ocaml: phase 6 List.take/drop/filter_map/flat_map (+6 tests, 400 total)

Common functional helpers written in OCaml. flat_map / concat_map
share an implementation. 400-test milestone.

lib/ocaml/runtime.sx

@@ -198,6 +198,35 @@
          match ys with
          | [] -> failwith \"List.map2: unequal\"
          | hy :: ty -> f hx hy :: map2 f tx ty
+
+     let rec take n xs =
+       if n <= 0 then []
+       else
+         match xs with
+         | [] -> []
+         | h :: t -> h :: take (n - 1) t
+
+     let rec drop n xs =
+       if n <= 0 then xs
+       else
+         match xs with
+         | [] -> []
+         | _ :: t -> drop (n - 1) t
+
+     let rec filter_map f xs =
+       match xs with
+       | [] -> []
+       | h :: t ->
+         match f h with
+         | None -> filter_map f t
+         | Some v -> v :: filter_map f t
+
+     let rec flat_map f xs =
+       match xs with
+       | [] -> []
+       | h :: t -> append (f h) (flat_map f t)
+
+     let concat_map = flat_map
    end ;;
 
    module Option = struct

lib/ocaml/test.sh

@@ -976,6 +976,20 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 2804)
 (eval "(ocaml-run \"match Blue with | (Red | Green) -> 1 | Blue -> 2\")")
 
+;; ── More List utilities (take/drop/filter_map/flat_map) ──────
+(epoch 2900)
+(eval "(ocaml-run \"List.take 3 [1;2;3;4;5]\")")
+(epoch 2901)
+(eval "(ocaml-run \"List.drop 2 [1;2;3;4;5]\")")
+(epoch 2902)
+(eval "(ocaml-run \"List.filter_map (fun x -> if x > 2 then Some (x * 10) else None) [1;2;3;4]\")")
+(epoch 2903)
+(eval "(ocaml-run \"List.flat_map (fun x -> [x; x]) [1;2;3]\")")
+(epoch 2904)
+(eval "(ocaml-run \"List.take 0 [1;2;3]\")")
+(epoch 2905)
+(eval "(ocaml-run \"List.take 100 [1;2;3]\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1544,6 +1558,14 @@ check 2802 "(1|2|3) match 5"              '0'
 check 2803 "(Red|Green) Red"              '1'
 check 2804 "(Red|Green) Blue"             '2'
 
+# ── List.take/drop/filter_map/flat_map ─────────────────────────
+check 2900 "List.take 3"                  '(1 2 3)'
+check 2901 "List.drop 2"                  '(3 4 5)'
+check 2902 "List.filter_map"              '(30 40)'
+check 2903 "List.flat_map double"         '(1 1 2 2 3 3)'
+check 2904 "List.take 0"                  '()'
+check 2905 "List.take overflow"           '(1 2 3)'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"