#15322: `KnownNat` does not imply `Typeable` any more when used with plugin -------------------------------------+------------------------------------- Reporter: chshersh | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: | typeable,knownnat Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #10348 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): My patch makes the reporting of unnecessary constraints more consistent -- in particular, accounting for built-in constraints. The original program still fails in the same way, for the reason explained in comment:1. What is the "real" fix? Well, despite comment:1 ''we'' know that `Typeable (n+1)` is never going to simplify to `Typeable n` (at least, not without using instances), because we know that `n+1 /= n`. But doesn't know that. The plug-in, I imagine, allows GHC to solve `KnownNat (n+1)` from `KnownNat n`. We want the plugin ''also'' to be able to say that `n` '''is apart from''' `n+1`; that is, they can never be equal. This is a property of `(+)`. So I think that perhaps domain-specific apartness should be part of what a typechecker plugin can specify. I'll leave this ticket open to discuss that idea. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15322#comment:13 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler