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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.
52 lines
1.4 KiB
Plaintext
52 lines
1.4 KiB
Plaintext
;; lib/minikanren/relations.sx — Phase 4 standard relations.
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;;
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;; Programs use native SX lists as data. Relations decompose lists via the
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;; tagged cons-cell shape `(:cons h t)` because SX has no improper pairs;
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;; the unifier treats `(:cons h t)` and the native list `(h . t)` as
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;; equivalent, and `mk-walk*` flattens cons cells back to flat lists for
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;; reification.
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;; --- pair / list shape relations ---
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(define nullo (fn (l) (== l (list))))
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(define pairo (fn (p) (fresh (a d) (== p (mk-cons a d)))))
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(define caro (fn (p a) (fresh (d) (== p (mk-cons a d)))))
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(define cdro (fn (p d) (fresh (a) (== p (mk-cons a d)))))
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(define conso (fn (a d p) (== p (mk-cons a d))))
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(define firsto caro)
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(define resto cdro)
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(define
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listo
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(fn (l) (conde ((nullo l)) ((fresh (a d) (conso a d l) (listo d))))))
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;; --- appendo: the canary ---
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;;
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;; (appendo l s ls) — `ls` is the concatenation of `l` and `s`.
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;; Runs forwards (l, s known → ls), backwards (ls known → all (l, s) pairs),
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;; and bidirectionally (mix of bound + unbound).
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(define
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appendo
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(fn
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(l s ls)
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(conde
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((nullo l) (== s ls))
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((fresh (a d res) (conso a d l) (conso a res ls) (appendo d s res))))))
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;; --- membero ---
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;; (membero x l) — x appears (at least once) in l.
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(define
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membero
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(fn
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(x l)
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(conde
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((fresh (d) (conso x d l)))
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((fresh (a d) (conso a d l) (membero x d))))))
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