Good afternoon Haskellers, So I'm trying to understand how STM works, and wrote a quick 'eating philosophers' example to see if I understood how it's supposed to work. The problem is that while it executes, it doesn't appear to *do* anything. Did I completely write things wrongheadedly or am I being bitten by something more subtle? Thanks. import Control.Concurrent.STM import Control.Concurrent import Data.Array import System.Random think :: IO () think = do ms <- randomRIO (20,1000) threadDelay ms data Philosopher = Philosopher {left::Bool,right::Bool,neighbors::(Int,Int)} deriving Show makeInitPhilosopher a = Philosopher {left=False,right=False,neighbors=a} initPhilosophers = listArray (0,4) (map makeInitPhilosopher [(1,4),(2,0),(3,1),(4,2),(0,3)]) main = do z <- atomically $ newTVar initPhilosophers mapM_ (\x -> forkIO (loop x z 0 10000)) [0,1,2,3,4] loop n tps c l | c > l = (atomically (readTVar tps)) >>= (\x -> print x) | otherwise = do think atomically $ eat n tps loop n tps (c+1) l eat :: Int -> TVar (Array Int Philosopher) -> STM () eat n tps = do takeLeft n tps takeRight n tps releaseLeft n tps releaseRight n tps takeLeft :: Int -> TVar (Array Int Philosopher) -> STM () takeLeft n tps = do ps <- readTVar tps let p = ps ! n if right (ps ! (fst $ neighbors p)) == False then (writeTVar tps $ ps // [(n,p{left=True})]) else retry takeRight :: Int -> TVar (Array Int Philosopher) -> STM () takeRight n tps = do ps <- readTVar tps let p = ps ! n if left (ps ! (snd $ neighbors p)) == False then (writeTVar tps $ ps // [(n,p{right=True})]) else retry releaseLeft n tps = do ps <- readTVar tps let p = ps ! n writeTVar tps $ ps // [(n,p{left=False})] releaseRight n tps = do ps <- readTVar tps let p = ps ! n writeTVar tps $ ps // [(n,p{right=False})]