RPN calculator in Haskell

RPN.hs

import Control.Monad
import Data.Char
import Data.List
 
op f (y:x:xs) = (f x y):xs                       -- Pop two, evaluate, push.

parse [value]  []     = value                    -- Done parsing, evaluate.
parse _any     []     = error "wrong stack size" -- This shouldn't happen!
parse stack (' ':xs)  = parse stack xs           -- Ignore spaces.
parse stack ('+':xs)  = parse (op (+) stack) xs  -- Addition.
parse stack ('-':xs)  = parse (op (-) stack) xs  -- Subtraction.
parse stack ('*':xs)  = parse (op (*) stack) xs  -- Multiplication.
parse stack ('/':xs)  = parse (op div stack) xs  -- Division.
parse stack xs = parse (value:stack) rest        -- Parse an integer.
  where (token, rest) = span isDigit xs
        value = read token :: Integer
 
main = forever $ do l <- getLine                 -- Read a line.
                    print (parse [] l)           -- Evaluate it and print.