[Design] Use higher-kinded data

[serras]

Inspired by some of the issues @cb372 raised about Proto3 and their "everything is optional" policy, I have been thinking of a new design to accomodate this scenario, but also others like GraphQL in which sometimes you want to use a data type only to indicate that you want to query some field.

Essentially the idea is to move from the usual first-order data:

data Person = Person { name :: Text, age :: Int }

to a higher-kinded version in which every field is wrapped in a type constructor:

data Person f = Person { name :: f Text, age :: f Int }

Now we can indicate that every field is optional by using Person Maybe, or required by setting Person Id. As I said before, it also opens possibilities to describe things like "I want this field":

data Wanted   a = Wanted | NotWanted deriving Functor
data Obtained a = HereYouAre a | DidNotAskForIt deriving Functor

queryGraphQL :: Person Wanted -> Person Obtained

Of course, I am more than happy to execute the plan.