#9334: Implement "instance chains" -------------------------------------+------------------------------------- Reporter: diatchki | Owner: diatchki Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.9 Component: Compiler | Keywords: (Type checker) | Architecture: Unknown/Multiple Resolution: | Difficulty: Unknown Operating System: | Blocked By: Unknown/Multiple | Related Tickets: Type of failure: | None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by diatchki): I've chatted with Mark and Garrett (the authors of the "Instance Chains" papers) and we've decided that there are really three separate features here: 1. "Instance Groups", which is what is outlined in this ticket, and enables programmers to order instances explicitly, rather than using more/less general realtions. 2. "Fails instances", which are of the form `instance Num Char fails`; they enable programmers to state explicitly that an instance should never exits. Interestingly, I just found a very related ticket asking for the same sort of thing (#7775). 3. "Using instance contexts when selecting instances (aka backtracking)": currently, if the head of an instance matches a goal, GHC commits to it and then fails if it encounters an error; an alternative design would be to back-track and try a different option (e.g., next member of an instance group, or a more general matching instance). I think that (1) and (2) are useful and shouldn't be too hard to implement in GHC. (3), however, seems like more work. Also, there are programs that rely on GHC's current behavior. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9334#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler