#16241: Avoid orphan instances with OVERLAPPABLE (sometimes) -------------------------------------+------------------------------------- Reporter: AntC | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.1 Resolution: | Keywords: Operating System: Windows | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: #15135 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by AntC): Ah, I see that I don't have orphan instances here, by the letter of the law: * `instance MyShow [a]` is declared in the same module as the class, so isn't an orphan; * `instance MyShow T` and `instance MyShow [T]` are declared in the same module as `data T`, so aren't orphans. Never the less, the problem is in the same ballpark as orphan instances: incoherence due to not all applicable instances being visible in all modules. Specifically, `instance MyShow [T]` is not visible in module `Help`, where constraint `MyShow [a] =>` on the signature for `showHelp` needs it. Then `OVERLAPPABLE` on `instance MyShow [a]` says 'beware! there are other instances in other scopes.' Is there a more accurate term or keyword I should use? ('orphan instance' seems to get bandied around on Stackoverflow in that more general sense.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16241#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler