ocaml: phase 6 expanded stdlib (+15 tests, 319 total)

List: concat/flatten, init, find/find_opt, partition, mapi/iteri,
assoc/assoc_opt. Option: iter/fold/to_list. Result: get_ok/get_error/
map_error/to_option.

Fixed skip-to-boundary! in parser to track let..in / begin..end /
struct..end / for/while..done nesting via a depth counter — without
this, nested-let inside a top-level decl body trips over the
decl-boundary detector. Stdlib functions like List.init / mapi / iteri
use begin..end to make their nested-let intent explicit.

lib/ocaml/runtime.sx

@@ -82,6 +82,67 @@
        match lst with
        | [] -> failwith \"List.nth: out of range\"
        | h :: t -> if n = 0 then h else nth t (n - 1)
+
+     let rec concat lst =
+       match lst with
+       | [] -> []
+       | h :: t -> append h (concat t)
+
+     let flatten = concat
+
+     let rec init n f =
+       if n = 0 then [] else
+       begin
+         let rec build i =
+           if i = n then [] else f i :: build (i + 1)
+         in build 0
+       end
+
+     let rec find_opt p lst =
+       match lst with
+       | [] -> None
+       | h :: t -> if p h then Some h else find_opt p t
+
+     let rec find p lst =
+       match find_opt p lst with
+       | None -> failwith \"List.find: not found\"
+       | Some x -> x
+
+     let rec partition p lst =
+       match lst with
+       | [] -> ([], [])
+       | h :: t ->
+         (match partition p t with
+          | (yes, no) ->
+            if p h then (h :: yes, no) else (yes, h :: no))
+
+     let rec mapi f lst =
+       begin
+         let rec go i xs =
+           match xs with
+           | [] -> []
+           | h :: t -> f i h :: go (i + 1) t
+         in go 0 lst
+       end
+
+     let rec iteri f lst =
+       begin
+         let rec go i xs =
+           match xs with
+           | [] -> ()
+           | h :: t -> f i h; go (i + 1) t
+         in go 0 lst
+       end
+
+     let rec assoc k lst =
+       match lst with
+       | [] -> failwith \"List.assoc: not found\"
+       | (k2, v) :: t -> if k = k2 then v else assoc k t
+
+     let rec assoc_opt k lst =
+       match lst with
+       | [] -> None
+       | (k2, v) :: t -> if k = k2 then Some v else assoc_opt k t
    end ;;
 
    module Option = struct
@@ -114,6 +175,21 @@
        match o with
        | None -> false
        | Some _ -> true
+
+     let iter f o =
+       match o with
+       | None -> ()
+       | Some x -> f x
+
+     let fold none_v f o =
+       match o with
+       | None -> none_v
+       | Some x -> f x
+
+     let to_list o =
+       match o with
+       | None -> []
+       | Some x -> [x]
    end ;;
 
    module Result = struct
@@ -136,6 +212,26 @@
        match r with
        | Ok _ -> false
        | Error _ -> true
+
+     let get_ok r =
+       match r with
+       | Ok x -> x
+       | Error _ -> failwith \"Result.get_ok: Error\"
+
+     let get_error r =
+       match r with
+       | Ok _ -> failwith \"Result.get_error: Ok\"
+       | Error e -> e
+
+     let map_error f r =
+       match r with
+       | Ok x -> Ok x
+       | Error e -> Error (f e)
+
+     let to_option r =
+       match r with
+       | Ok x -> Some x
+       | Error _ -> None
    end ;;
 
    module String = struct