[Haskell-cafe] Mutable data structures and asynchronous exceptions

[info at maximka.de]

I hope you'll find some answers to you questions in the "Asynchronous Exceptions" chapter http://chimera.labs.oreilly.com/books/1230000000929/ch09.html

Regards,
Alexei
> On 28 September 2017 at 13:00 Станислав Черничкин <schernichkin at gmail.com> wrote:
> 
> 
> It's quite hard to implement mutable data structures in presence of
> asynchronous exceptions. Since exception can arise at any point, it is not
> possible to guarantee atomicity of operation, hence mutable data structure
> may remain in incorrect state in case of interruption. One can certainly
> use maskAsyncExceptions# and friends to protect critical regions, but
> masking function are living in IO, mutable data structures on other hand
> trend to be state-polymorphic (to allow it usage in ST).
> 
> This lead to conflicting requirements:
> - One should not care about asynchronous exceptions inside ST (it is not
> possible to catch exception in ST, hence not possible to use something in
> invalid state). More over, it is not even possible to do write
> “exception-safe” code, because masking functions not available.
> - One should provide accurate masking then using same data structures in IO.
> 
> So I want do discuss several questions topics on this case.
> 
> 1. Impact. Are async exceptions really common? Would not be easier to say:
> “ok, things can go bad if you combine async exceptions with mutable data
> structures, just don't do it”.
> 
> 2. Documentation. Should library authors explicitly mention async
> exceptions safety? For example
> https://hackage.haskell.org/package/hashtables – is it async exceptions
> safe when used in IO? Or even worse
> https://hackage.haskell.org/package/ghc-prim-0.5.1.0/docs/GHC-Prim.html#v:resizeMutableByteArray-35-
> - what will happened in case of async exception? This functions is
> sate-polimorphic, will it implicitly mask exceptions if used from IO?
> 
> 3. Best practices. How should we deal with problem? Is creating separate
> versions of  code for ST and IO is the only way? Probably it is possible to
> add “mask” to something like
> https://hackage.haskell.org/package/primitive-0.6.2.0/docs/Control-Monad-Primitive.html#t:PrimMonad
> emit mask in IO instance and NOOP in ST version? Or maybe somebody know
> better patterns for async exeption safe code?
> 
> -- 
> Sincerely, Stanislav Chernichkin.
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