Simple RPN calculator in Haskell.

rpn.hs

module Main where

import System.Environment(getArgs)

type Value     = Double
type Operand   = Value
type Result    = Value
type BinaryOp  = Operand -> Operand -> Result
type Arguments = [String]
type Stack     = [Value]
type Name      = String


main :: IO ()
main = do
    args <- getArgs
    putStrLn $ show $ process args []

functions :: [(Name, BinaryOp)];
functions = [("+", (+)), ("-", (-)), ("*", (*)), ("/", (/))]

process :: Arguments -> Stack -> Double
process [] stack = head stack
process input stack = case lookup symbol functions of
    Nothing -> plugNext readOperand
    Just op -> plugNext $ binaryAppl op
    where symbol       = head input
          plugNext     = process (tail input)
          binaryAppl f = (f (head stack) (stack !! 1)) : (drop 2 stack)
          readOperand  = (read $ head input) : stack