open-recursion

open_recursion.ml

module Eval_base = struct
  let eval eval_rec = function
    | `Int n -> n
    | _ ->
      failwith "unknown expression"

  let show show_rec = function
    | `Int n -> string_of_int n
    | _ ->
      failwith "unknown expression"
end

module Eval_add = struct
  let eval eval_rec = function
    | `Add (e0, e1) -> eval_rec e0 + eval_rec e1
    | e -> eval_rec e

  let show show_rec = function
    | `Add (e0, e1) -> Printf.sprintf "(%s + %s)" (show_rec e0) (show_rec e1)
    | e -> show_rec e
end

module Eval_sub = struct
  let eval eval_rec = function
    | `Sub (e0, e1) -> eval_rec e0 - eval_rec e1
    | e -> eval_rec e

  let show show_rec = function
    | `Sub (e0, e1) -> Printf.sprintf "(%s - %s)" (show_rec e0) (show_rec e1)
    | e -> show_rec e

end

module Eval = struct
  let rec eval e = Eval_sub.eval (Eval_add.eval (Eval_base.eval eval)) e
  let rec show e = Eval_sub.show (Eval_add.show (Eval_base.show show)) e
end

module Eval_example = struct
  open Eval

  let res0 = eval (`Sub (`Add (`Int 1, `Int 2), `Int 3));;
  let res1 = show (`Sub (`Add (`Int 1, `Int 2), `Int 3));;
end