Given that each comes at the cost of the other, if I had to choose between which of these two STRef features I could have, I would pick being able to use it to recover equality over being able to lift newtype coercions through it. The former is increases expressivity while the latter accomplishes something that is achievable by simply using less newtypes in APIs where the need for this arises (not that I've ever actually needed to coerce an STRef in this way, but it would be interesting to hear from anyone who has needed this).

On Tue, Dec 4, 2018 at 12:26 AM Edward Kmett <ekmett@gmail.com> wrote:
Mostly because it means I wind up needing another construction to make it all go and can't just kick it all upstream. ;)

-Edward

On Mon, Dec 3, 2018 at 10:11 PM Richard Eisenberg <rae@cs.brynmawr.edu> wrote:
Why is it unfortunate? This looks like desired behavior to me. That is: I think these reference types should allow coercions between representationally equal types. Of course, that means that TestEquality is out.

Richard

On Dec 2, 2018, at 5:04 PM, Edward Kmett <ekmett@gmail.com> wrote:

On Sun, Dec 2, 2018 at 7:55 PM David Feuer <david.feuer@gmail.com> wrote:
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.

That is rather unfortunate, as it means most if not all of these would be limited to TestCoercion.

-Edward 

 
On Sun, Dec 2, 2018, 7:16 PM Edward Kmett <ekmett@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
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-Andrew Thaddeus Martin