Control.Concurrent.STM.atomically was nested

[mitchellwrosen]

foo.hs:

{-# LANGUAGE OverloadedStrings #-}

import Control.Concurrent.STM
import Data.Yaml

main = do
  atomically (let x = decode "" :: Maybe Value in x `seq` x)
  pure ()

result:

$ stack runghc --resolver lts-6.3 foo.hs
foo.hs: Control.Concurrent.STM.atomically was nested

Uh...? I don't believe this is actually a yaml bug because it doesn't depend on stm, but one of its dependencies, somewhere, is calling some unsafePerformIO (atomically foo)-type thing. Not too sure where to look from here.

[mitchellwrosen]

Hmm... I looked into this a bit more. I think it's the tryAny in Data.Yaml.Internal.decodeHelper_, which uses waitCatch, inside of decode's unsafePerformIO. Plus, you know, laziness and stuff.

[mitchellwrosen]

I'm guessing rewriting decode to not use unsafePerformIO would be a big undertaking, so I think some kind of comment disclaimer saying "do not use decode inside of an STM computation" would be sufficient.

[snoyberg]

Actually, there's probably a better solution available: modify tryAny to not use the async package, and instead of using TVars use MVars for the synchronization, which have no nesting issue. Are you interested in sending a PR to enclosed-exception to do that?

[mitchellwrosen]

Sure, I can give it a go.
On Jun 15, 2016 1:01 AM, "Michael Snoyman" notifications@github.com wrote:

Actually, there's probably a better solution available: modify tryAny to
not use the async package, and instead of using TVars use MVars for the
synchronization, which have no nesting issue. Are you interested in sending
a PR to enclosed-exception to do that?

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[mitchellwrosen]

jcristovao/enclosed-exceptions#9

[snoyberg]

Can you confirm that this issue does not exist with the newly released enclosed-exceptions version?

[snoyberg]

Closing as believed to be fixed.