[Haskell-cafe] Automatically infer Newtype instance

[Alexander Solla]

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
and c?
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