Proposal: add Control.Monad.whenJust ∷ (Monad m) ⇒ Maybe α→(α→ m ()) → m ()

This particular pattern crops up a lot for me. Does it for anyone else? I (half-heartedly) propose adding to Control.Monad the following:

-- | A variation of 'when' for 'Maybe a'. whenJust :: (Monad m) => Maybe a -> (a -> m ()) -> m () whenJust = flip (maybe (return ()))

so that I can write:

whenJust (Map.lookup k m) $ \ a -> do ...

instead of say:

case Map.lookup k m of Nothing -> return () Just a -> do ...

which is not only nicer on the eyes, but also saves me a valuable level of indentation. Discussion period: 2 weeks. Cheers, /Liyang