Ryan Ingram wrote:
Actually, at least in GHC, associated types are just syntax sugar for
type families.
That is, this code:
class Container c where
type Element c :: *
view :: c -> Maybe (Element c,c)
instance Container [a] where
type Element [a] = a
view [] = Nothing
view (x:xs) = Just (x,xs)
is the same as this code:
type family Element c :: *
class Container c where
view :: c -> Maybe (Element c, c)
type instance Container [a] = a
instance Container [a] where
view [] = Nothing
view (x:xs) = Just (x,xs)
OK, well in that case, I'm utterly puzzled as to why both forms exist in
the first place. If TFs don't allow you to do anything that can't be
done with ATs, why have them?
My head hurts...
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