Re: seemingly inconsistent behavior for "kind-indexed" type constraints in GHC 7.6
Richard Eisenberg <eir <at> cis.upenn.edu>
2012-10-06 02:25:48 GMT
I think GHC is right in rejecting your instance, but perhaps not for the reasons you expect.
It would be possible to add another instance to NUnits:
instance NUnits Z '[()]
This would require OverlappingInstances, but because of the possibility of further instances, GHC
rightly does not assume that a matching instance is the only possible matching instance. So, without that
assumption, there is no reason GHC should unify ts with '.
For similar reasons, GHC does not bring the constraints on an instance into the context when an instance
matches. So, even if GHC did select the instance you want, it would not bring (' ~ ps) into the context.
To me, both of these problems have solutions: introduce a functional dependency on NUnits, n -> ts; and
declare the wanted instance to be "instance NUnits Z '", without the equality constraint. What's
interesting is that doing both of these changes didn't get the instance accepted. This *might* be a bug in
GHC, but I'd love another opinion before filing, because I'm really not sure.
Another issue at work here is that GHCi, by default, does not enable PolyKinds, even when it has loaded a file
with PolyKinds. The effect of this is that printing out polymorphic kinds will see all type variables
default to * without :set -XPolyKinds. That may have had a role to play in the commentary in the file, but I'm
Thanks for posting!
On Oct 5, 2012, at 5:49 PM, Nicolas Frisby wrote: