Polymorphic variants expansion power example

pretty_expr.ml

type 'e expr = [
  | `Var of string
  | `Ctr of string * 'e list
]

let string_of_expr string_of_e = function
  | `Var vname -> vname
  | `Ctr (cname, args) ->
      cname ^ "(" ^ String.concat ", " (List.map string_of_e args) ^ ")"

let rec string_of_pure expr = string_of_expr string_of_pure expr

let test_pure () =
  let pure = `Ctr ("S", [`Var "x"]) in
  print_endline (string_of_pure pure)

(* 10 years passed *)

let string_of_if string_of_e = function
  | `If (cond, t, f) ->
      "if " ^ string_of_e cond ^
      " then " ^ string_of_e t ^
      " else " ^ string_of_e f
  | #expr as e -> string_of_expr string_of_e e

let rec string_of_if_expr expr = string_of_if string_of_if_expr expr

let test_if_expr () =
  let if_expr = `Ctr ("S", [`If (`Var "c", `Var "t", `Var "f")]) in
  print_endline (string_of_if_expr if_expr)


let () =
  test_pure ();
  test_if_expr ()

S(x)
S(if c then t else f)