#4259: Relax restrictions on type family instance overlap ----------------------------------------+----------------------------------- Reporter: lilac | Owner: Type: feature request | Status: new Priority: normal | Milestone: 7.8.1 Component: Compiler (Type checker) | Version: 6.12.1 Keywords: | Os: Unknown/Multiple Architecture: Unknown/Multiple | Failure: None/Unknown Difficulty: Unknown | Testcase: Blockedby: | Blocking: Related: | ----------------------------------------+----------------------------------- Comment(by goldfire): Yes. The desire for injective type families (#6018) and this report seem something like treating symptoms instead of the disease. (Don't take that simile too far -- I'm not suggesting GHC's current treatment is somehow diseased, just not as powerful as some want.) What we all seem to want (even if we don't know it) is the ability to define an arbitrary rewrite system over types and for GHC to somehow, magically check that our rewrite system is consistent and then to apply it. I'm pretty sure that goal is impossible. But, we can certainly make inroads in that direction. Relaxing restrictions on instance overlap as described here is one way, and injective type families is another. My current thought on all of this is that we should consider some unified approach with a unified goal, instead of attacking this problem piecemeal. This unified approach may end up devolving into solving these two problems separately, but there should be an articulated reason why these two problems (this ticket and injectivity) are the two problems to solve. -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/4259#comment:27 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler