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.
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