Proposal: GHC.Generics marked UNSAFE for SafeHaskell

John Lato jwlato
Mon Oct 7 05:13:02 UTC 2013

On Sun, Oct 6, 2013 at 10:14 PM, Ryan Newton <rrnewton at> wrote:

> On Sun, Oct 6, 2013 at 6:28 PM, Ganesh Sittampalam <ganesh at>wrote:
>>  - Referential transparency: e.g. no unsafePerformIO
>  - Module boundary control: no abstraction violation like Template
>> Haskell and GeneralizedNewtypeDeriving
>>  - Semantic consistency: importing a safe module can't change existing
>> code, so no OverlappingInstances and the like
> Is this change necessary to preserve the existing properties, or are you
>> hoping to add a new one?
> I'm not currently aware of ways to break these invariants *just* with
> GHC.Generics.  Hmm, but I would like to know why it is marked trustworthy
> and not inferred-safe...

How about this demo repo?

I'm really not a safe haskell expert, but I believe this is a demonstration
of using GHC.Generics to violate a module's abstraction boundaries with
SafeHaskell enabled.

If I'm incorrect, I would appreciate if somebody could explain my error.
If, however, I'm correct, then I think that Ryan's proposal of marking
GHC.Generics Unsafe is the best way to remedy the problem.

A possible stumbling block may involve base and package-trust, but I'm not
certain of the current status.

John L.
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