On Mon, Oct 7, 2013 at 10:22 AM, Edward Kmett <ekmett@gmail.com> wrote:

Am I correct in understanding your issue arises from manually rolled instances of Generic, not from Generic itself?

Exactly.

Wouldn't then perhaps the better fix be to resurrect the old rule for derived Typeable instances and apply it to Generic and Generic1 instead?

The new rule would be that if you hand-implemented Generic or Generic1 in your module it isn't Safe.

Ah... so you're proposing adding an extra rule to the GHC compiler itself?

Isn't it simpler to do it just with a library-level fix? I.e. my proposal is that GHC.Generics is marked as Unsafe. And then if you really want you can mark your module as TrustWorthy even with manual instances. (And then, yes, we'd need a GHC.Generics.Safe to pull the Generic symbol itself from when we just want to derive it.)

That would make it so that derived Generic, Generic1 would be considered Safe, and you wouldn't break literally every user of the library who care about Safe Haskell.

Isn't that also accomplished just by "import GHC.Generic.Safe (Generic)"? And the handful of hypothetical people that are using this right now can make that one-line fix. I will *personally* make that fix for all those people if they would come forward and share the link to their code ;-).

As it stands the things are damn-near impossible to get right instantiating them by hand anyways, so I expect this would affect only 1 or 2 users rather than all of them!

Exactly!