From b8dfc080dd90fb183857f129c70ae15edf64ce9f Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Sat, 9 May 2026 11:33:30 +0000
Subject: [PATCH] ocaml: phase 5.1 zip_unzip.ml baseline (zip/unzip round-trip,
 sum-product = 1000)

zip walks both lists in lockstep, truncating at the shorter. unzip
uses tuple-pattern destructuring on the recursive result.

  let pairs = zip [1;2;3;4] [10;20;30;40] in
  let (xs, ys) = unzip pairs in
  List.fold_left (+) 0 xs * List.fold_left (+) 0 ys
  = 10 * 100
  = 1000

Exercises:
  - tuple-cons patterns in match scrutinee: 'match (xs, ys) with'
  - tuple constructor in return value: '(a :: la, b :: lb)'
  - the iter-98 let-tuple destructuring: 'let (la, lb) = unzip rest'

53 baseline programs total.
---
 lib/ocaml/baseline/expected.json |  1 +
 lib/ocaml/baseline/zip_unzip.ml  | 18 ++++++++++++++++++
 plans/ocaml-on-sx.md             |  9 +++++++++
 3 files changed, 28 insertions(+)
 create mode 100644 lib/ocaml/baseline/zip_unzip.ml

diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 3f93f13d..2f291158 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -48,6 +48,7 @@
   "safe_div.ml": 20,
   "shuffle.ml": 55,
   "word_freq.ml": 8,
+  "zip_unzip.ml": 1000,
   "sieve.ml": 15,
   "sum_squares.ml": 385,
   "unique_set.ml": 9,
diff --git a/lib/ocaml/baseline/zip_unzip.ml b/lib/ocaml/baseline/zip_unzip.ml
new file mode 100644
index 00000000..b64787db
--- /dev/null
+++ b/lib/ocaml/baseline/zip_unzip.ml
@@ -0,0 +1,18 @@
+let rec zip xs ys =
+  match (xs, ys) with
+  | ([], _) -> []
+  | (_, []) -> []
+  | (x :: xs', y :: ys') -> (x, y) :: zip xs' ys'
+
+let rec unzip pairs =
+  match pairs with
+  | [] -> ([], [])
+  | (a, b) :: rest ->
+    let (la, lb) = unzip rest in
+    (a :: la, b :: lb)
+
+;;
+
+let pairs = zip [1;2;3;4] [10;20;30;40] in
+let (xs, ys) = unzip pairs in
+List.fold_left (+) 0 xs * List.fold_left (+) 0 ys
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 8fdf4adc..82eee160 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,15 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-09 Phase 5.1 — zip_unzip.ml baseline (list zip/unzip
+  round-trip, sum-product = 1000). zip walks both lists in lockstep
+  truncating at the shorter; unzip uses tuple-pattern destructuring
+  on the recursive result. After zip [1;2;3;4] [10;20;30;40] +
+  unzip, sums are 10 and 100 → product 1000. Exercises tuple-cons
+  patterns in match scrutinee `(xs, ys)`, tuple constructor in
+  return value `(a :: la, b :: lb)`, and the iter-98 let-tuple
+  destructuring `let (la, lb) = unzip rest in`. 53 baseline programs
+  total.
 - 2026-05-09 Phase 5.1 — bigint_add.ml baseline (digit-list big-num
   add, 28 = 1+18+9). Recursive 4-arm match on `(a, b)` tuples
   threading a carry: `(x::xs, y::ys) -> (s mod 10) :: aux xs ys (s