ocaml: phase 6 List.combine/split/iter2/fold_left2/map2 (+4, 367 total)

Mechanical pair-walk OCaml implementations. failwith on length
mismatch matches Stdlib semantics. List module now covers 30+
functions.

lib/ocaml/runtime.sx

@@ -157,6 +157,47 @@
        end
 
      let stable_sort = sort
+
+     let rec combine xs ys =
+       match xs with
+       | [] -> (match ys with
+                | [] -> []
+                | _ -> failwith \"List.combine: unequal lengths\")
+       | hx :: tx ->
+         match ys with
+         | [] -> failwith \"List.combine: unequal lengths\"
+         | hy :: ty -> (hx, hy) :: combine tx ty
+
+     let rec split lst =
+       match lst with
+       | [] -> ([], [])
+       | (a, b) :: t ->
+         (match split t with
+          | (xs, ys) -> (a :: xs, b :: ys))
+
+     let rec fold_left2 f acc xs ys =
+       match xs with
+       | [] -> (match ys with [] -> acc | _ -> failwith \"List.fold_left2: unequal\")
+       | hx :: tx ->
+         match ys with
+         | [] -> failwith \"List.fold_left2: unequal\"
+         | hy :: ty -> fold_left2 f (f acc hx hy) tx ty
+
+     let rec iter2 f xs ys =
+       match xs with
+       | [] -> (match ys with [] -> () | _ -> failwith \"List.iter2: unequal\")
+       | hx :: tx ->
+         match ys with
+         | [] -> failwith \"List.iter2: unequal\"
+         | hy :: ty -> f hx hy; iter2 f tx ty
+
+     let rec map2 f xs ys =
+       match xs with
+       | [] -> (match ys with [] -> [] | _ -> failwith \"List.map2: unequal\")
+       | hx :: tx ->
+         match ys with
+         | [] -> failwith \"List.map2: unequal\"
+         | hy :: ty -> f hx hy :: map2 f tx ty
    end ;;
 
    module Option = struct