module E = struct
  type bound = int
  type env = char -> bound
end
module R_E = Monad.Reader(E)
module R = R_E.M


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) : int R.t = R.asks (fun e -> let x = e var in x)

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

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

(* monadic version of `3 + x` *)
let (expr2 : int R.t) = R.(map2 (+) (mid 3) getx)

let letx xx body = R.(xx >>= fun x -> shift (insert 'x' x) body)

(* monadic version of `let x = 2 in 3 + x` *)
let (expr3 : int R.t) = R.(letx (mid 2) expr2)

let res = R.run expr3 env0 (* will be 5 *)