#14938: Pattern matching on GADT does not refine type family parameters -------------------------------------+------------------------------------- Reporter: kcsongor | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.2 checker) | Keywords: GADTs, Resolution: | TypeFamilies, TypeInType Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by kcsongor): Replying to [comment:1 goldfire]:
But if you have a type-level `Refl` here, then `x` and `y` really will be the same. The equation you want is `R x x Refl = Refl`, like in those other dependently typed languages.
In the end I did end up using that equation, but my original idea was to try and avoid non-linear patterns, and passing in an explicit proof seemed like a sensible idea. For example, Idris lets me write {{{#!idris r : x -> y -> x = y -> x = y r x y Refl = Refl }}} (But of course non-linear patterns are not supported) Is there something obvious I missed that would explain why this form of matching should not accepted? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14938#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler