#8141: Liberalising IncoherentInstances -------------------------------------+------------------------------------ Reporter: nomeata | Owner: Type: feature request | 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: -------------------------------------+------------------------------------ Changes (by simonpj): * status: closed => new * resolution: fixed => Comment: I can't quite see from the diffs, but I would like the user manual to pretty much give the specification in my email: {{{ More precisely, when trying to find an instance matching a target constraint (C tys), a) Find all instances matching (C tys); these are the candidates b) Eliminate any candidate X for which another candidate Y is strictly more specific (ie Y is a substitution instance of X), if either X or Y was complied with -XOverlappingInstances c) Check that any non-candidate instances that *unify* with (C tys) were compiled with -XIncoherentInstances d) If only one candidate remains, pick it. Otherwise if all remaining candidates were compiled with -XInccoherentInstances, pick an arbitrary candidate }}} I don't think it yet does. I'll take a look. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8141#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler