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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.
40 lines
1.4 KiB
Plaintext
40 lines
1.4 KiB
Plaintext
;; lib/minikanren/conde.sx — Phase 2 piece C: `conde`, the canonical
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;; miniKanren and-or form, with implicit Zzz inverse-eta delay so recursive
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;; relations like appendo terminate.
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;;
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;; (conde (g1a g1b ...) (g2a g2b ...) ...)
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;; ≡ (mk-disj (Zzz (mk-conj g1a g1b ...))
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;; (Zzz (mk-conj g2a g2b ...)) ...)
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;;
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;; `Zzz g` wraps a goal expression in (fn (S) (fn () (g S))) so that
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;; `g`'s body isn't constructed until the surrounding fn is applied to a
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;; substitution AND the returned thunk is forced. This is what gives
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;; miniKanren its laziness — recursive goal definitions can be `(conde
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;; ... (... (recur ...)))` without infinite descent at construction time.
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;;
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;; Hygiene: the substitution parameter is gensym'd so that user goal
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;; expressions which themselves bind `s` (e.g. `(appendo l s ls)`) keep
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;; their lexical `s` and don't accidentally reference the wrapper's
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;; substitution. Without gensym, miniKanren relations that follow the
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;; common (l s ls) parameter convention are silently miscompiled.
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(defmacro
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Zzz
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(g)
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(let
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((s-sym (gensym "zzz-s-")))
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(quasiquote
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(fn ((unquote s-sym)) (fn () ((unquote g) (unquote s-sym)))))))
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(defmacro
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conde
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(&rest clauses)
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(quasiquote
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(mk-disj
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(splice-unquote
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(map
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(fn
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(clause)
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(quasiquote (Zzz (mk-conj (splice-unquote clause)))))
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clauses)))))
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