default roles

[Miguel]

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, Jos? Pedro Magalh?es <dreixel at gmail.com>wrote:

> 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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>
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