Haskell - 用防护罩替换外壳

Question

我想知道这部分代码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 )

提前谢谢您!

Answer 1

您可以使用 pattern guard [Haskell-wiki] ，例如:

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

但是你在这里所做的基本上是“fmap”firstFunction的结果，例如:

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

fmap::Functor f => (a -> b) -> f a -> f b 用于“映射”仿函数。现在也许是一个仿函数，defined as :

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

这基本上就是您在此处编写的逻辑。