ocaml: phase 6 List.equal / List.compare (+5 tests, 589 total)

Both take an inner predicate / comparator and walk both lists in
lockstep:

  equal eq a b     short-circuits on first mismatch
  compare cmp a b  -1 if a is a strict prefix
                    1 if b is
                    0 if both empty
                    otherwise first non-zero element comparison

Mirrors real OCaml's signatures.

  List.equal (=) [1;2;3] [1;2;3]            = true
  List.equal (=) [1;2;3] [1;2;4]            = false
  List.compare compare [1;2;3] [1;2;4]      = -1
  List.compare compare [1;2] [1;2;3]        = -1
  List.compare compare [] []                = 0

lib/ocaml/runtime.sx

@@ -194,6 +194,24 @@
          | Some v -> Some v
          | None -> find_map f t
 
+     let rec equal eq a b =
+       match a with
+       | [] -> (match b with [] -> true | _ -> false)
+       | h :: t ->
+         (match b with
+          | [] -> false
+          | h2 :: t2 -> if eq h h2 then equal eq t t2 else false)
+
+     let rec compare cmp a b =
+       match a with
+       | [] -> (match b with [] -> 0 | _ -> -1)
+       | h :: t ->
+         (match b with
+          | [] -> 1
+          | h2 :: t2 ->
+            let c = cmp h h2 in
+            if c <> 0 then c else compare cmp t t2)
+
      let rec combine xs ys =
        match xs with
        | [] -> (match ys with

lib/ocaml/test.sh

@@ -1460,6 +1460,18 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 5214)
 (eval "(ocaml-run \"Option.compare compare (Some 5) (Some 3)\")")
 
+;; ── List.equal / List.compare ────────────────────────────────
+(epoch 5220)
+(eval "(ocaml-run \"List.equal (=) [1;2;3] [1;2;3]\")")
+(epoch 5221)
+(eval "(ocaml-run \"List.equal (=) [1;2;3] [1;2;4]\")")
+(epoch 5222)
+(eval "(ocaml-run \"List.compare compare [1;2;3] [1;2;4]\")")
+(epoch 5223)
+(eval "(ocaml-run \"List.compare compare [1;2] [1;2;3]\")")
+(epoch 5224)
+(eval "(ocaml-run \"List.compare compare [] []\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -2322,6 +2334,13 @@ check 5212 "Option.equal Some 1 Some 1"   'true'
 check 5213 "Option.equal Some 1 None"     'false'
 check 5214 "Option.compare Some 5 Some 3" '1'
 
+# ── List.equal / List.compare ───────────────────────────────────
+check 5220 "List.equal [1;2;3] [1;2;3]"   'true'
+check 5221 "List.equal [1;2;3] [1;2;4]"   'false'
+check 5222 "List.compare [1;2;3] [1;2;4]" '-1'
+check 5223 "List.compare [1;2] [1;2;3]"   '-1'
+check 5224 "List.compare [] []"           '0'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"

plans/ocaml-on-sx.md

@@ -407,6 +407,13 @@ _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 6 — List.equal / List.compare (+5 tests, 589
+  total). Both take an inner predicate / comparator and walk both
+  lists in lockstep. equal short-circuits on first mismatch.
+  compare returns -1 if a is a strict prefix, 1 if b is, 0 if both
+  empty, otherwise the first non-zero element comparison. Mirrors
+  real OCaml's signatures: `List.equal eq a b`, `List.compare cmp
+  a b`.
 - 2026-05-09 Phase 6 — Bool module + Option.equal / Option.compare
   (+5 tests, 584 total). Bool: equal, compare (false < true via if
   ladder), to_string, of_string, not_, to_int. Option additions