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