The issue with such an explicit false is that it requires more magic on behalf of the compiler. 

It would have to be filled in whenever an explicit `instance Eq Blah` was written. 

Recall that 

deriving instance Eq Blah

can occur after the data type declaration site and may have to be used for many more complicated recursive data types, so it isn't sufficient to fill in at the data type declaration.

-Edward


On Mon, Oct 14, 2013 at 11:41 PM, Ryan Newton <rrnewton@gmail.com> wrote:
But what stops the user from defining their own instances if they in fact did not derive it?

The explicit "False" in Pedro's formulation seems to serve this purpose.



On Mon, Oct 14, 2013 at 11:20 PM, Nicolas Frisby <nicolas.frisby@gmail.com> wrote:
The formulation as a type family seems to conflict with the open-world principle. Would a class-based encoding instead be sufficient?

-- the proposed, special wired-in class
class Derives (t :: k1) (c :: k2)

-- GHC would infer these from Pedro's example's declarations
instance Derives MyData Eq
instance Derives MyData Generic
instance Derives MyData Show

NB that there is no instance Derives MyData Ord, but standalone deriving could yield one "later"

On Mon, Oct 14, 2013 at 10:02 PM, Ryan Newton <rrnewton@gmail.com> wrote:
Hey, that's an awesome formulation!  Thanks Pedro.

Any idea how much work this would be to implement in GHC, if it did garner approval?


On Tue, Oct 8, 2013 at 3:48 AM, José Pedro Magalhães <dreixel@gmail.com> wrote:
Hi,

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?


Cheers,
Pedro

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