module rec E : sig
  type bound = Int of int | Fun of (int R.t -> int R.t)
  type env = char -> bound
end = E
and R : Monad.READER with type env = E.env = struct
  type env = E.env
  module R_E = Monad.Reader(E)
  include = R_E.M
end


let env0 = fun var -> raise Not_found
let insert var value e = fun sought -> if sought = var then value else e sought

let getint (var : char) = R.asks (fun e -> let (E.Int x) = e var in x)

let getfun (var : char) = R.asks (fun e -> let (E.Fun f) = e var in f)

(* monadic versions of `x` and `y` and `f` *)
let getx = getint 'x'
let gety = getint 'y'
let getf = getfun 'f'

(* monadic version of `y + x` *)
let (expr1 : int R.t) = R.(map2 (+) gety getx)

(* monadic version of `\y -> y + x` *)
let lambda1 = R.(fun yy -> yy >>= fun y -> shift (insert 'y' (E.Int y)) expr1)

let letx xx body = R.(xx >>= fun x -> shift (insert 'x' (E.Int x)) body)
let letf ff body = R.(ff >>= fun f -> shift (insert 'f' (E.Fun f)) body)

(* monadic version of `let x = 2 in let f = \y -> y + x in f 3` *)
let (expr4 : int R.t) = R.(letx (mid 2) (letf (mid lambda1) (getf >>= fun f -> f (mid 3))))

let res = R.run expr4 env0 (* will be 5 *)