Superclass Cycle via Associated Type

Edward Kmett ekmett at
Fri Jul 22 21:06:43 CEST 2011

2011/7/22 Simon Peyton-Jones <simonpj at>

> 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!

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