#13773: Types are not normalized in instance declarations -------------------------------------+------------------------------------- Reporter: augustss | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: 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): It's a bit simpler than I realised. I think you are asking * That in any type pattern, the compiler should fully normalise * And then complain if any type functions remain * The evidence from this normalisation can be complete discarded. So {{{ type instance F (G Int) = blah }}} would normalise `(G Int)` to (say) `Tree [Bool]`, and then behave exactly as if you'd written {{{ type instance F (Tree [Bool]) = blah }}} If we later tried to solve a constraint involving `F (G Int)` we'd reduce the `G Int` to `Tree [Boo]]` and then (and only then) see the match with the `type instance`. I suppose that'd be straightforward, and clearly backward compatible. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13773#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler