Haskell - 用防护罩替换外壳

我想知道在这部分代码中是否可以用守卫替换 case 语句：

firstFunction  :: String -> Maybe MyType
secondFunction :: MyType -> Integer
myFunction     :: String -> Maybe Integer
myFunction xs = case firstFunction xs of
    Nothing -> Nothing
    Just x  -> Just( secondFunction x )

先感谢您！

你可以使用模式保护[Haskell 维基] https://wiki.haskell.org/Pattern_guard, like:

myFunction :: String -> Maybe Integer
myFunction xs | Just x <- firstFunction xs = Just (secondFunction x)
              | otherwise = Nothing

但你在这里所做的基本上是“fmap“ 的结果firstFunction, like:

myFunction :: String -> Maybe Integer
myFunction xs = fmap secondFunction (firstFunction xs)

fmap :: Functor f => (a -> b) -> f a -> f b用于“映射”函子。现在Maybe是一个函子，定义为 http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Base.html#line-836:

instance Functor Maybe  where
    fmap _ Nothing = Nothing
    fmap f (Just a) = Just (f a)

这基本上就是你在这里写的逻辑。