From 3842496f3bdf409007663af68275be7f5c88d072 Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Fri, 8 May 2026 12:21:42 +0000
Subject: [PATCH] =?UTF-8?q?mk:=20foldr-o=20=E2=80=94=20relational=20right?=
 =?UTF-8?q?=20fold?=
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Takes a 3-arg combiner relation, a list, and an initial accumulator,
produces the right-fold result. (rel a tail-result result) combines
the head with the recursive result.

Examples:
  (foldr-o appendo (list (list 1 2) (list 3) (list 4 5)) (list) q)
    -> ((1 2 3 4 5))                ; flatten

  (foldr-o conso (list 1 2 3) (list) q)
    -> ((1 2 3))                    ; rebuild list

4 new tests, 505/505 cumulative.
---
 lib/minikanren/relations.sx     | 10 ++++++++-
 lib/minikanren/tests/foldr-o.sx | 38 +++++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+), 1 deletion(-)
 create mode 100644 lib/minikanren/tests/foldr-o.sx

diff --git a/lib/minikanren/relations.sx b/lib/minikanren/relations.sx
index 23a1356f..ac340736 100644
--- a/lib/minikanren/relations.sx
+++ b/lib/minikanren/relations.sx
@@ -56,6 +56,14 @@
       ((nullo l) (nullo yes) (nullo no))
       ((fresh (a d y-rest n-rest) (conso a d l) (conde ((pred a) (conso a y-rest yes) (== no n-rest) (partitiono pred d y-rest n-rest)) ((nafc (pred a)) (== yes y-rest) (conso a n-rest no) (partitiono pred d y-rest n-rest))))))))
 
+(define
+  foldr-o
+  (fn
+    (rel l acc result)
+    (conde
+      ((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
   membero
   (fn
@@ -80,6 +88,7 @@
       ((nullo l1))
       ((fresh (a d) (conso a d l1) (membero a l2) (subseto d l2))))))
 
+
 (define
   reverseo
   (fn
@@ -88,7 +97,6 @@
       ((nullo l) (nullo r))
       ((fresh (a d res-rev) (conso a d l) (reverseo d res-rev) (appendo res-rev (list a) r))))))
 
-
 (define
   rev-acco
   (fn
diff --git a/lib/minikanren/tests/foldr-o.sx b/lib/minikanren/tests/foldr-o.sx
new file mode 100644
index 00000000..7a24ca5e
--- /dev/null
+++ b/lib/minikanren/tests/foldr-o.sx
@@ -0,0 +1,38 @@
+;; lib/minikanren/tests/foldr-o.sx — relational right fold.
+
+(mk-test
+  "foldr-o-empty"
+  (run* q (foldr-o conso (list) (list 99) q))
+  (list (list 99)))
+
+(mk-test
+  "foldr-o-conso-rebuilds-list"
+  (run* q (foldr-o conso (list 1 2 3) (list) q))
+  (list (list 1 2 3)))
+
+(mk-test
+  "foldr-o-appendo-flattens"
+  (run*
+    q
+    (foldr-o
+      appendo
+      (list
+        (list 1 2)
+        (list 3)
+        (list 4 5))
+      (list)
+      q))
+  (list (list 1 2 3 4 5)))
+
+(mk-test
+  "foldr-o-with-acc-init"
+  (run*
+    q
+    (foldr-o
+      conso
+      (list 1 2)
+      (list 9 9)
+      q))
+  (list (list 1 2 9 9)))
+
+(mk-tests-run!)