#10318: Cycles in class declaration (via superclasses) sometimes make sense. -------------------------------------+------------------------------------- Reporter: ekmett | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.10.1 checker) | Keywords: Resolution: | Architecture: Operating System: Unknown/Multiple | Unknown/Multiple Type of failure: GHC rejects | Test Case: valid program | Blocking: Blocked By: | Differential Revisions: Related Tickets: | -------------------------------------+------------------------------------- Comment (by simonpj): OK, well I don't understand much of comment:2, but I'll take it on trust. What is difficult about recursive superclasses? I'll use the example in the Description. * '''"Given" superclasses'''. Suppose we have {{{ f :: C a => .... }}} and on f's RHS we have the constraint `(D a)`. GHC treats `(C a)` as a "given", and then adds all of `C`'s superclasses as "givens" too, and their superclasses, and so on; in case they are useful to prove the constraints arising from f's RHS. For example, `(D a)` might be a distant superclass of `(C a)`. If there are an infinite number of superclasses, this "add all superclasses" idea isn't going to work. I expect you'll say "well, in my case `Frac (Frac a) ~ Frac a`, so the second-level superclass is the same as the first". That may be true, but it requires somewhat-sophisticated reasoning; a kind of fixpoint calculation. I can see it might be possible, but the penalty for (the programmer) getting it wrong is severe; the type checker goes into a loop. * '''"Wanted" superclasses'''.Similarly, for "wanted" constraints we fluff up the superclases in case one of them has a functional dependency that will help us make progress. * '''Instance declarations'''. In an instance declaration we must cough up a witness for the superclass. This part might not be too hard. At lesat in the monomorphic cases you give, like `instance IntegralDomain Integer`, we seek an instance for `IntegralDomain (Frac Integer)`, which is just `IntegralDomain Rational`, and we have that. Again things could go badly wrong, but I think the existing superclass code might "just work". -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10318#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler