Haskell: some and many [duplicate]

问题:

This question already has an answer here:

• What are Alternative's “some” and “many” useful for? 4 answers

What are some and many in Control.Applicative.Alternative good for? If I write something like some $ Just 42, it seems to cause infinite recursion, which seems not very useful...

回答1:

They make sense, when used as a parser combinator. some means, that the parser is applied as often as possible, but at least once. many is similar, but allows no parse. In case of Maybe, Just ... never "fails", thus your parser loops.