#14832: QuantifiedConstraints: Strengthening context causes failure -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Resolution: | Keywords: | QuantifiedConstraints wipT2893 Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): GHC is very careful about picking instances. See http://downloads.haskell.org/~ghc/master/users-guide/glasgow_exts.html #overlapping-instances, esp the bit about "all instances that unify". The constraint `rel xx xx` matches both `(Refl rel)` and `(Trans rel)`; but the (head of) the former is an instance of the (head of) the latter so it should win. At least with overlapping instances on. (There's no way to add an OVERLAPPING pragama to an individual quantified constraint -- yet anyway.) The constraint `rel xx zz` could (if `zz` was further instantiated) match`(Refl rel)`, so GHC refains from picking it. `IncoherentInstnaces` might help. In short, right now I'm being very conservative about ambiguity. Once we pick the "wrong" path there is no way back -- GHC does no backtracking. I'm inclined to park this one until we have the basics nailed. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14832#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler