Document for review: evaluation order and state tokens

[Michael Snoyman]

On Mon, Aug 25, 2014 at 9:37 PM, Bertram Felgenhauer <
bertram.felgenhauer at googlemail.com> wrote:

> Dear Michael,
>
> Michael Snoyman wrote:
> > As part of trac ticket 9390[1], Simon PJ recommended that we try to get a
> > document written that clarifies some of the issues regarding evaluation
> > order, and get it included in the GHC wiki. After a few iterations with
> > review from Simon, I've got a first "publicly consumable" version
> available
> > at:
> >
> >
> https://www.fpcomplete.com/user/snoyberg/general-haskell/advanced/evaluation-order-and-state-tokens
>
> Thanks for writing this, it looks very useful.
>
> > I'd appreciate any feedback on this document before I add it to the wiki.
>
> [on unsafeDupablePerformIO]
> | And when the first thread completes the action, it may terminate the
> | execution of the other thread.
>
> I think it's worthwhile to stress that "bracket" does not help, because
> evaluation of the thunk in the other thread simply stops, without
> throwing an exception.
>
>
> Could you add something about unsafePerformIO inside STM? The upshot is
> that you get the behaviour of unsafeDupablePerformIO, except for the
> parallelism: When an STM transaction is interrupted to be retried, its
> evaluation simply stops. This is a known bug which causes real problems
> with FFI bindings, see
>
>   https://ghc.haskell.org/trac/ghc/ticket/2401
>   http://www.haskell.org/pipermail/haskell-cafe/2014-February/112555.html
>
> but it has been around for a long time.
>
> Cheers,
>
> Bertram
>

Thanks for the feedback, I've added information on both of these points to
the document (STM in its own section at the bottom, as I couldn't find a
good place where it fit with the existing flow).

Michael
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