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Motivation

To the best of my knowledge (I could not find any information about it in the GHC Standalone Deriving documentation), it is not possible to derive multiple classes in a single standalone deriving declaration.

Repeating an identical deriving declaration only to change the derived instance seems like unnecessary boilerplate:

deriving instance Eq (MyType 'Foo)
deriving instance Show (MyType 'Foo)
deriving instance Read (MyType 'Foo)

But this situation can actually become bug-prone when used together with via strategy, as it becomes easy to have discrepancies. Example using deriving-aeson library:

deriving via
  ( CustomJSON
      '[ OmitNothingFields,
         UnwrapUnaryRecords,
         TagSingleConstructors,
         FieldLabelModifier (StripPrefix "user", CamelToSnake)
       ]
  )
  instance
    ToJSON (MyType 'Foo)

deriving via
  ( CustomJSON
      '[ OmitNothingFields, -- Oh, we forgot to add 'UnwrapUnaryRecords' here!
         TagSingleConstructors,
         FieldLabelModifier (StripPrefix "user", CamelToSnake)
       ]
  )
  instance
    FromJSON (MyType 'Foo)

Proposal

I propose GHC supports deriving several instances in a single standalone declaration. I don't have a strong opinion on the syntax (nor have I put a lot of effort in thinking about it, I have to admit), though I give an example of what it could look like below:

deriving instance (Eq, Show, Read) (MyType 'Foo)

deriving via
  ( CustomJSON
      '[ OmitNothingFields,
         UnwrapUnaryRecords,
         TagSingleConstructors,
         FieldLabelModifier (StripPrefix "user", CamelToSnake)
       ]
  )
  instance
    (ToJSON, FromJSON) (MyType 'Foo)

Should this proposal get rejected, I think we should at least document this limitation.