#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 diatchki): Replying to [comment:3 simonpj]:
* 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
Yeah, I thought that we should start simple :) I'll try to meet with Mark to understand better how the full system should work. For now, I just wrote up the part that I think fits easily with GHC. 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. Interestingly, even in my simple version, instance chains are a bit more expressive, because of the explicit ordering of instances. So we can write things like this: {{{ instance C Int a where ... else C a Int where ... }}} I am not sure how common cases like these are, but it is worth noting.
* 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.
Ah, not yet! I think the two can coexist for a while. Once we have a working version of instance chains we can see if existing overlapping instances code can be replaced, and if not, why not.
If I have this right, its just a question of whether to support a chained syntax.
For the simple implementation, I think I'll start by adding the syntax (in all passes of the front-end), and then modifying `InstEnv` to keep track of instance-chains rather than individual instances. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9334#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler