问题
I was looking at the Applicative class within Haskell libraries and stumbled across Alternative.
What is this class good for? A google search did not reveal anything particularly insightful. And it seems to be completely out of place, bundled as it is with the Applicative package.
Could someone please post a possible scenario where you would use this class?
回答1:
It's commonly used with parser combinators. For example, if space is a parser combinator that matches a single whitespace character, many space would be one that would match consecutive whitespace.
I can agree that it's slightly out of place in Control.Applicative, though.
回答2:
Sometimes the <|> operator is quite useful:
foldl1 (<|>) [Nothing, Just 5, Just 3]
-- Just 5
来源：https://stackoverflow.com/questions/7203686/haskell-what-is-control-applicative-alternative-good-for