On Mon, Oct 7, 2013 at 10:32 AM, Dag Odenhall <dag.odenhall@gmail.com> wrote:

Here‘s a thought: doesn’t Generic already have an unused phantom type that's only there “just in case we need to add something in the future”?

No, it doesn't.

Just a thought: what if we had a type family

type family Derives (t :: k1) (c :: k2) :: Bool

which would automatically be instantiated by GHC appropriately? E.g., if the user had the following code:

data MyData = MyData deriving (Eq, Generic)

deriving instance Show MyData

instance Ord MyData

GHC would automatically instantiate:

type instance Derives MyData Eq      = True
type instance Derives MyData Generic = True
type instance Derives MyData Show    = True
type instance Derives MyData Ord     = False

Would this be something Ryan could use for detecting safe instances for LVish?