Re: [Haskell-cafe] Automatically infer Newtype instance

you should checkout genealizednewtype deriving :) https://ghc.haskell.org/trac/haskell-prime/wiki/NewtypeDeriving GHC has had it for quite some time also the Coerce Machinery in 7.8 GHC provides a stronger version of your NewType style class On Tue, Apr 8, 2014 at 1:05 PM, Dmitry Bogatov 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 , Free Software supporter, esperantisto and netiquette guardian. git://kaction.name/rc-files.git GPG: 54B7F00D _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe