Incomplete type signature

Lets say we've got a function like f below, that returns a monad. However, where you see `Int`, pretend it's a really complicated type. ``` f :: (Monad m) => m Int -- Pretend this isn't Int but something complicated f = return 42 ``` Now lets say we want to force this into the `Maybe` monad. We don't need to write the full type of `f` to do this, we can just do the following: ``` g :: Maybe a -> Maybe a g = id main = print $ (g f) ``` The dummy function `g` forces `f` to become `Maybe`. I think the above is rather messy. What I'd rather write is this: ``` main = print $ (f :: Maybe a) ``` But it fails with the following error: ``` Couldn't match expected type `a' against inferred type `Int' `a' is a rigid type variable bound by the polymorphic type `forall a. Maybe a' at prog.hs:7:16 Expected type: Maybe a Inferred type: Maybe Int In the second argument of `($)', namely `(f :: Maybe a)' In the expression: print $ (f :: Maybe a) ``` Is there a way to do what `g` above does in a less messy way that doesn't involve creating a new function? I don't want to write `f :: Maybe Int`, as it becomes a maintenance problem if the return type changes. GHC extensions are okay in answers.