We have a constraint (a~b) that can appear in a type signature f :: (a~b) => a -> b We can also have a type-level equality function type family Eq a b where Eq a a = True Eq a b = False The type-level Boolean reflects the (a~b) constraint precisely. You sound as if you want a type family Satisfied, so you type family Satisfied :: Constraint -> Bool Then you could have instance Ord T where ... type instance Satisfied (Ord T) = True The difficulty is that we can never return False, because you can always make T an instance of Ord "later". And you can't have Satisfied (Ord T) returning True in one place and Falsie in another. It can get stuck (not reduce) but it can't return False. I don't know if this would be useful to you. And for more complex instances lie instance Ord a => Ord [a] I don't know what you want to say for type instance Satisfied (Ord a) = ??? Simon From: Libraries [mailto:libraries-bounces@haskell.org] On Behalf Of Ryan Newton Sent: 15 October 2013 04:03 To: Pedro Magalhães (dreixel@gmail.com) Cc: Haskell Libraries; Andres Löh; Ganesh Sittampalam; ghc-devs@haskell.org Subject: Re: Proposal: GHC.Generics marked UNSAFE for SafeHaskell 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<mailto:dreixel@gmail.com>> wrote: Hi, On Mon, Oct 7, 2013 at 10:32 AM, Dag Odenhall <dag.odenhall@gmail.com<mailto: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 _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org<mailto:ghc-devs@haskell.org> http://www.haskell.org/mailman/listinfo/ghc-devs