a little demo GHCI transcript illustrating STM and concurrency

STM-FUNN.hs

GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
Prelude> import Control.
Prelude> import Control.Concurrent.STM
Prelude Control.Concurrent.STM> vars <- atomically $ mapM (\ _ ->  newTVar False) [1..10]
Prelude Control.Concurrent.STM> length vars
10
Prelude Control.Concurrent.STM> :t vars
vars :: [TVar Bool]
Prelude Control.Concurrent.STM> import Control.Concurrent as CC
Prelude Control.Concurrent.STM CC> CC.forkIO ( do  x <- atomically (do  {  xs <- mapM readT   }  ) ; if x then putStrLn "wippeee" else putStrLn "wattt")
readTBQueue  readTChan    readTMVar    readTQueue   readTVar     readTVarIO
Prelude Control.Concurrent.STM CC> CC.forkIO ( do  x <- atomically (do  {  xs <- mapM readTVar vars ; if and xs then return True else retry   }  ) ; if x then putStrLn "wippeee" else putStrLn "wattt")
ThreadId 489
Prelude Control.Concurrent.STM CC> atomically $ writeT
writeTBQueue  writeTChan    writeTQueue   writeTVar
Prelude Control.Concurrent.STM CC> atomically $ writeTVar (vars ! 6) False

<interactive>:7:30: error:
    Variable not in scope: (!) :: [TVar Bool] -> Integer -> TVar Bool
Prelude Control.Concurrent.STM CC> atomically $ writeTVar (vars ! 6 :) False
Prelude Control.Concurrent.STM CC> :t (!)

<interactive>:1:1: error: Variable not in scope: !
Prelude Control.Concurrent.STM CC> import Data.List (!)

<interactive>:9:19: error: parse error on input ‘!’
Prelude Control.Concurrent.STM CC> import Data.List ((!))

<interactive>:1:19: error: Module ‘Data.List’ does not export ‘(!)’
Prelude Control.Concurrent.STM CC> :t (!!)
(!!) :: [a] -> Int -> a
Prelude Control.Concurrent.STM CC> atomically $ writeTVar (vars !! 6) False
Prelude Control.Concurrent.STM CC> atomically $ writeTVar (vars !! 3) True
Prelude Control.Concurrent.STM CC> atomically $ writeTVar (vars !! 3) True
Prelude Control.Concurrent.STM CC> atomically $ writeTVar (vars !! 3) True
Prelude Control.Concurrent.STM CC> mapM (\x -> atomically $ writeTVar x True) vars
[(),(),(),()w,i(p)p,e(e)e,
(),(),(),()]
Prelude Control.Concurrent.STM CC> vars <- atomically $ mapM (\ _ ->  newTVar False) [1..10]
Prelude Control.Concurrent.STM CC> CC.forkIO ( do  x <- atomically (do  {  xs <- mapM readTVar vars ; if or xs then return True else retry   }  ) ; if x then putStrLn "wippeee" else putStrLn "wattt")
ThreadId 826
Prelude Control.Concurrent.STM CC> mapM (\x -> atomically $ writeTVar x True) vars
Prelude Control.Concurrent.STM CC> x <- atomically $ writeTVar (vars !! 3) True
wippeePrelude Control.Concurrent.STM CC> e