I don't understand it either. Type family solution, however, seems wrong. See, if we, somehow, make something nominal when it has to be representational — well, some code won't compile, but nothing really bad happens. If, on the other hand, we by some miracle make something representational when in should be nominal — we can get a runtime error. It seems to be very similar to how type classes work, with "nominal" being the default, and "representational" a type class.
Consider, for example, the "Tricky" example from the slides, slightly changed: data Tricky2 a b c = MkTricky2 (a c) (b c) Currently parameter c would be nominal. I suggest that it should be representational if and only if it's representational for BOTH a and b. WIth type classes it would be very simple: instance (HasRepresentationalParameter a, HasRepresentationalParameter b) => HasRepresentationalParameter (Tricky2 a b) With type families... well, apparently I don't have enough milliolegs to figure out how to do it. On Tue, Oct 8, 2013 at 12:01 PM, José Pedro Magalhães <drei...@gmail.com>wrote: > Hi, > > On Tue, Oct 8, 2013 at 3:21 AM, Richard Eisenberg <e...@cis.upenn.edu>wrote: > >> We considered this for a while, but it led to a strange design -- to do >> it right, you would have to import all constructors for all datatypes >> *recursively* out to the leaves, starting at the datatypes mentioned in the >> class for which you wanted to use GND. This would mean potentially a whole >> lot of imports for symbols not actually used in the text of a program. >> > > I'm not sure I understand why constructors are involved in this. Wouldn't > something like > the following potentially be useful? > > data Role = Nominal | Representational | Phantom | Fun Role Role > > type family HasRole (t :: k) :: Role > > data MyData a b = MyData a > data MyGADT a b where MyGADT :: MyGADT a Int > > type instance HasRole MyData = Fun Representational Phantom > type instance HasRole MyGADT = Fun Representational Nominal > type instance HasRole Traversable = Nominal > > HasRole instances would be automatically given by GHC. > > > Cheers, > Pedro > > > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > >
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