#8338: Incoherent instances without -XIncoherentInstances -------------------------------------+------------------------------------ Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by goldfire): In my comment, I was referring to overlapping instances from modules that ''don't'' use `OverlappingInstaces`. Here's how I can see it happening: * Suppose package `dayta` exports a datatype `T` and package `klass` exports a class `C`. * Now, packages `a` and `b` both import `T` and `C` from their respective packages and define an instance `C T`. * Package `c` wants to use the relevant modules from both `a` and `b`. Disaster with eager checking of overlap. I've used packages here instead of modules to emphasize that the authors of all of these parts may be different. From a technical standpoint, replacing "package" with "module" would also work here. I should say I'm not really disagreeing with illissius about eager checking, just drawing out the example of where this might be a problem. I find it very hard to come up with an opinion on this issue that I would feel comfortable defending! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8338#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler