If one wants to declare instances for a type schema `a -> _`, one writes something like this: instance Context => Class parameters ... ((->) a) where { ... } But what if you want to declare instances for a type schema `a -> b _`, that is, where the slot to be filled is not just the result of (->) a, but rather the composition of that together with another type operation b :: * -> *? I'd like to write something like this: instance Context => Class parameters ... ((->) a (b :: *->*)) where { ...} or: instance Context => Class parameters ... ((->) a (b *)) where { ...} but these don't work. Is it possible to do this? Concretely, here's what I'm trying to achieve. I wrote a typeclass for MonadReaders that are embedded inside (one level) of other MonadReaders, like this: {-# LANGUAGE FunctionalDependencies FlexibleInstances UndecidableInstances #-} class MonadReader w m => DeepMonadReader w r m | m -> r where { deepask :: m r ; deepask = deepreader id ; deeplocal :: (r -> r) -> m a -> m a ; deepreader :: (r -> a) -> m a ; deepreader f = do { r <- deepask; return (f r) } } instance MonadReader r m => DeepMonadReader w r (ReaderT w m) where { deepask = lift ask ; deeplocal = mapReaderT . local ; deepreader = lift . reader } It'd be nice to also provide an instance something like this: instance MonadReader r m => DeepMonadReader w r ((->) w (m :: * -> *)) where { deepask = \w -> ask ; deeplocal f xx = \w -> local f (xx w) ; deepreader xx = \w -> reader xx } -- Jim Pryor jim@jimpryor.net