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Three coupled fixes plus a new relations module land together because
each is required for the next: appendo can't terminate without all
three.
1. unify.sx — added (:cons h t) tagged cons-cell shape because SX has no
improper pairs. The unifier treats (:cons h t) and the native list
(h . t) as equivalent. mk-walk* re-flattens cons cells back to flat
lists for clean reification.
2. stream.sx — switched mature stream cells from plain SX lists to a
(:s head tail) tagged shape so a mature head can have a thunk tail.
With the old representation, mk-mplus had to (cons head thunk) which
SX rejects (cons requires a list cdr).
3. conde.sx — wraps each clause in Zzz (inverse-eta delay) for laziness.
Zzz uses (gensym "zzz-s-") for the substitution parameter so it does
not capture user goals that follow the (l s ls) convention. Without
gensym, every relation that uses `s` as a list parameter silently
binds it to the substitution dict.
relations.sx is the new module: nullo, pairo, caro, cdro, conso,
firsto, resto, listo, appendo, membero. 25 new tests.
Canary green:
(run* q (appendo (list 1 2) (list 3 4) q))
→ ((1 2 3 4))
(run* q (fresh (l s) (appendo l s (list 1 2 3)) (== q (list l s))))
→ ((() (1 2 3)) ((1) (2 3)) ((1 2) (3)) ((1 2 3) ()))
(run 3 q (listo q))
→ (() (_.0) (_.0 _.1))
152/152 cumulative.
67 lines
2.0 KiB
Plaintext
67 lines
2.0 KiB
Plaintext
;; lib/minikanren/stream.sx — Phase 2 piece A: lazy streams of substitutions.
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;;
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;; SX has no improper pairs (cons requires a list cdr), so we use a
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;; tagged stream-cell shape for mature stream elements:
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;;
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;; stream ::= mzero empty (the SX empty list)
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;; | (:s HEAD TAIL) mature cell, TAIL is a stream
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;; | thunk (fn () ...) → stream when forced
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;;
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;; HEAD is a substitution dict. TAIL is again a stream (possibly a thunk),
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;; which is what gives us laziness — mk-mplus can return a mature head with
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;; a thunk in the tail, deferring the rest of the search.
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(define mzero (list))
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(define s-cons (fn (h t) (list :s h t)))
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(define
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s-cons?
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(fn (s) (and (list? s) (not (empty? s)) (= (first s) :s))))
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(define s-car (fn (s) (nth s 1)))
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(define s-cdr (fn (s) (nth s 2)))
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(define unit (fn (s) (s-cons s mzero)))
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(define stream-pause? (fn (s) (and (not (list? s)) (callable? s))))
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;; mk-mplus — interleave two streams. If s1 is paused we suspend and
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;; swap (Reasoned Schemer "interleave"); otherwise mature-cons head with
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;; mk-mplus of the rest.
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(define
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mk-mplus
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(fn
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(s1 s2)
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(cond
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((empty? s1) s2)
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((stream-pause? s1) (fn () (mk-mplus s2 (s1))))
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(:else (s-cons (s-car s1) (mk-mplus (s-cdr s1) s2))))))
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;; mk-bind — apply goal g to every substitution in stream s, mk-mplus-ing.
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(define
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mk-bind
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(fn
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(s g)
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(cond
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((empty? s) mzero)
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((stream-pause? s) (fn () (mk-bind (s) g)))
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(:else (mk-mplus (g (s-car s)) (mk-bind (s-cdr s) g))))))
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;; stream-take — force up to n results out of a (possibly lazy) stream
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;; into a flat SX list of substitutions. n = -1 means take all.
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(define
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stream-take
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(fn
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(n s)
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(cond
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((= n 0) (list))
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((empty? s) (list))
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((stream-pause? s) (stream-take n (s)))
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(:else
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(cons
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(s-car s)
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(stream-take
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(if (= n -1) -1 (- n 1))
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(s-cdr s)))))))
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