#15379: Don't reject user-written instances of KnownNat and friends in hsig files -------------------------------------+------------------------------------- Reporter: ezyang | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: backpack Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by ppk): Replying to [comment:7 simonpj]:
The current implementation only ever has instances for `KnownNat 1`, `KnownNat 2` etc. This is described in `Note [KnownNat & KnownSymbol and EvLit]` in `TcInteract`.
Yes that note was quite useful in fact.
But now you want to allow `instance KnownNat T` for some abstract type `T`; you explain this well in comment:4.
Very well; but I really dislike messing with `matchInstEnv`. Better: make `mathKnownNat` have an `otherwise` case that, instead of returning `NoInstance` uses `matchInstEnv`. With a careful `Note` to explain.
[snip]
But `Typeable` can fail; I wonder if you might end up wanting a `Typeable` instance in a `hsig` file. I think that's all... but the big
Okey I will think about it and try it out. The only thing puzzling for me is that the old code is returning Overlapping instances, which to me appears to be finding too many instances rather than finding too few. So there is some problem with my comment:4 which I cannot really pin-point. point is that GHC's treatment of built-in classes may need attention because of Backpack; KnownNat is just an example. Again the `Typeable` instances in backpack works fine //even with the old version// of the code (unlike `KnownNat` or `KnownSymbol`). In fact, I realised and has tweaked the test case to have all the three problematic classes (KnownNat, KnownSymbol and Typeable) but only the first two was giving the overlapping instance error. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15379#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler