case on monadic value

问题

Is there a way to perform a case on the value stored within a monad without having to bind a name to it?

i.e. instead of doing this:

c <- getChar
case c of
  ...

Is there a way to do this:

mcase getChar of
  ...

Alternatively, it would be nice if the case statement could be partially applied so:

case of
  ...

would be desugared to:

\a -> case a of
  ...

So you could do this:

getChar >>= case of
              ...

回答1:

The answer is no. In Haskell 98, you can't use a case statement without using a name inside it. But there is a proposal for adding support for case-lambdas. The syntax they propose is the same you propose too.

回答2:

The proposal mentioned by FUZxxl was now implemented in GHC since 7.6.1, it's called LambdaCase.

Now you can do:

{-# LANGUAGE LambdaCase #-}
getChar >>= \case
   ...

Note the \ before the case keyword and the fact that there is no of in that case.

回答3:

No, not really, but you can move the case into another function and apply it to the result of a monadic action.

f x = case  x of ...

main = do
  f <$> getChar

Alternativly, the following is possible:

getChar >>= \x -> case x of ...

来源：https://stackoverflow.com/questions/5395517/case-on-monadic-value