From eb69039935c91eda42ed2c94645985a0a4011089 Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Fri, 8 May 2026 11:59:41 +0000
Subject: [PATCH] =?UTF-8?q?mk:=20swap-firsto=20=E2=80=94=20swap=20first=20?=
 =?UTF-8?q?two=20list=20elements?=
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Four conso calls express the (a b . rest) -> (b a . rest) rewrite as a
purely relational constraint. Self-inverse on length-2+ lists; runs
forward (swap given input) and backward (recover original from the
swapped form). Fails on lists shorter than 2.

6 new tests, 437/437 cumulative.
---
 lib/minikanren/relations.sx         | 11 ++++++++++
 lib/minikanren/tests/swap-firsto.sx | 32 +++++++++++++++++++++++++++++
 plans/minikanren-on-sx.md           |  1 +
 3 files changed, 44 insertions(+)
 create mode 100644 lib/minikanren/tests/swap-firsto.sx

diff --git a/lib/minikanren/relations.sx b/lib/minikanren/relations.sx
index ce49586c..73bfd532 100644
--- a/lib/minikanren/relations.sx
+++ b/lib/minikanren/relations.sx
@@ -198,6 +198,17 @@
       ((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
+  swap-firsto
+  (fn
+    (l result)
+    (fresh
+      (a b rest mid-l mid-r)
+      (conso a mid-l l)
+      (conso b rest mid-l)
+      (conso b mid-r result)
+      (conso a rest mid-r))))
+
 (define
   everyo
   (fn
diff --git a/lib/minikanren/tests/swap-firsto.sx b/lib/minikanren/tests/swap-firsto.sx
new file mode 100644
index 00000000..773a57c2
--- /dev/null
+++ b/lib/minikanren/tests/swap-firsto.sx
@@ -0,0 +1,32 @@
+;; lib/minikanren/tests/swap-firsto.sx — swap first two elements.
+
+(mk-test
+  "swap-firsto-pair"
+  (run* q (swap-firsto (list 1 2) q))
+  (list (list 2 1)))
+
+(mk-test
+  "swap-firsto-with-tail"
+  (run* q (swap-firsto (list 1 2 3 4) q))
+  (list (list 2 1 3 4)))
+
+(mk-test
+  "swap-firsto-singleton-fails"
+  (run* q (swap-firsto (list 1) q))
+  (list))
+
+(mk-test "swap-firsto-empty-fails" (run* q (swap-firsto (list) q)) (list))
+
+(mk-test
+  "swap-firsto-self-inverse"
+  (run*
+    q
+    (fresh (mid) (swap-firsto (list :a :b :c :d) mid) (swap-firsto mid q)))
+  (list (list :a :b :c :d)))
+
+(mk-test
+  "swap-firsto-backward"
+  (run* q (swap-firsto q (list :y :x :z)))
+  (list (list :x :y :z)))
+
+(mk-tests-run!)
diff --git a/plans/minikanren-on-sx.md b/plans/minikanren-on-sx.md
index 9b096dd9..f77aa993 100644
--- a/plans/minikanren-on-sx.md
+++ b/plans/minikanren-on-sx.md
@@ -173,6 +173,7 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **swap-firsto**: swap the first two elements of a list. Built via four conso constraints. Self-inverse on length-2+ lists; runs forward and backward. 6 new tests, 437/437 cumulative.
 - **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.