2011/7/22 Simon Peyton-Jones
I talked to Dimitrios. Fundamentally we think we should be able to handle recursive superclasses, albeit we have a bit more work to do on the type inference engine first.
The situation we think we can handle ok is stuff like Edward wants (I've removed all the methods):
class LeftModule Whole m => Additive m class Additive m => Abelian m class (Semiring r, Additive m) => LeftModule r m class Multiplicative m where (*) :: m -> m -> m class LeftModule Natural m => Monoidal m class (Abelian m, Multiplicative m, LeftModule m m) => Semiring m class (LeftModule Integer m, Monoidal m) => Group m class Multiplicative m => Unital m class (Monoidal r, Unital r, Semiring r) => Rig class (Rig r, Group r) => Ring r
The superclasses are recursive but a) They constrain only type variables b) The variables in the superclass context are all mentioned in the head. In class Q => C a b c fv(Q) is subset of {a,b,c}
Question to all: is that enough?
This would perfectly address all of the needs that I have had! -Edward