Haskell 字符串到可能列表

Question

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

我只想得到 [L,L,R,R]。但看起来我失败了:(这是错误信息!

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

这样做的模块化方法是首先定义 readSquareTurn，它定义如何将 Char 转换为单个 SquareTurn(使用失败的可能性):

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

然后使用 mapM::(a -> Maybe b) -> [a] -> Maybe [b] 来处理整个 String ，如下所示:

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