From f8b9bde1a559d62852f48105c7715ba1c7aeeae9 Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Fri, 8 May 2026 12:31:53 +0000
Subject: [PATCH] =?UTF-8?q?mk:=20zip-with-o=20=E2=80=94=20element-wise=20c?=
 =?UTF-8?q?ombine=20of=20two=20lists?=
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Like Haskell's zipWith but relational. (zip-with-o rel l1 l2 result)
applies a 3-arg combiner relation pointwise: rel l1[i] l2[i] result[i].

  (zip-with-o pluso-i (list 1 2 3) (list 10 20 30) q)
    -> ((11 22 33))

  (zip-with-o (fn (a b r) (== r (list a b))) (list :x :y) (list 1 2) q)
    -> (((:x 1) (:y 2)))

Different-length lists fail.

5 new tests, 551/551 cumulative.
---
 lib/minikanren/relations.sx        |  8 +++++
 lib/minikanren/tests/zip-with-o.sx | 52 ++++++++++++++++++++++++++++++
 2 files changed, 60 insertions(+)
 create mode 100644 lib/minikanren/tests/zip-with-o.sx

diff --git a/lib/minikanren/relations.sx b/lib/minikanren/relations.sx
index 758d1df1..35d87cb5 100644
--- a/lib/minikanren/relations.sx
+++ b/lib/minikanren/relations.sx
@@ -277,6 +277,14 @@
       ((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
+  zip-with-o
+  (fn
+    (rel l1 l2 result)
+    (conde
+      ((nullo l1) (nullo l2) (nullo result))
+      ((fresh (a1 d1 a2 d2 a-result d-result) (conso a1 d1 l1) (conso a2 d2 l2) (rel a1 a2 a-result) (conso a-result d-result result) (zip-with-o rel d1 d2 d-result))))))
+
 (define
   swap-firsto
   (fn
diff --git a/lib/minikanren/tests/zip-with-o.sx b/lib/minikanren/tests/zip-with-o.sx
new file mode 100644
index 00000000..c3cea9ab
--- /dev/null
+++ b/lib/minikanren/tests/zip-with-o.sx
@@ -0,0 +1,52 @@
+;; lib/minikanren/tests/zip-with-o.sx — element-wise combine of two lists.
+
+(mk-test
+  "zip-with-o-empty"
+  (run* q (zip-with-o pluso-i (list) (list) q))
+  (list (list)))
+
+(mk-test
+  "zip-with-o-pluso-i"
+  (run*
+    q
+    (zip-with-o
+      pluso-i
+      (list 1 2 3)
+      (list 10 20 30)
+      q))
+  (list (list 11 22 33)))
+
+(mk-test
+  "zip-with-o-times-i"
+  (run*
+    q
+    (zip-with-o
+      *o-i
+      (list 2 3 4)
+      (list 5 6 7)
+      q))
+  (list (list 10 18 28)))
+
+(mk-test
+  "zip-with-o-different-length-fails"
+  (run*
+    q
+    (zip-with-o
+      pluso-i
+      (list 1 2)
+      (list 1 2 3)
+      q))
+  (list))
+
+(mk-test
+  "zip-with-o-non-arith-rel"
+  (run*
+    q
+    (zip-with-o
+      (fn (a b r) (== r (list a b)))
+      (list :x :y)
+      (list 1 2)
+      q))
+  (list (list (list :x 1) (list :y 2))))
+
+(mk-tests-run!)