[Haskell-cafe] STM and unsafePerformIO/bracket

Alberto G. Corona agocorona at gmail.com
Fri Feb 7 14:51:38 UTC 2014

I use this trick, that unlike the previous workarounds permits concurrency
and does handle the IO computation in a civilized way, that is, it permits
brackets and so on:

notReallySafeButNotAsUnsafeAsUnsafeIOToSTM= safeIOToSTM

safeIOToSTM ∷  IO a → STM a
safeIOToSTM req= unsafeIOToSTM  $ do
  tv   ←  newEmptyMVar
  forkIO $ (req  ↠ putMVar  tv . Right)
            (λ(e ∷  SomeException) → putMVar tv $ Left e )
  r ←  takeMVar tv
  case r of
   Right x → return x
   Left e → throw e

Here the IO computation is run in another thread. Even If the STM
transaction is aborted, the IO computation finalizes. If the STM
transaction is retried, the IO computation is re-executed, so it is up to
you to take this into account depending on what you intend to do.
If the IO computation throws an exception the STM transaction throws it. As
far as I remember, this trick was used in a package time ago to do solve
the problem.

2014-02-04 Bertram Felgenhauer <bertram.felgenhauer at googlemail.com>:

> Rob Leslie wrote:
> > I’ve run into a difficulty I’d appreciate some advice to solve.
> [unsafePerformIO and bracket, within an STM transaction]
> There's a long-standing bug report for this issue,
>   https://ghc.haskell.org/trac/ghc/ticket/2401
> So the current situation is that bracket within unsafePerformIO
> (and unsafeIOToSTM) and STM transactions don't mix. I'm a bit
> surprised that this doesn't bite more people.
> Cheers,
> Bertram
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