#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 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`. 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. Interestingly, `matchHasField` already does this, but without explanation :-(. I also worry about the other classes treated specially in `matchGlobalInst`. For some (`matchCTuple` , `matchLiftedEquality`, `matchLiftedCoercible`) they can't fail, so that's fine. 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 point is that GHC's treatment of built-in classes may need attention because of Backpack; KnownNat is just an example. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15379#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler