On further reflection I have a question. Under the limited design below, which Edward says will do all he wants:
· The mutually recursive classes (call them A, B, C) must be defined all together. Like class B a => A a; class C a => B a; class A a => C a · If a type T is an instance of any of those classes, it must be a member of all of them · If a function f has type f :: A a => blah, then the signature f :: B a => blah and f :: C a => blah would work equally well In short, I see no advantage to making A,B,C separate classes compared to simply unioning them into a single class. Bottom line: adding recursive superclasses with the restrictions I describe below would add no useful expressive power. But it would cost effort to implement. So why do it? Maybe I’m missing something. Simon From: Edward Kmett [mailto:ekm...@gmail.com] Sent: 22 July 2011 20:07 To: Simon Peyton-Jones Cc: Gábor Lehel; glasgow-haskell-users@haskell.org Subject: Re: Superclass Cycle via Associated Type 2011/7/22 Simon Peyton-Jones <simo...@microsoft.com<mailto:simo...@microsoft.com>> 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 r 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
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