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Michael Lafayette February 2016

Haskell get set thread name

When I print something in Java, I usually append the name of the thread to the print statement so I know what thread is running what print statement. How do I get and set the thread name in Haskell? If I can't set the thread name, how do I make a thread that has a given name?

Note: I usually give my threads descriptive names like "GUI_thread", "main_thread", "database_thread", etc


Carsten February 2016

As I said in a comment you can use labelThread to give the thread a label. Sadly I could not find a way to pull this label back out (all I found was this invalid ticket where someone wanted to do just this).

But as descriped in Parallel and Concurrent Programming in Haskell you can get an event-log view on this:

(straight from there – I hope nobody minds):

main = do
  t <- myThreadId
  labelThread t "main"
  m <- newEmptyMVar
  t <- forkIO $ putMVar m 'a'
  labelThread t "a"
  t <- forkIO $ putMVar m 'b'
  labelThread t "b"
  traceEventIO "before takeMVar"
  takeMVar m
  takeMVar m

You then should compile it with the threaded and eventlog switches set:

ghc mvar4.hs -threaded -eventlog

and can call it with +RTS and -l:

./mvar4 +RTS -l

Finally you can look at the output with ghc-evetns:

ghc-events show mvar4.eventlog

Naive self-implementation

That seems not to be exactly what you want, so here is a (very basic) example of how you could do it yourself (using just lists and an MVar to keep the association between the ThreadId and its name):

import Control.Concurrent

type Labels = MVar [(ThreadId, String)]

initLabels :: IO Labels
initLabels = newMVar []

labelThread :: Labels -> ThreadId -> String -> IO ()
labelThread labels threadId label =
  modifyMVar_ labels (\assocs -> return $ (threadId, label):assocs)

getLabel :: Labels - 

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Asked in February 2016
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