逆ポーランド記法電卓

rpn.hs

solveRPN :: String -> Double
-- solveRPN expression = head (fold foldingFunction [] (words expression)) をポインタフリースタイルで書いた ↓
solveRPN = head . foldl foldingFunction [] . words
  where
    -- foldingFunction stack operator
    -- stack は x:y:ys に分割し, x と y を operator で評価してスタックの先頭を置き換える
    foldingFunction (x:y:ys) "*" = (y * x):ys
    foldingFunction (x:y:ys) "+" = (y + x):ys
    foldingFunction (x:y:ys) "-" = (y - x):ys
    foldingFunction (x:y:ys) "/" = (y / x):ys
    foldingFunction (x:y:ys) "^" = (y ** x):ys
    foldingFunction xs numberString = read numberString:xs

main = print $ solveRPN "10 4 3 + 2 * - 2 ^ 2 /"