confused-async.hs

module Chapter8.GetURLTwo where

import Chapter8.MFGetURL
import Control.Concurrent
import qualified Data.ByteString as B

data Async a = Async (MVar a)

async' :: IO a -> IO (Async a)
async' action = do
  var <- newEmptyMVar
  forkIO (do r <- action; putMVar var r)
  return (Async var)


async'' :: IO a -> IO (Async a)
async'' action = newEmptyMVar >>= \var ->
  (forkIO (action >>= \r -> putMVar var r)) >>
  (return $ Async var)

wait :: Async a -> IO a
wait (Async var) = readMVar var

-- how is this async? 
blah = do
  a1 <- async'' (getURL "http://www.wikipedia.org/wiki/Shovel")
  a2 <- async'' (getURL "http://www.wikipedia.org/wiki/Spade")
  r1 <- wait a1
  r2 <- wait a2
  print (B.length r1, B.length r2)

-- how is this async? I'm flatmapping ... it looks like I always need a1 to finish before a2? 
blah' = async''(getURL "http://www.wikipedia.org/wiki/Shovel") >>= \a1 ->
  async''(getURL "http://www.wikipedia.org/wiki/Spade") >>= \a2 ->
  wait a1 >>= \r1 ->
  wait a2 >>= \r2 ->
  (print (B.length r1, B.length r2))