CODEWITHSUNDEEP
haskell
Luigi Minardi Maia
Luigi Minardi Maia

Reputation: 13

How can I create and kill a list of threads in Haskell?

I don't know how to put the ThreadIds running in a list and, then, killing all the thread created.

I've tried some Monads operations but I could't understand the type erros in ghci.

import Control.Concurrent

main = do 
  ts <- [forkIO(write 'a'), forkIO(write 'b')];
  putStrLn "Stop printing as and bs!";
  map killThread ts;
  where
    write c = do 
      putChar c;
      write c;

I expected the code to run two other threads in parallel and then killing this two threads.

Upvotes: 1

Views: 297

Answers (1)

Rinne Hmm
Rinne Hmm

Reputation: 171

I think you need to use Control.Monad.mapM :: (Monad m) => (a -> m b) -> [a] -> m [b]

Example: 

main = do 
  ts <- mapM forkIO [(write 'a'), (write 'b')]
  putStrLn "Stop printing as and bs!"
  _ <- mapM killThread ts -- or use mapM_ for no returning result effect
  pure () -- return ()
  where
    write c = do 
      putChar c
      write c

And you don't have to use semicolon in haskell ;. Just correction indentation is enough.

Upvotes: 2

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