Proposal: Make unsafeInterleaveST safer

[David Feuer]

We ended up just making this change unilaterally. The previous
situation was just too horrible. Sorry for the noise.

David

On Tue, Mar 21, 2017 at 3:46 PM, David Feuer <david.feuer at gmail.com> wrote:
> Currently, unsafeInterleaveST is considerably less safe than
> unsafeInterleaveIO. Specifically, very careful discipline is required
> to make code using it thread-safe. This means that unsafeInterleaveST
> actually behaves like unsafeDupableInterleaveIO. See Edward Kmett's
> blog post from several years ago [*] for details.
>
> I propose that we should
>
> 1. Make unsafeInterleaveST behave only as unsafely as unsafeInterleaveIO, and
>
> 2. Add a new unsafeDupableInterleaveST function implementing the
> current unsafeInterleaveST behavior.
>
> The alternative would be to leave unsafeInterleaveST alone and add a
> new unsafeNonDupableInterleaveST, but that seems like an absolutely
> terrible mismatch of names. Since any currently-correct code using
> unsafeInterleaveST will continue to work (albeit more slowly), I don't
> think that's justified.
>
> Thanks,
> David
>
> [*] https://www.schoolofhaskell.com/school/to-infinity-and-beyond/older-but-still-interesting/deamortized-st#newsflash--unsafeinterleavest-is-unsafe-
> f