How to apply overlapping pragma to derived instance

Question

Pragma OverlappingInstances is deprecated in GHC for quite awhile and OVERLAPPING pragma is a substitute for that.

instance {-# OVERLAPPING #- } ...

Though this is not the only way in Haskell to define a class instance. I cannot define overlapping instance through deriving and avoid pesky warning about deprecated OverlappingInstances.

None of below cases are working:

  deriving {-# OVERLAPPING #-} (Lift)
  deriving ({-# OVERLAPPING #-} Lift)

Of course, my real opinion about overlapping instances is "don't do that", but I know others see it differently. — dfeuer, Apr 28 '21 at 16:31

score 3 · Accepted Answer · answered Apr 28 '21 at 17:46

You need to use StandaloneDeriving for such instances, as well as for ones that need specialized instance contexts.

{-# language StandaloneDeriving, FlexibleInstances #-}

data T a = T

deriving instance {-# OVERLAPPING #-} Show (T Int)
instance {-# OVERLAPPABLE #-} Show (T a) where
  show ~T = "Tee-hee"

main = do
  print (T :: T Int)
  print (T :: T Char)

How to apply overlapping pragma to derived instance

1 Answers1