The following code compiles happily in GHC:
{-# LANGUAGE TypeFamilies #-}
class C a where data D a m :: D a -> Bool
test :: C a => D a -> Bool test = m
My question is why do I need the context in the function "test"? It seems like since "D" is associated with class "C", the compiler can safely assume that any time I have a "D a", "a" must be an instance of C. But GHC complains if the context is removed. At first I thought that maybe the associated type was just syntactic sugar for a non-associated data type family. But this doesn't seem to be the case since I cannot instantiate D outside of an instance of C. Google and the type family documentation provided no other leads. Am I missing something here? Is the context assumption invalid? Or is it just an assumption that GHC doesn't make (yet)? Thanks in advance for any replies. -Eric