[Haskell-cafe] Automatically infer Newtype instance
alex.solla at gmail.com
Wed Apr 9 18:06:55 UTC 2014
On Wed, Apr 9, 2014 at 11:05 AM, Alexander Solla <alex.solla at gmail.com>wrote:
> Try using scoped type variables, and explicitly declare the types for pack
> and unpack at the call site:
> 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
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