Excercise from "Learn You a Haskell for Great Good!"

RPNWriter.hs

module RPNWriter where
-- 逆ポーランド記法で書かれた式を計算する関数 solveRPN (p.214) に、
-- Writerモナドを使って計算経過ログをとる機能を追加してみよう。

import Control.Monad.Writer
import Control.Applicative ((<$>))
import Data.Char (isDigit)

solveRPNwithLog :: String -> Writer String Double
solveRPNwithLog s = do
  head <$> (foldM foldingFunctionWithLog [] $ words s)
  where
    foldingFunction (x:y:ys) "*" = (y * x) : ys
    foldingFunction (x:y:ys) "+" = (y + x) : ys
    foldingFunction (x:y:ys) "-" = (y - x) : ys
    foldingFunction xs       str = read str : xs

    foldingFunctionWithLog :: [Double] -> String -> Writer String [Double]
    foldingFunctionWithLog xs str
      | all isDigit str = do
        tell $ "Read " ++ str ++ ".\n"
        return $ foldingFunction xs str
      | otherwise = do
        let x:y:_ = xs
        tell $ "Apply " ++ str ++ " on " ++ show x ++ " and " ++ show y ++ ".\n"
        return $ foldingFunction xs str