#9334: Implement "instance chains" -------------------------------------+------------------------------------- Reporter: diatchki | Owner: diatchki Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.9 Component: Compiler | Keywords: (Type checker) | Operating System: Unknown/Multiple Resolution: | Type of failure: None/Unknown Differential Revisions: | Test Case: Architecture: | Blocking: Unknown/Multiple | Difficulty: Unknown | Blocked By: | Related Tickets: | -------------------------------------+------------------------------------- Comment (by simonpj): Some thoughts * The instance chains described in the instance-chain paper are much more elaborate than your proposal here; in particular they involve backtracking search and a "fails" possibility. I imagine that is a deliberate narrowing of the specification on your part. * The behaviour you specify for instance chains is, I think, precisely what GHC does for overlappping instances ''when they are all declared in the same module''. See the bullets at the end of [http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class- extensions.html#instance-overlap 7.6.3.5 in the user manual]. I grant that putting all the overlapping equations together in one place is clearer, just as with closed type families. But you have the behaviour you want right now, I think. * I think you are arguing that we should ''replace'' overlapping instances with instance chains. That would render illegal any program that uses overlaping instnaces spread across modules. I suspect that would make many people cry, so we'd end up with both. If I have this right, its just a question of whether to support a chained syntax. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9334#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler