Make ComposeT poly-kinded

[RyanGlScott]

ComposeT's kinds can be generalized quite a bit more, allowing it to be used with more exotic data structures like TaggedT from tagged-transformer, e.g.,

> newtype TaggedT s m a = TaggedT (m a)
> :kind ReaderT Int `ComposeT` TaggedT Maybe
ReaderT () `ComposeT` TaggedT Int :: (k2 -> *) -> k2 -> *

[Gabriella439]

This one I'm not so sure about, for a few reasons:

• Use of poly-kinded types really deteriorates the generated Haddock documentation because it will now show explicit kind applications everywhere in type signatures and instance lists. This is going to be a particular problem for this type because it has has three kind variables
• The type is too general, so it doesn't guide the user to the the intended use case where the two parameters are monad transformers. The excessive flexibility means that error messages and inferred types will be less informative
• The kind variable k2 is for all practical purposes not free because all of the instances in the module only work when k2 is *. This means, for example, that you won't be able to use any of those instances for your example ComposeT (ReaderT ()) (TaggedT Int) type constructor

[RyanGlScott]

Those are all valid points. For the sake of keeping ComposeT simple, I wouldn't have any issue with keeping the kinds monomorphic.