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

Olaf Klinke olf at aatal-apotheke.de
Mon Mar 7 09:56:49 UTC 2022

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:

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
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. 


-- 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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