[Haskell-cafe] Automatically infer Newtype instance

[Alexander Solla]

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.


On Tue, Apr 8, 2014 at 10:05 AM, Dmitry Bogatov <KAction at gnu.org> wrote:

> Hello!
> I think, that Newtype instances are transitive, and there is only one
> sane definiton is (pack . pack). But in example below I have to use
> UndecidableInstances and they seems to loop somewhere
> (Context reduction stack overflow; size = 132).
>
> To my understanding, Ghc tries to prove, that exists only one type b,
> and somewhy fail at it(It is unclear, why), but is it any way to say to
> it that I take responsibility, that ANY b would be nice?
>
> I know about TH, but interested in more elegant solution.
>
> {-# LANGUAGE MultiParamTypeClasses #-}
> {-# LANGUAGE FunctionalDependencies #-}
> {-# LANGUAGE FlexibleInstances #-}
> {-# LANGUAGE TypeFamilies #-}
> {-# LANGUAGE UndecidableInstances #-}
>
> import Control.Newtype
>
> newtype A1 = A1 Int deriving (Eq, Show)
> newtype A2 = A2 A1  deriving (Eq, Show)
>
> instance Newtype A1 Int where
>     pack = A1
>     unpack (A1 a) = a
>
> instance Newtype A2 A1 where
>     pack = A2
>     unpack (A2 a) = a
>
> -- Here comes UndecidableInstances
> instance (Newtype a b, Newtype b c) => Newtype a c where
>     pack   = pack . pack
>     unpack = unpack . unpack
>
> main = let foo :: A2
>            foo = pack "46" in
>        print foo
>
>
> --
> Best regards, Dmitry Bogatov <KAction at gnu.org>,
> Free Software supporter, esperantisto and netiquette guardian.
>         git://kaction.name/rc-files.git
>         GPG: 54B7F00D
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>
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