#12837: Disallow users to write instances for KnownNat and KnownSymbol -------------------------------------+------------------------------------- Reporter: adamgundry | Owner: sean Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: newcomer Operating System: Unknown/Multiple | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D2898 Wiki Page: | -------------------------------------+------------------------------------- Description changed by sean: @@ -9,3 +9,3 @@ - This is accepted (albeit with a warning about the missing `natSing` - method). I think it should be rejected, as should an instance of - `KnownSymbol`. Compare doing the same thing with `Typeable`, which says: + Originally this was accepted (albeit with a warning about the missing + `natSing` method). It is now rejected, as with instances of `KnownSymbol`, + in the same way as instances of `Typeable`, which says: New description: Consider the following patent nonsense: {{{#!hs {-# LANGUAGE FlexibleInstances #-} import GHC.TypeLits instance KnownNat n }}} Originally this was accepted (albeit with a warning about the missing `natSing` method). It is now rejected, as with instances of `KnownSymbol`, in the same way as instances of `Typeable`, which says: {{{ Class ‘Typeable’ does not support user-specified instances }}} -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12837#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler