ocaml: phase 6 Either module + Hashtbl.copy (+4 tests, 602 total)

Either module (mirrors OCaml 4.12+ stdlib):
  left x / right x
  is_left / is_right
  find_left / find_right (return Option)
  map_left / map_right (single-side mappers)
  fold lf rf e            (case dispatch)
  equal eq_l eq_r a b
  compare cmp_l cmp_r a b (Left < Right)

Constructors are bare 'Left x' / 'Right x' (OCaml 4.12+ exposes them
directly without an explicit type-decl).

Hashtbl.copy:
  build a fresh cell with _hashtbl_create
  walk _hashtbl_to_list and re-add each (k, v)
  mutating one copy doesn't touch the other
  (Hashtbl.length t + Hashtbl.length t2 = 3 after fork-and-add
   verifies that adds to t2 don't appear in t)

lib/ocaml/runtime.sx

@@ -1030,6 +1030,48 @@
      let remove t k = _hashtbl_remove t k
      let reset t = _hashtbl_clear t
      let clear t = _hashtbl_clear t
+
+     let copy t =
+       let t' = _hashtbl_create 8 in
+       List.iter
+         (fun (k, v) -> _hashtbl_add t' k v)
+         (_hashtbl_to_list t);
+       t'
+   end ;;
+
+   module Either = struct
+     let left x = Left x
+     let right x = Right x
+
+     let is_left e = match e with Left _ -> true | Right _ -> false
+     let is_right e = match e with Left _ -> false | Right _ -> true
+
+     let find_left e =
+       match e with Left x -> Some x | Right _ -> None
+     let find_right e =
+       match e with Left _ -> None | Right x -> Some x
+
+     let map_left f e =
+       match e with Left x -> Left (f x) | Right x -> Right x
+     let map_right f e =
+       match e with Left x -> Left x | Right x -> Right (f x)
+
+     let fold lf rf e =
+       match e with Left x -> lf x | Right x -> rf x
+
+     let equal eq_l eq_r a b =
+       match a with
+       | Left x ->
+         (match b with Left y -> eq_l x y | Right _ -> false)
+       | Right x ->
+         (match b with Left _ -> false | Right y -> eq_r x y)
+
+     let compare cmp_l cmp_r a b =
+       match a with
+       | Left x ->
+         (match b with Left y -> cmp_l x y | Right _ -> -1)
+       | Right x ->
+         (match b with Left _ -> 1 | Right y -> cmp_r x y)
    end ;;
 
    module Map = struct

lib/ocaml/test.sh

@@ -1494,6 +1494,16 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 5245)
 (eval "(ocaml-run \"Float.zero +. Float.one\")")
 
+;; ── Either module + Hashtbl.copy ─────────────────────────────
+(epoch 5250)
+(eval "(ocaml-run \"Either.is_left (Left 5)\")")
+(epoch 5251)
+(eval "(ocaml-run \"Either.fold (fun x -> x + 100) (fun x -> x * 10) (Left 7)\")")
+(epoch 5252)
+(eval "(ocaml-run \"Either.fold (fun x -> x + 100) (fun x -> x * 10) (Right 7)\")")
+(epoch 5253)
+(eval "(ocaml-run \"let t = Hashtbl.create 4 in Hashtbl.add t \\\"a\\\" 1; let t2 = Hashtbl.copy t in Hashtbl.add t2 \\\"b\\\" 2; Hashtbl.length t + Hashtbl.length t2\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -2376,6 +2386,12 @@ check 5243 "Float.max 3.14 2.71"          '3.14'
 check 5244 "Float.equal 1.5 1.5"          'true'
 check 5245 "Float.zero +. Float.one"      '1'
 
+# ── Either module + Hashtbl.copy ────────────────────────────────
+check 5250 "Either.is_left Left"          'true'
+check 5251 "Either.fold Left 7+100"       '107'
+check 5252 "Either.fold Right 7*10"       '70'
+check 5253 "Hashtbl.copy independent"     '3'
+
 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 — Either module + Hashtbl.copy (+4 tests, 602
+  total). Either: left, right, is_left, is_right, find_left,
+  find_right, map_left, map_right, fold, equal, compare. Constructors
+  are bare `Left x` / `Right x` (per OCaml 4.12+). Hashtbl.copy
+  builds a fresh cell, walks `_hashtbl_to_list`, and re-adds; mutating
+  one copy doesn't touch the other (verified by `Hashtbl.length t +
+  Hashtbl.length t2 = 3` after a fork-and-add).
 - 2026-05-09 Phase 5.1 — json_pretty.ml baseline (recursive ADT
   serialization). Defines a JSON-like ADT (JNull / JBool / JInt /
   JStr / JList) and recursively pretty-prints to a string, then