Re: [Haskell-cafe] Automatically infer Newtype instance

* Alexander Solla [2014-04-09 11:06:55-0700]

On Wed, Apr 9, 2014 at 11:05 AM, Alexander Solla 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 and c?

In fact, in case of Newtype there is fundep: class Newtype n o | n -> o where ... so I should have c -> a. Unfortunatelly, somewhy it do not work. But also I am interested in even more unsafe case, when there is no fundep. -- Best regards, Dmitry Bogatov , Free Software supporter, esperantisto and netiquette guardian. git://kaction.name/rc-files.git GPG: 54B7F00D