On Wed, Apr 9, 2014 at 2:06 PM, Alexander Solla
instance (Newtype a b, Newtype b c) => Newtype a c where pack = (pack :: b -> c) . (pack :: a -> b)
Did I get those types right? They look free to me.
Actually, I'm not sure if that will work. How will GHC know which b to choose for the pair a and c, unless there are functional dependencies on a and c?
Hi Alex, If there was no fundep on Newtype, that 'b' could also be inferred when the 'a' is actually Int, and we have an instance: instance (b ~ Key) => Newtype Int b You tend to need many type annotations when you have types that look like they are ambiguous but turn out to be unambiguous because of your particular instances: https://ghc.haskell.org/trac/ghc/ticket/8477 Regards, Adam