Generalized newtype deriving doesn't work on non-newtypes [GHC-10333]

Language extension: GeneralizedNewtypeDeriving, DerivingStrategies

Generalized newtype deriving implements typeclass derivation for newtypes by coercing them to and from their underlying datatypes. But types that aren’t declared with newtype cannot be coerced, so other deriving strategies must be used to derive instances for them.

Examples

Attempt to use newtype deriving for an ordinary datatype

Message

DeriveNewtypeData.hs:5:20: error: [GHC-10333]
    • Can't make a derived instance of
        ‘Show Weekend’ with the newtype strategy:
        GeneralizedNewtypeDeriving cannot be used on non-newtypes
    • In the data declaration for ‘Weekend’
  |
5 |   deriving newtype Show
  |                    ^^^^

Explanation

Generalized newtype deriving works only on types defined with newtype, but Weekend is defined with data. Ordinary deriving, however, works with types defined with data.

DeriveNewtypeData.hs

Before

{-# LANGUAGE DerivingStrategies #-}
module DeriveNewtypeData where

data Weekend = Saturday | Sunday
  deriving newtype Show

After


module DeriveNewtypeData where

data Weekend = Saturday | Sunday
  deriving Show