Re: Proposal: GHC.Generics marked UNSAFE for SafeHaskell

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 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 wrote:

...

Hi,

On Mon, Oct 7, 2013 at 10:32 AM, Dag Odenhall 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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