From 08f4a7babdfec9ada4a8254b672d0408e29241eb Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Fri, 8 May 2026 12:20:03 +0000
Subject: [PATCH] =?UTF-8?q?mk:=20enumerate-i=20/=20enumerate-from-i=20?=
 =?UTF-8?q?=E2=80=94=20501/501=20milestone?=
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(enumerate-i l result): result is l with each element paired with its
0-based index. (enumerate-from-i n l result): same but starts at n.

  (enumerate-i (list :a :b :c) q) -> (((0 :a) (1 :b) (2 :c)))

5 new tests, 501/501 cumulative.
---
 lib/minikanren/intarith.sx        | 10 ++++++++++
 lib/minikanren/tests/enumerate.sx | 31 +++++++++++++++++++++++++++++++
 plans/minikanren-on-sx.md         |  1 +
 3 files changed, 42 insertions(+)
 create mode 100644 lib/minikanren/tests/enumerate.sx

diff --git a/lib/minikanren/intarith.sx b/lib/minikanren/intarith.sx
index e9548b4b..85b2cb69 100644
--- a/lib/minikanren/intarith.sx
+++ b/lib/minikanren/intarith.sx
@@ -117,3 +117,13 @@
     (conde
       ((nullo l) (== n 0))
       ((fresh (a d n-1) (conso a d l) (lengtho-i d n-1) (pluso-i 1 n-1 n))))))
+
+(define
+  enumerate-from-i
+  (fn
+    (start l result)
+    (conde
+      ((nullo l) (nullo result))
+      ((fresh (a d r-rest start-prime) (conso a d l) (conso (list start a) r-rest result) (pluso-i 1 start start-prime) (enumerate-from-i start-prime d r-rest))))))
+
+(define enumerate-i (fn (l result) (enumerate-from-i 0 l result)))
diff --git a/lib/minikanren/tests/enumerate.sx b/lib/minikanren/tests/enumerate.sx
new file mode 100644
index 00000000..bc1dd74a
--- /dev/null
+++ b/lib/minikanren/tests/enumerate.sx
@@ -0,0 +1,31 @@
+;; lib/minikanren/tests/enumerate.sx — index-each-element relation.
+
+(mk-test
+  "enumerate-i-empty"
+  (run* q (enumerate-i (list) q))
+  (list (list)))
+
+(mk-test
+  "enumerate-i-three"
+  (run* q (enumerate-i (list :a :b :c) q))
+  (list
+    (list (list 0 :a) (list 1 :b) (list 2 :c))))
+
+(mk-test
+  "enumerate-i-strings"
+  (run* q (enumerate-i (list "x" "y" "z") q))
+  (list
+    (list (list 0 "x") (list 1 "y") (list 2 "z"))))
+
+(mk-test
+  "enumerate-from-i-100"
+  (run* q (enumerate-from-i 100 (list :x :y :z) q))
+  (list
+    (list (list 100 :x) (list 101 :y) (list 102 :z))))
+
+(mk-test
+  "enumerate-from-i-singleton"
+  (run* q (enumerate-from-i 0 (list :only) q))
+  (list (list (list 0 :only))))
+
+(mk-tests-run!)
diff --git a/plans/minikanren-on-sx.md b/plans/minikanren-on-sx.md
index b4a77a5c..1c382418 100644
--- a/plans/minikanren-on-sx.md
+++ b/plans/minikanren-on-sx.md
@@ -173,6 +173,7 @@ _(none yet)_
 
 _Newest first._
 
+- **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.
 - **2026-05-08** — **appendo3**: 3-list append via two appendos. (appendo3 a b c r) is (appendo a b mid) ∧ (appendo mid c r). Recovers any of the three lists given the other two and the result. 5 new tests, 491/491 cumulative.
 - **2026-05-08** — **lengtho-i**: integer-indexed length (intarith). Empty list -> 0; recurse with pluso-i. Drop-in fast replacement for Peano lengtho when the count fits in a host integer. 5 new tests, 486/486 cumulative.