[Haskell-cafe] GHC.Conc.threadStatus - documentation of ThreadDied :: ThreadStatus

[coot at coot.me]

Hi Olaf,

`forkIO` is rather a low level.  It's more common to use async package (https://hackage.haskell.org/package/async).  Async has `waitCatch` which allows you to wait for a thread to finish and get access to either an exception which killed the thread or the result of the thread handler.

Best regards,
Marcin Szamotulski

Sent with ProtonMail secure email.

------- Original Message -------

On Monday, March 7th, 2022 at 10:56, Olaf Klinke <olf at aatal-apotheke.de> wrote:

> Dear Cafe,
> 

> I had expected to see ThreadDied in the small example below.
> 

> But when I compile with
> 

> ghc --make -threaded -with-rtsopts=-N2
> 

> The output is:
> 

> threadStatus: user error (child thread is crashing!)
> 

> The status of my child is:
> 

> ThreadFinished
> 

> The output is not really a lie. But how do I determine whether a child
> 

> thread has exited normally or not? Wouldn't you say a call to fail (or
> 

> any other throwIO) should count as ThreadDied?
> 

> The documentation of GHC.Conc.forkIO says:
> 

> "... passes all other exceptions to the uncaught exception handler."
> 

> and the documentation for GHC.Conc.ThreadStatus says:
> 

> ThreadDied -- the thread received an uncaught exception
> 

> One can provoke ThreadDied by using throwTo from the parent thread. So
> 

> the emphasis in the documentation of ThreadDied should be on the word
> 

> "received".
> 

> This is a case of misleading documentation, in my humble opinion.
> 

> The constructor should not be named ThreadDied because that suggests
> 

> inclusion of internal reasons.
> 

> Olaf
> 

> -- begin threadStatus.hs
> 

> import Control.Concurrent
> 

> import GHC.Conc
> 

> main = mainThread
> 

> childThread :: IO ()
> 

> childThread = fail "child thread is crashing!"
> 

> mainThread :: IO ()
> 

> mainThread = do
> 

> child <- forkIO childThread
> 

> threadDelay 5000
> 

> status <- threadStatus child
> 

> putStr "The status of my child is: "
> 

> print status
> 

> -- end threadStatus.hs
> 

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