Haskell - What is Control.Applicative.Alternative good for?
-
13-01-2021 - |
質問
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?
解決
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.
他のヒント
Sometimes the <|>
operator is quite useful:
foldl1 (<|>) [Nothing, Just 5, Just 3]
-- Just 5
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