#9858: Typeable instances should be kind-aware -------------------------------------+------------------------------------- Reporter: dreixel | Owner: Type: bug | Status: new Priority: highest | Milestone: 7.10.1 Component: Compiler | Version: 7.9 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Changes (by rwbarton): * owner: dreixel => * status: merge => new Comment: (Reopening for discussion) Sorry for getting here a bit late; but I have some concerns about this change as it stands currently. Abstractly my main concern is that we've made a change (implicitly deriving Typeable everywhere) for the wrong reasons, and concretely my main fear, given the timing of this change, is that we'll find the new approach is untenable for some reason or other and be forced back into a world in which explicit deriving Typeable is required. I'm certainly not trying to say that implicitly deriving Typeable is the wrong decision; but it seems that it arose as fallout from the issue in this ticket and #10000 for technical reasons, and not because we decided that we really want it. We will always be able to transition to implicit deriving Typeable in the future, but transitioning back to explicit deriving Typeable loses compatibility with programs written for an earlier version of the compiler that had implicit deriving Typeable. So, seeing as we are in the RC phase for 7.10, my recommendation would be: - Keep the requirement to explicitly derive Typeable (and keep AutoDeriveTypeable, etc.) as in 7.10 RC 2 - Don't actually generate evidence terms when deriving Typeable. Instead mark the type as "Typeable-allowed". - When generating evidence in the Typeable solver, check that all of the types involved are marked as "Typeable-allowed". -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9858#comment:64 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler