In an attempt to dredge out the mud from these waters, I have updated the wiki page at http://hackage.haskell.org/trac/ghc/wiki/NewAxioms to have the correct details about the current implementation (which has been merged with HEAD). A few salient points: * Open (normal, old-fashioned) type families are essentially unchanged. In particular, coincident overlap and non-linear patterns *are* allowed. * The overlap check between open type family instances now does a unification without an "occurs check" to mark (x, x) and ([y], y) as overlapping, as necessary for type soundness. * Coincident overlap within closed families works just fine, the way you might expect. In particular, in both the theory and the implementation, any set of type instances you could write in an open family can be combined in a closed family, and they will behave in exactly the same way. This is a marked improvement over the last implementation. As for AntC's finer-grained UndecidableInstances: I think that would be great, too. In general, I'm of the opinion that the brutal termination checker could use some improvements. But, there's little incentive to do so, with the ease of just saying UndecidableInstances. Even then, though, some reasonable type-level computations really don't terminate in all cases (division) and these still have a role to play. Richard PS: Sorry to those of you who have used "type instance where" and now find that your code doesn't compile. We (Simon, Dimitrios, and I) put a good deal of thought into the original design. But, we really liked the new one more. Given that the feature never made it into a released GHC, we thought it was OK to make this breaking change before it became impossible. Note that you can simulate the openness of "type instance where" by using an open family with closed family helpers.
-----Original Message----- From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell- users-bounces@haskell.org] On Behalf Of AntC Sent: 24 June 2013 03:59 To: glasgow-haskell-users@haskell.org Subject: Re: Repeated variables in type family instances - UndecidableInstances
Richard Eisenberg
writes: ... The plan of action is to use the check labeled (B) on the wiki page. This check does *not* ban all nonlinear type families.
Thanks Richard, great! Then the focus of attention moves to infinite types.
I don't think anybody intentionally wants infinite types, so UndecidableInstances ought to be switched off, to catch unintended instances.
But often there's a need for one or two instances to break the coverage conditions. (For example one of Oleg's standard techniques is to introduce a 'helper class' that has the same parameters as the based-on class, plus some extra parameter that drives instance selection, and is computed from the types of the arguments. It's not easy to see at this stage how that technique will translate into 'closed type families'.)
The trouble with the UndecidableInstances flag is that it's a very blunt instrument module-wide. A 'nice to have' would be to make it finer-grained: - set Undecidableness on a per-instance or per-family basis. - or even: validate that the RHS of this instance uses a decidable family but allow the RHS to break cover compared to LHS
(OK, I know that for a 'decidable' family there could be instances declared in other modules that get compiled with a different flag setting. But with 'closed type families', that can't happen, right?)
AntC
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