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
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