Attempt at recursive map type in OCaml

types-fixed.ml

module MakeVal (MapType : Map.S) =
struct
  let compare = Pervasives.compare
  type map_type = t MapType.t
  and 'a with_meta = { value : 'a; meta : map_type }
  and t =
    | List of t list with_meta
    | Vector of t list with_meta
    | Map of map_type with_meta
    | Int of int
    | Symbol of string with_meta
    | Keyword of string
    | Nil
    | Bool of bool
    | String of string
    | Fn of (t list -> t)
end

module type MakeValSig =
sig
  val compare : 'a -> 'a -> int
  type map_type
  and 'a with_meta
  and t =
      List of t list with_meta
    | Vector of t list with_meta
    | Map of map_type with_meta
    | Int of int
    | Symbol of string with_meta
    | Keyword of string
    | Nil
    | Bool of bool
    | String of string
    | Fn of (t list -> t)
end

module rec MalVal : MakeValSig = MakeVal(MalMap)
and MalMap : Map.S = Map.Make(MalVal)

let to_bool x = match x with
  | MalVal.Nil | MalVal.Bool false -> false
  | _ -> true

types.ml

module MakeVal (MapType : Map.S) =
struct
  let compare = Pervasives.compare
  type 'a with_meta = { value : 'a; meta : t MapType.t }
  and t =
    | List of t list with_meta
    | Vector of t list with_meta
    | Map of t MapType.t with_meta
    | Int of int
    | Symbol of string with_meta
    | Keyword of string
    | Nil
    | Bool of bool
    | String of string
    | Fn of (t list -> t)
end

module rec MalVal : Map.OrderedType = MakeVal(MalMap)
and MalMap : Map.S = Map.Make(MalVal)

let to_bool x = match x with
  | MalVal.Nil | MalVal.Bool false -> false
  | _ -> true