逆ポーランド記法電卓 Reverse Polish Notation Calcurator

Rpn.hs

import Text.Read
import Stack

--
readWithEffort :: Read r => String -> Either String r
readWithEffort src = case readMaybe src of
    (Just a) -> Right a
    Nothing  -> Left  src

--
solveRPN :: String -> Double
solveRPN = head . foldl loop [] . map readWithEffort . words
    where
        loop :: Stack Double -> Either String Double -> Stack Double
        
        loop stack (Left operator) = push newStack $ case operator of
            "+" -> operand1 + operand2
            "-" -> operand1 - operand2
            "*" -> operand1 * operand2
            _   -> error "invalid operator"
            
            where
                (operand2, stack')   = pop stack
                (operand1, newStack) = pop stack'
        
        loop stack (Right num) = push stack num

--
main :: IO()
main = print $ solveRPN "10 4 3 + 2 * -"

Stack.hs

module Stack(
    Stack, pop, push
) where


--
type Stack a = [a]

pop :: Stack a -> (a, Stack a)
pop stack = (e, newStack)
    where
        e        = last stack
        newStack = init stack
        
push :: Stack a -> a -> Stack a
push stack e = newStack
    where
        newStack = stack ++ [e]