pretty.ml

type exp =
  | Int of int
  | Div of exp * exp
  | Sub of exp * exp
  | Or of exp * exp
  | Pair of exp * exp

type assoc = AscLeft | AscNon | AscRight

let rec string_of_exp parent_prec exp =
  let string_of_binop e1 e2 op prec assoc =
    let p1, p2 =
      match assoc with
      | AscLeft -> (prec, succ prec)
      | AscNon -> (succ prec, succ prec)
      | AscRight -> (succ prec, prec)
    in
    let str = string_of_exp p1 e1 ^ op ^ string_of_exp p2 e2 in
    if prec < parent_prec then "(" ^ str ^ ")" else str
  in
  match exp with
  | Int i -> string_of_int i
  | Div (e1, e2) -> string_of_binop e1 e2 " / " 12 AscLeft
  | Sub (e1, e2) -> string_of_binop e1 e2 " - " 11 AscLeft
  | Or (e1, e2) -> string_of_binop e1 e2 " || " 6 AscRight
  | Pair (e1, e2) -> string_of_binop e1 e2 ", " 5 AscNon

let string_of_exp = string_of_exp 0

(* Some expressions for the tests *)
let e1 = Sub (Sub (Sub (Int 1, Int 2), Int 3), Int 4)
let e2 = Sub (Sub (Int 1, Sub (Int 2, Int 3)), Int 4)
let e3 = Sub (Sub (Int 1, Int 2), Sub (Int 3, Int 4))
let e4 = Sub (Int 1, Sub (Int 2, Sub (Int 3, Int 4)))
let e5 = Pair (Int 1, Sub (Int 2, Sub (Int 3, Int 4)))
let e6 = Pair (Sub (Int 1, Int 2), Sub (Int 3, Int 4))
let e7 = Sub (Pair (Int 1, Int 2), Sub (Int 3, Int 4))
let e8 = Sub (Pair (Int 1, Int 2), Or (Int 3, Int 4))
let e9 = Or (Pair (Int 1, Int 2), Or (Int 3, Int 4))
let e10 = Or (Pair (Int 1, Int 2), Sub (Int 3, Int 4))
let e11 = Or (Or (Int 1, Int 2), Or (Int 3, Int 4))
let e12 = Div (Sub (Int 1, Int 2), Div (Int 3, Int 4))

(* Test *)
print_string @@ string_of_exp e1;;