逆ポーランド計算機

S式ストリームパーサ版を作ろうと思って書いてたら id:mzp が書いてしまったので方向転換．
逆ポーランド計算機

#load "camlp4o.cma"

type token = Op of string | Num of int

let lexer = Genlex.make_lexer ["+"; "-"; "*"; "/"]
let rec parse = parser 

  | [<'Int n; e = parse>] -> (Num n) :: e

  | [<'Kwd k; e = parse>] -> (Op k) :: e

  | [<>] -> []
exception NotOperation

let op (Op s) (Num n1) (Num n2) = 

  if s = "+" then Num (n1 + n2)

  else if s = "-" then Num (n1 - n2)

  else if s = "*" then Num (n1 * n2)

  else if s = "/" then Num (n1 / n2)

  else raise NotOperation
let s = Stack.create ()

let rec calc = function

    Num n :: l -> (Stack.push (Num n) s); calc l

  | Op o :: l ->

      let n1 = Stack.pop s

      and n2 = Stack.pop s

      in (Stack.push (op (Op o) n2 n1) s); calc l

  | [] -> Stack.top s
let inv_polish s = 

  let n = calc (parse (lexer (Stream.of_string s)))

  in 

    match n with

	Num n -> n

Stack モジュールを初めて使ってみた．

実行結果

# inv_polish "3 1 - 10 * 2 /";;


: int = 10