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condu.sx: defmacro `condu` folds clauses through a runtime `condu-try` walker. First clause whose head yields a non-empty stream commits its single first answer; later clauses are not tried. `onceo` is the simpler sibling — stream-take 1 over a goal's output. 10 tests cover: onceo trimming success/failure/conde, condu first-clause wins, condu skips failing heads, condu commits-and-cannot-backtrack to later clauses if the rest of the chosen clause fails. 110/110 cumulative. Phase 2 complete.
53 lines
1.7 KiB
Plaintext
53 lines
1.7 KiB
Plaintext
;; lib/minikanren/condu.sx — Phase 2 piece D: `condu` and `onceo`.
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;;
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;; Both are commitment forms (no backtracking into discarded options):
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;;
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;; (onceo g) — succeeds at most once: takes the first answer
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;; stream-take produces from (g s).
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;;
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;; (condu (g0 g ...) (h0 h ...) ...)
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;; — first clause whose head goal succeeds wins; only
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;; the first answer of the head is propagated to the
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;; rest of that clause; later clauses are not tried.
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;; (Reasoned Schemer chapter 10; Byrd 5.4.)
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;;
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;; `conda` (the variant that propagates ALL answers of the head) lives in
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;; Phase 5 with `project` and `matche`.
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(define onceo (fn (g) (fn (s) (stream-take 1 (g s)))))
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;; condu-try — runtime walker over a list of clauses (each clause a list of goals).
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;; Forces the head with stream-take 1; if head fails, falls to next clause;
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;; if head succeeds, commits its single answer through the rest of the clause.
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(define
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condu-try
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(fn
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(clauses s)
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(cond
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((empty? clauses) mzero)
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(:else
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(let
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((cl (first clauses)))
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(let
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((head-goal (first cl)) (rest-goals (rest cl)))
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(let
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((peek (stream-take 1 (head-goal s))))
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(if
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(empty? peek)
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(condu-try (rest clauses) s)
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((mk-conj-list rest-goals) (first peek))))))))))
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(defmacro
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condu
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(&rest clauses)
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(quasiquote
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(fn
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(s)
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(condu-try
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(list
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(splice-unquote
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(map
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(fn (cl) (quasiquote (list (splice-unquote cl))))
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clauses)))
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s))))
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