#8827: Inferring Safe mode with GeneralizedNewtypeDeriving is wrong -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.12.1 Component: Compiler | Version: 7.9 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #8226, #8745 | Differential Revisions: -------------------------------------+------------------------------------- Comment (by goldfire): A few reactions: * I think this proposal holds water, from a technical standpoint. * I'm worried about the efficiency of `Safe`-inference. This proposal requires doing a recursive check, potentially loading new interface files, just for inferring `Safe`ty. This makes me sad. I have no suggestion for improvement, however. * Along similar lines, just expanding a module's import list can change it from un`Safe`-inferred to `Safe`-inferred. Are the extra imported data constructors considered used in the module? That is, do they get "redundant import" warnings? Either answer to that last question seems wrong. * The reason that the default role is representational (phantom, actually) is solely for backward compatibility. In a newly-minted language, I think choosing nominal here would be rather uncontroversial. Is it worth recasting this debate as a move toward a nominal-default? With this change, the `Safe`ty issue goes away. This would be socially harder, but perhaps better in the long run. ------------- Here's a stab at that last point: Plan to introduce, in GHC 7.14, a new extension `PhantomDefaultRoles`. With this extension, roles would work as they do today. Roles would also be `Safe`, because the author declares their knowledge of roles by using this extension. Without the extension enabled, a nominal role is default. We would introduce a warning in GHC 7.12 about this impending change. I don't have a good proposal for how to implement the warning, though -- it would amount to an inefficient recursive check. Maybe someone has a better idea. By making this change via an extension, it's easy for library-writers to upgrade: include a straightforward conditional block in a .cabal file. All packages could be upgrade by something like (I didn't look up concrete syntax) `if impl(ghc >= 7.12) default-extensions: PhantomDefaultRoles`. Easy! ------- In sum, I think that the proposal in comment:42 is feasible, but I'm worried about efficiency. But I think that tackling the larger problem of default roles is better in the long run. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8827#comment:43 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler