for_ is whenJust

Sometimes, I defined a utility function whenJust as follows:

whenJust :: Monad m => Maybe a -> (a -> m ()) -> m ()
whenJust = flip $ maybe (return ())

and use it like:

whenJust x $ print

But finally I found that Data.Foldable.forM_ is equivalent to this function thanks to Proposal: add Control.Monad.whenJust ∷ (Monad m) ⇒ Maybe α→(α→ m ()) → m (). Actually, this is obvious because Maybe is a container that may have one element. You can also use Data.Foldable.for_ for applicative functors. Now you can write:

for_ x $ print

without defining your whenJust.