Answer by Greg Bacon for Lifting class instance in Haskell

The generalized newtype deriving extension is what you want here:

{-# LANGUAGE GeneralizedNewtypeDeriving #-}module Main wherenewtype SomeType a = SomeCons a deriving (Num, Show, Eq)main = do  let a = SomeCons 2      b = SomeCons 3  print $ a + b

Output:

*Main> mainSomeCons 5