure.
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;;
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;; (nafc g)
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;; succeeds (yields the input substitution) if g has zero answers
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;; against that substitution; fails (mzero) if g has at least one.
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;;
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;; Caveat: `nafc` is unsound under the open-world assumption. It only
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;; makes sense for goals over fully-ground terms, or with the explicit
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;; understanding that adding more facts could flip the answer. Use
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;; `(project (...) ...)` to ensure the relevant vars are ground first.
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;;
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;; Caveat 2: stream-take forces g for at least one answer; if g is
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;; infinitely-ground (say, a divergent search over an unbound list),
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;; nafc itself will diverge. Standard miniKanren limitation.
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(define
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nafc
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(fn
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(g)
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(fn
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(s)
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(let
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((peek (stream-take 1 (g s))))
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(if (empty? peek) (unit s) mzero)))))
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