Field accessor type inference woes

[Barney Hilken]

The two points in AntC's message suggest a possible compromise solution. Unless I've missed something,
this allows nested fields, fixed type projections, and HR fields. The only restriction is that HR fields must be
fixed type, though they can still be used in multiple records.

1. Use an associated type class for Has, to solve the nesting problem:

	class Has (r :: *) (x :: Symbol) where
		type GetField r x :: *
		getField :: r -> GetField r x

(Maybe a fundep would also work, but I'm more used to thinking with associated types.)


2. Introduce a declaration for fixed type fields:

	field bar :: Bar

is translated as:

	class Has_bar r where
		bar :: r -> Bar

	instance Has_bar r => Has r "bar" where
		GetType r "bar" = Bar
		getField = bar


3. Undeclared fields and those declared typeless don't have their own class:

	field bar

is translated as

	bar :: Has r "bar" => r -> GetType r "bar"
	bar = getField


4. Now you can use HR fields, provided you declare them first:

	field bar :: forall b. b -> b

is translated as:

	class Has_bar r where
		bar :: r -> forall b. b -> b

	instance Has_bar r => Has r "bar" where
		GetType r "bar" = forall b. b -> b
		getField = bar

which doesn't look impredicative to me.

Does this work, or have I missed something?

Barney.