TestEquality for references

[David Feuer]

Unfortunately, testEquality for STRef is not at all safe, for reasons we've
previously discussed in another context.

    testEquality :: STRef s a -> STRef s b -> Maybe (a :~: b)

  let x = [1, 2]
  foo :: STRef s [Int] <- newSTRef x
  let bar :: STRef s (ZipList Int) = coerce foo
  case testEquality foo bar of UH-OH

I suspect testCoercion actually will work here.

You could patch up the problem by giving STRef (and perhaps MutVar#) a
stricter role signature:

type role STRef nominal nominal

That might not break enough code to worry about; I'm not sure.

On Sun, Dec 2, 2018, 7:16 PM Edward Kmett <ekmett at gmail.com wrote:

> I'd like to propose adding a bunch of instances for TestEquality and
> TestCoercion to base and primitive types such as: IORef, STRef s, MVar as
> well as MutVar and any appropriately uncoercible array types we have in
> primitive.
>
> With these you can learn about the equality of the types of elements of an
> STRef when you go to
>
>      testEquality :: STRef s a -> STRef s b -> Maybe (a :~: b)
>
> I've been using an ad hoc versions of this on my own for some time, across
> a wide array of packages, based on Atze van der Ploeg's paper:
> https://dl.acm.org/citation.cfm?id=2976008 and currently I get by by
> unsafeCoercing reallyUnsafePointerEquality# and unsafeCoercing the witness
> that I get back in turn. =/
>
> With this the notion of a "Key" introduced there can be safely modeled
> with an STRef s (Proxy a).
>
> This would make it {-# LANGUAGE Safe #-} for users to construct
> heterogeneous container types that don't need Typeable information about
> the values.
>
> Implementation wise, these can either use the value equality of those
> underlying primitive types and then produce a witness either by
> unsafeCoerce, or by adding new stronger primitives in ghc-prim to produce
> the witness in a type-safe manner, giving us well typed core all the way
> down.
>
> -Edward
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20181202/46bbedf1/attachment.html>