AST to Postfix string representation

postfix.ml

open Printf
 
type node =
  | Call of string * node list
  | Int of int
 
let rec toString = function
  | Call (fn, args) -> sprintf "(%s %s)" fn (String.concat " " (List.map toString args))
  | Int i -> sprintf "%d" i
 
let rec toPostfix = function
  | Call (fn, args) -> sprintf "%s %s" (String.concat " " (List.map toPostfix args)) fn
  | Int i -> sprintf "%d" i
 
let () =
  let ast = Call ("print", [
                            Call ("+", [Int 2; Int 3])
                          ]) in
  let r = toPostfix ast in
  printf "%s\n" r