Re: Proposal: GHC.Generics marked UNSAFE for SafeHaskell

I'd much rather have code that can compile as Safe rather than Trustworthy. Trustworthy code is a pain to annotate the safety of correctly. A one line change to GHC to ensure the safety of GHC.Generics and to maximize the number of operations you can use safely seems entirely reasonable. Otherwise I have to mark perfectly Safe-by-construction code Trustworthy, enlarging the trusted code base considerably. Using properly constructed Generic instances is perfectly safe. That library level fix has knock-on effects across the entire ecosystem for anyone who cares about SafeHaskell. I tend to instantiate GHC.Generics for any type in most of my packages. A quick grep finds 209 modules broken in just the projects I have open in my working directory. That is a mixture of public and private repos. The majority of which are public. So, I first have to change a couple hundred import statements... conditionally on compiler version, since I maintain wide support windows. This fix would literally break SafeHaskell for me silently across almost everything I have written, forcing me to run through tons of packages trying to figure out what I have to change to get it back. I've done this song and dance before trying to get lambdabot to build, since it uses lens these days. If I don't get every one of them right lens builds as unsafe. There isn't a good debugging process for this, as the only real way I have to know it worked is to go through each one and make sure I didn't compromise safe haskell by looking at the resulting haddocks for each case. The code that *uses* GHC.Generics uses their internals and is safe by construction. It is only the instance construction that is unsafe, so lets mark it as such. -Edward On Mon, Oct 7, 2013 at 11:11 AM, Ryan Newton wrote:

On Mon, Oct 7, 2013 at 10:22 AM, Edward Kmett 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!