#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 simonpj):
I think only if the instance is not marked OVERLAPPABLE
I'm not sure that's right. Currently [http://downloads.haskell.org/~ghc/master/users-guide/glasgow_exts.html #overlapping-instances the user manual for instance resolution] says this {{{ Eliminate any candidate IX for which there is another candidate IY such that both of the following hold: * IY is strictly more specific than IX. That is, IY is a substitution instance of IX but not vice versa. * Either IX is overlappable, or IY is overlapping. (This “either/or” design, rather than a “both/and” design, allow a client to deliberately override an instance from a library, without requiring a change to the library.) }}} The second bullet means that you don't need to say "OVERLAPPABLE" for an instance to be overlappable. It's enough to say "OVERLAPPING" in the overlapping instance. So you can't deduce anything much from the absence of "OVERLAPPABLE", sadly. I don't know if this is the right design. If we you could only overlap an explicitly-overlappable instance, we could commit to them more aggressively. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16241#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler