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我正在通過練習wikibooks/haskell進行練習,並且在MonadPlus-chapter中有一個練習,它要求您編寫這個hexChar函數。我的函數如下所示,但是當我嘗試切換函數週圍的2個幫助器解析器(digitParse和alphaParse)時,它停止正常工作。如果我切換它們,我只能解析數字而不是字母字符。也許MonadPlus解析器需要以某種順序?
這是爲什麼呢?
char :: Char -> String -> Maybe (Char, String)
char c s = do
let (c':s') = s
if c == c' then Just (c, s') else Nothing
digit :: Int -> String -> Maybe Int
digit i s | i > 9 || i < 0 = Nothing
| otherwise = do
let (c:_) = s
if read [c] == i then Just i else Nothing
hexChar :: String -> Maybe (Char, String)
hexChar s = alphaParse s `mplus` digitParse s -- cannot switch these to parsers around!!
where alphaParse s = msum $ map ($ s) (map char (['a'..'f'] ++ ['A'..'F']))
digitParse s = do let (c':s') = s
x <- msum $ map ($ s) (map digit [0..9])
return (intToDigit x, s')
另外,還有一個函數'readMaybe',它返回'Read a =>也許是',因爲它存在於Maybe'monad中,所以我喜歡使用它。 – bheklilr