Haskell String to Maybe List

Question

readSquareTransition :: String -> Maybe [SquareTurn] 
readSquareTransition [] = Just [] 
readSquareTransition (x:xs) = case x of 
     'L' -> Just (L : readSquareTransition xs) 
     'R' -> Just (R : readSquareTransition xs) 
     _  -> Nothing 

我想要得到只是[L，L，R，R]。不過貌似我沒:(以下是錯誤信息！

src/StudentSources/LangtonsAnt.hs:231:24: 
Couldn't match expected type ‘[SquareTurn]’ 
      with actual type ‘Maybe [SquareTurn]’ 
In the second argument of ‘(:)’, namely ‘readSquareTransition xs’ 
In the first argument of ‘Just’, namely 
    ‘(L : readSquareTransition xs)’ 

src/StudentSources/LangtonsAnt.hs:232:24: 
Couldn't match expected type ‘[SquareTurn]’ 
      with actual type ‘Maybe [SquareTurn]’ 
In the second argument of ‘(:)’, namely ‘readSquareTransition xs’ 
In the first argument of ‘Just’, namely 
    ‘(R : readSquareTransition xs)’ 

Answer 1

這樣做的一個模塊化的方式是定義readSquareTurn第一限定如何將一個Char成單個SquareTurn（具有故障的可能性）：

readSquareTurn :: Char -> Maybe SquareTurn 
readSquareTurn x = case x of 
    'L' -> Just L 
    'R' -> Just R 
    _ -> Nothing 

，然後使用mapM :: (a -> Maybe b) -> [a] -> Maybe [b]處理整個String像這樣：

readSquareTransition :: String -> Maybe [SquareTurn] 
readSquareTransition = mapM readSquareTurn 

Answer 2

更改此

'L' -> Just (L : readSquareTransition xs) 
'R' -> Just (R : readSquareTransition xs) 

這個

'L' -> fmap (L :) $ readSquareTransition xs 
'R' -> fmap (R :) $ readSquareTransition xs 

的問題是，readSquareTransition返回Maybe [SquareTurn]，這樣你就可以不適用(:)（(:)需要一個列表）。fmap但是可以讓你申請到Just （同時保留Nothing）。