default roles

[José Pedro Magalhães]

Hi,

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