mk: pairlisto — relational zip

(pairlisto l1 l2 pairs): pairs is the zipped list of pairs (l1[i] l2[i]).
Recurses on both l1 and l2 in lockstep, building pairs in parallel.

Runs forward, can recover l1 given l2 and pairs, can recover l2 given
l1 and pairs. Different-length lists fail.

5 new tests, 431/431 cumulative.

lib/minikanren/relations.sx

@@ -190,6 +190,14 @@
       ((== n :z) (== result x))
       ((fresh (n-1 mid) (== n (list :s n-1)) (rel x mid) (iterate-no rel n-1 mid result))))))
 
+(define
+  pairlisto
+  (fn
+    (l1 l2 pairs)
+    (conde
+      ((nullo l1) (nullo l2) (nullo pairs))
+      ((fresh (a1 d1 a2 d2 d-pairs) (conso a1 d1 l1) (conso a2 d2 l2) (conso (list a1 a2) d-pairs pairs) (pairlisto d1 d2 d-pairs))))))
+
 (define
   everyo
   (fn

lib/minikanren/tests/pairlisto.sx

@@ -0,0 +1,41 @@
+;; lib/minikanren/tests/pairlisto.sx — zip two lists into pair list.
+
+(mk-test
+  "pairlisto-empty"
+  (run* q (pairlisto (list) (list) q))
+  (list (list)))
+
+(mk-test
+  "pairlisto-equal-lengths"
+  (run*
+    q
+    (pairlisto (list 1 2 3) (list :a :b :c) q))
+  (list
+    (list (list 1 :a) (list 2 :b) (list 3 :c))))
+
+(mk-test
+  "pairlisto-recover-l1"
+  (run*
+    q
+    (pairlisto
+      q
+      (list :a :b :c)
+      (list (list 10 :a) (list 20 :b) (list 30 :c))))
+  (list (list 10 20 30)))
+
+(mk-test
+  "pairlisto-recover-l2"
+  (run*
+    q
+    (pairlisto
+      (list 1 2 3)
+      q
+      (list (list 1 :x) (list 2 :y) (list 3 :z))))
+  (list (list :x :y :z)))
+
+(mk-test
+  "pairlisto-different-lengths-fails"
+  (run* q (pairlisto (list 1 2) (list :a :b :c) q))
+  (list))
+
+(mk-tests-run!)

plans/minikanren-on-sx.md

@@ -173,6 +173,7 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **pairlisto**: relational zip — pairs is the zipped list of (l1[i] l2[i]). Runs forward, recovers l1 given l2 and pairs, recovers l2 given l1 and pairs. 5 new tests, 431/431 cumulative.
 - **2026-05-08** — **iterate-no**: relational iterated application. Applies a 2-arg relation n times (Peano n) starting from x to produce result. Generalises succ-iteration / list-cons-iteration / etc. 4 new tests, 426/426 cumulative.
 - **2026-05-08** — **rev-acco / rev-2o**: accumulator-style reversal — faster than appendo-driven reverseo for forward queries because no quadratic appendos. Trade-off: rev-acco is asymmetric (cannot run cleanly backward in run*). 6 new tests, 422/422 cumulative.
 - **2026-05-08** — **even-i / odd-i (intarith)**: ground-only parity goals via project. Composes with membero for filtered enumeration:  -> . 5 new tests, 416/416 cumulative.