mk: foldl-o — relational left fold

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

@@ -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

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!)

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.