Even more generalized newtype deriving

Question

Newtypes are often used to change the behavior of certain types when used in certain class contexts. For example, one would use the Data.Monoid.All wrapper to change the behavior of Bool when used as a Monoid.

I'm currently writing such a newtype wrapper that would apply to a large range of different types. The wrapper is supposed to change the behavior of one specific class instance. It might look like this:

newtype Wrapper a = Wrapper a

instance Special a => Special (Wrapper a) where
  -- ...

However, adding this wrapper will often change the usability of the wrapped type. For example, if I previously was able to use the function mconcat :: Monoid a => [a] -> a, I am not able to now use it for a list of wrapped values.

I can of course use -XGeneralizedNewtypeDeriving and newtype Wrapper a = Wrapper a deriving (Monoid). However, this only solves the problem for Monoid and no other class, while I will be dealing with an open world full of different classes, and standalone orphan generalized newtype deriving is not really a practical option. Ideally, I'd like to write deriving hiding (Special) (deriving every class except Special), but that's not valid Haskell, of course.

Is there some way of doing this or am I just screwed and need to add a GHC feature request?

Answer 1

Look, GeneralizedNewtypeDeriving is unsafe. In that vein, here is an unsafe way of doing it

{-# LANGUAGE GADTs, ConstraintKinds #-}
import Data.Monoid
import Unsafe.Coerce

data Dict c where
  Dict :: c => Dict c

newtype Wrapper a = Wrapper a

addDictWrapper :: Dict (f a) -> Dict (f (Wrapper a))
addDictWrapper = unsafeCoerce

you can then use it anytime you need the typeclass instance

intWrapperNum :: Dict (Num (Wrapper Int))
intWrapperNum = addDictWrapper Dict

two :: Wrapper Int
two = case intWrapperNum of
           Dict -> 1 + 1

this system of passing around explicit dictionaries is highly general, and has a pretty good (although experimental) library to support it called Data.Constraint