On 2 Dec 2010, at 08:04, Roman Leshchinskiy wrote:
On 01/12/2010, at 13:04, Conor McBride wrote:
I was being slightly facetious. I was also wondering what value the Pointed class would add, by asking which instances of Functor would fail to be Pointed. The constantly Zero (or Void, if you insist) Functor? What else?
Any tuple type. Or, more generally, any multifunctor or anything that carries more information than just the a.
I can see that we're not going to get instance Pointed ((,) a) anytime soon, but is that the thing we'd want anyway? Seems a bit peculiar to me, given that Pointed f basically asserts the inhabitation of f (). Why don't we just have an Inhabited class? class Inhabited x where something :: x Then we can build Pointed once for all instance Inhabited (f ()) => Pointed f where pure x = fmap (uconst x) something where uconst :: x -> () -> x uconst = const Now, write a theorem prover! instance Inhabited () where something = () instance (Inhabited a, Inhabited b) => Inhabited (a, b) where something = (something, something) Wouldn't that give us instance Inhabited a => Pointed ((,) a) ? Or, slightly more scarily, but without an extra class instance Pointed (Const a) => Pointed ((,) a) where pure b = (getConst (pure ()), b) And we can all have fun writing Djinn in type class prolog (didn't Oleg do that already?) until somebody points out that every type is inhabited by bottom. However, fun though it may be, I see no evidence yet that the game is worth the candle. All the best Conor