chapter14_problem1

foldm_writer.hs

-- Writerを返す2引数関数を用いたfoldMによる整数の畳込みを行うために、9より大きい数が表れた場合は9より小さい数が表れた場合とは異なるログを残すような関数を実装せよ

import Control.Monad.Writer

binSmallsWriter :: Int -> Int -> Writer [String] Int
binSmallsWriter acc x
    | x > 9 = do
        tell [show x ++ " too big, not added"]
        return acc
    | otherwise = do
        tell [show x ++ " is added"]
        return (acc + x)

{-
 GHCi> mapM_ putStrLn $ snd $ runWriter $ foldM binSmallsWriter
 0 [2,8,3,1]
 2 is added
 8 is added
 3 is added
 1 is added
 GHCi> mapM_ putStrLn $ snd $ runWriter $ foldM binSmallsWriter
 0 [2,11,3,1]
 2 is added
 11 too big, not added
 3 is added
 1 is added
-}