From f5ab66e1a3dcbf4b518647c8f6eacef0adc59017 Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Fri, 8 May 2026 12:23:40 +0000
Subject: [PATCH] =?UTF-8?q?mk:=20foldl-o=20=E2=80=94=20relational=20left?=
 =?UTF-8?q?=20fold?=
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Complement to foldr-o. The combiner relation has signature
(acc head new-acc) — accumulator first.

Examples:
  (foldl-o pluso-i (list 1 2 3 4 5) 0 q)  -> (15)
  (foldl-o *o-i    (list 1 2 3 4)   1 q)  -> (24)
  (foldl-o (fn (acc x r) (conso x acc r))  ; flipped conso
           (list 1 2 3 4) (list) q)        -> ((4 3 2 1))   ; reverse

5 new tests, 510/510 cumulative.
---
 lib/minikanren/relations.sx     | 10 ++++++-
 lib/minikanren/tests/foldl-o.sx | 48 +++++++++++++++++++++++++++++++++
 plans/minikanren-on-sx.md       |  1 +
 3 files changed, 58 insertions(+), 1 deletion(-)
 create mode 100644 lib/minikanren/tests/foldl-o.sx

diff --git a/lib/minikanren/relations.sx b/lib/minikanren/relations.sx
index ac340736..c9b8b24e 100644
--- a/lib/minikanren/relations.sx
+++ b/lib/minikanren/relations.sx
@@ -64,6 +64,14 @@
       ((nullo l) (== result acc))
       ((fresh (a d r-rest) (conso a d l) (foldr-o rel d acc r-rest) (rel a r-rest result))))))
 
+(define
+  foldl-o
+  (fn
+    (rel l acc result)
+    (conde
+      ((nullo l) (== result acc))
+      ((fresh (a d new-acc) (conso a d l) (rel acc a new-acc) (foldl-o rel d new-acc result))))))
+
 (define
   membero
   (fn
@@ -80,6 +88,7 @@
       ((nullo l))
       ((fresh (a d) (conso a d l) (nafc (== a x)) (not-membero x d))))))
 
+
 (define
   subseto
   (fn
@@ -88,7 +97,6 @@
       ((nullo l1))
       ((fresh (a d) (conso a d l1) (membero a l2) (subseto d l2))))))
 
-
 (define
   reverseo
   (fn
diff --git a/lib/minikanren/tests/foldl-o.sx b/lib/minikanren/tests/foldl-o.sx
new file mode 100644
index 00000000..2e196e43
--- /dev/null
+++ b/lib/minikanren/tests/foldl-o.sx
@@ -0,0 +1,48 @@
+;; lib/minikanren/tests/foldl-o.sx — relational left fold.
+
+(mk-test
+  "foldl-o-empty"
+  (run* q (foldl-o pluso-i (list) 42 q))
+  (list 42))
+
+(mk-test
+  "foldl-o-sum"
+  (run*
+    q
+    (foldl-o
+      pluso-i
+      (list 1 2 3 4 5)
+      0
+      q))
+  (list 15))
+
+(mk-test
+  "foldl-o-product"
+  (run*
+    q
+    (foldl-o
+      *o-i
+      (list 1 2 3 4)
+      1
+      q))
+  (list 24))
+
+(mk-test
+  "foldl-o-reverse-via-flip-conso"
+  (run*
+    q
+    (foldl-o
+      (fn (acc x r) (conso x acc r))
+      (list 1 2 3 4)
+      (list)
+      q))
+  (list (list 4 3 2 1)))
+
+(mk-test
+  "foldl-o-with-init"
+  (run*
+    q
+    (foldl-o pluso-i (list 1 2 3) 100 q))
+  (list 106))
+
+(mk-tests-run!)
diff --git a/plans/minikanren-on-sx.md b/plans/minikanren-on-sx.md
index d0903e32..30519136 100644
--- a/plans/minikanren-on-sx.md
+++ b/plans/minikanren-on-sx.md
@@ -173,6 +173,7 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **foldl-o (relational left fold)**: complement to foldr-o. Combiner has args (acc, head) -> new-acc. (foldl-o pluso-i (1 2 3 4 5) 0 q) -> 15; (foldl-o flipped-conso l () q) reverses l. 5 new tests, 510/510 cumulative.
 - **2026-05-08** — **foldr-o (relational right fold)**: takes a 3-arg combiner relation rel, a list, an initial accumulator, produces the result. (foldr-o appendo lists () q) is a flatten; (foldr-o conso l () q) rebuilds l. 4 new tests, 505/505 cumulative.
 - **2026-05-08** — **enumerate-i / enumerate-from-i — 500-test milestone**: index-each-element relations. (enumerate-i l result) -> result is l with each element paired with its 0-based index. (enumerate-from-i n l result) starts at n. 5 new tests, **501/501** cumulative.
 - **2026-05-08** — **partitiono**: relational partition. (partitiono pred l yes no) splits l so yes contains elements where pred succeeds and no contains the rest. Composes with intarith for numeric predicates. 5 new tests, 496/496 cumulative.