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 <dreixel@gmail.com> wrote:

Hi,

On Tue, Oct 8, 2013 at 3:21 AM, Richard Eisenberg <eir@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

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