make avar puts block

[jdegoes]

If an AVar is full, putting should block until it's empty.

[jdegoes]

This is mostly to match Haskell MVar semantics and avoid treating AVars as a queue.

[syaiful6]

i try to implements it, but look like Haskell does not specify an order in which it executes threads. My haskell code like this:

import Control.Concurrent
import Control.Monad

roundRobin :: IO ()
roundRobin = do
  x <- newEmptyMVar :: IO (MVar Int)
  void $ forkIO $ forever $ do
    val <- takeMVar x
    putStrLn $ "Received value (Thread #1): " ++ show val
  void $ forkIO $ forever $ do
    val <- takeMVar x
    putStrLn $ "Received value (Thread #2): " ++ show val
  mapM_ (putMVar x) [0..10]
  threadDelay 100000

roundRobinPut :: IO ()
roundRobinPut = do
  x <- newEmptyMVar :: IO (MVar Int)
  void $ forkIO $ mapM_ (putMVar x) [0..10]
  void $ forkIO $ mapM_ (putMVar x) [11..20]
  replicateM_ 21 $ do
    val <- takeMVar x
    putStrLn $ "Received value: " ++ show val
  threadDelay 100000

main :: IO ()
main = roundRobin >> roundRobinPut

execute on 4 core yield:

Received value (Thread #1): 0
Received value (Thread #2): 1
Received value (Thread #2): 3
Received value (Thread #2): 4
Received value (Thread #1): 2
Received value (Thread #2): 5
Received value (Thread #2): 7
Received value (Thread #2): 8
Received value (Thread #1): 6
Received value (Thread #2): 9
Received value (Thread #1): 10
Received value: 0
Received value: 1
Received value: 2
Received value: 11
Received value: 12
Received value: 13
Received value: 3
Received value: 14
Received value: 15
Received value: 16
Received value: 4
Received value: 17
Received value: 18
Received value: 19
Received value: 5
Received value: 20
Received value: 6
Received value: 7
Received value: 8
Received value: 9
Received value: 10

similliar code on Purescript which block when putting full AVar yield different results, it just switching. I guess it because Javascript is single threaded.

received value (Thread #1): 0
received value (Thread #2): 1
received value (Thread #1): 2
received value (Thread #2): 3
received value (Thread #1): 4
received value (Thread #2): 5
received value (Thread #1): 6
received value (Thread #2): 7
received value (Thread #1): 8
received value (Thread #2): 9
received value (Thread #1): 10
received: 0
received: 1
received: 11
received: 2
received: 12
received: 3
received: 13
received: 4
received: 14
received: 5
received: 15
received: 6
received: 16
received: 7
received: 17
received: 8
received: 18
received: 9
received: 19
received: 10
received: 20