Ahoy, I have the following type class. -- | An entity whose underlying information spans zero or more columns class ResultEntity a where -- | Build an instance of @a@. parseEntity :: RowParser a default parseEntity :: (EligibleDataType meta cons a) => RowParser a parseEntity = parseGeneric The EligibleDataType constraint allows me to construct a RowParser with the help of generics. It is so minimal that I am considering to transform the hole thing into this. -- | An entity whose underlying information spans zero or more columns class ResultEntity a where -- | Build an instance of @a@. parseEntity :: RowParser a instance {-# OVERLAPPABLE #-} (EligibleDataType meta cons a) => ResultEntity a where parseEntity = parseGeneric This makes defining new instances of ResultEntity obsolete for types that already satisfy the EligibleDataType constraint (basically everything that has a Generic instance). It is still possible to provide a hand-rolled instance. Undecidable and overlapping instances seem like a big no-no. Even after reading up on them, I can't get rid of the feeling that this might be dangerous. *Is it dangerous? *I am targeting GHC 8.0.1 with this. - Ole