GADT based integer/boolean evaluator

gadt_eval.ml

type _ expr =
  | Int_lit : int -> int expr
  | Bool_lit : bool -> bool expr
  | Add : int expr * int expr -> int expr
  | Sub : int expr * int expr -> int expr
  | Mul : int expr * int expr -> int expr
  | Div : int expr * int expr -> int expr
  | Lt : int expr * int expr -> bool expr
  | Gt : int expr * int expr -> bool expr
  | And : bool expr * bool expr -> bool expr
  | Or : bool expr * bool expr -> bool expr

let rec eval : type a. a expr -> a = function
  | Int_lit n -> n
  | Bool_lit n -> n
  | Add (a, b) -> (eval a) + (eval b)
  | Sub (a, b) -> (eval a) - (eval b)
  | Mul (a, b) -> (eval a) * (eval b)
  | Div (a, b) -> (eval a) / (eval b)
  | Lt  (a, b) -> (eval a) < (eval b)
  | Gt  (a, b) -> (eval a) > (eval b)
  | And (a, b) -> (eval a) && (eval b)
  | Or  (a, b) -> (eval a) || (eval b)