Shorter way to write this code

Question

The following pattern appears very frequently in Haskell code. Is there a shorter way to write it?

if pred x
then Just x
else Nothing

Answer 1

You're looking for mfilter in Control.Monad:

mfilter :: MonadPlus m => (a -> Bool) -> m a -> m a

-- mfilter odd (Just 1) == Just 1
-- mfilter odd (Just 2) == Nothing

Note that if the condition doesn't depend on the content of the MonadPlus, you can write instead:

"foo" <$ guard (odd 3) -- Just "foo"
"foo" <$ guard (odd 4) -- Nothing