[Haskell-cafe] Timeouts inside a ConduitParser

[Michael Snoyman]

Due to the coroutine-based nature of conduit, this kind of approach isn't
possible. In short, when you're inside the `ConduitT` transformer, you
cannot use `timeout` on a `ConduitT` action because you need to yield
control of execution to a different coroutine. Instead, you should move the
`timeout` call to _outside_ the `ConduitT` parts, e.g.:

    timeout foo $ runConduit $ src .| satisfy .| sink

It seems like in this case, that kind of timeout usage is going to be too
coarse-grained. I haven't used it personally, but the `stm-conduit` package
probably has something in the direction you're looking for. Alternatively,
I put together an example of how this might be done using some standard
Haskell libraries like stm and async here:

https://gist.github.com/snoyberg/7e5dd52109b03c8bf1aa8fe1a7e522b9

The basic idea is to have two sibling threads: one running the original
source and writing its values to a queue, and another running the full
conduit pipeline with a modified source that will time out on reads from
that queue.

On Thu, Jun 21, 2018 at 4:08 PM Luke Lau <luke_lau at icloud.com> wrote:

> I have a ConduitParser
> <https://hackage.haskell.org/package/conduit-parse-0.2.1.0/docs/Data-Conduit-Parser.html#t:ConduitParser> (a
> Sink with some parsing state) with a version of satisfy that can time out:
>
> satisfy :: MonadIO m => (a -> Bool) -> ConduitParser a m a
> satisfy pred = do
> tId <- liftIO myThreadId
> timeoutThread <- liftIO $ forkIO $ do
> threadDelay 1000000
> throwTo tId TimeoutException
> x <- await
> liftIO $ killThread timeoutThread
> if pred x
> then return x
> else empty
>
> However I would rather not deal with the risks involved with handling
> concurrency myself and use a system library like System.Timeout:
>
> satisfy :: MonadIO m => (a -> Bool) -> ConduitParser a m a
> satisfy pred = do
> x <- timeout 1000000 await
> if pred x
> then return x
> else empty
>
> This doesn’t work though since I need to be able to both lift and unlift
> await from IO, and ConduitParser lies on top of ConduitT, which is one of
> the types of monads that UnliftIO cannot be an instance of
> <https://github.com/fpco/unliftio#limitations>. Are there any better
> approaches to this?
> <https://github.com/fpco/unliftio#limitations>
> <https://github.com/fpco/unliftio#limitations>
> <https://github.com/fpco/unliftio#limitations>
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