[Haskell] Re: [Haskell-cafe] SimonPJ and Tim Harris explain STM - video

[Chris Kuklewicz]

I posted an improved version of the new monad to the wiki at
http://haskell.org/haskellwiki/New_monads/MonadAdvSTM

Observations:

** This idiom made it easy for the retrying case to queue an action which
ensures success in the next attempt.

** More than one operation can be queued for both the commit and the retry
possibilities.

** Reading the TVar in the onRetry/retryWith  branch sees the "rolled back"
value, which luckily is the initialization value instead of undefined in the
case where the TVar was created in the aborted block.

** The new code includes unlift* operations which makes the STM code in
testUnlift much easier to write.

The relevant example its output are now:

-- Example code using the above, lifting into MonadAdvSTM:
test ::(Monad m, MonadAdvSTM m) => TVar Bool -> m [Char]
test todo = do
  onCommit (print "onCommit Start")
  onRetry (print "onRetry Start")
  v <- liftAdv $ newTVar 7
  liftAdv $ writeTVar v 42
  onCommit (atomically (readTVar v) >>= \x->print ("onCommit v",x))
  onRetry (atomically (readTVar v) >>= \x->print ("onRetry v",x))
  choice <- liftAdv $ readTVar todo
  case choice of
    True -> return "foo"
    False -> retryWith $ do
      atomically (writeTVar todo True)
      print "Flipped choice to True to avoid infinite loop"

-- Same example as test, but unlifting from AdvSTM
testUnlift :: TVar Bool -> AdvSTM [Char]
testUnlift todo = do
  onCommit <- unlift1 onCommit
  onRetry <- unlift1 onRetry
  retryWith <- unlift1 retryWith
  liftAdv $ do
    onCommit (print "onCommit Start")
    onRetry (print "onRetry Start")
    v <- newTVar 7
    writeTVar v 42
    onCommit (atomically (readTVar v) >>= \x->print ("onCommit v",x))
    onRetry (atomically (readTVar v) >>= \x->print ("onRetry v",x))
    choice <- readTVar todo
    case choice of
      True -> return "foo"
      False -> retryWith $ do
        atomically (writeTVar todo True)
        print "Flipped choice to True to avoid infinite loop"

-- Example similar to Simon's suggested example:
countRetries :: (MonadAdvSTM m, Monad m, Enum a) => IORef a -> m a1 -> m a1
countRetries ioref action =
  let incr = do old <- readIORef ioref
                writeIORef ioref $! (succ old)
  in action `orElseAdv` (retryWith incr)

-- Load this file in GHCI and execute main to run the test:
main = do
  counter <- newIORef 0
  todo <- newTVarIO False
  print "test"
  result <- runAdvSTM (countRetries counter $ test todo)
  retries <- readIORef counter
  print ("result",result,"retries",retries)
  atomically (writeTVar todo False)
  print "testUnlift"
  result <- runAdvSTM (countRetries counter $ testUnlift todo)
  retries <- readIORef counter
  print ("result",result,"retries",retries)
  print "bye world"

The output in GHCI is

*AdvSTM> main
"test"
"onRetry Start"
("onRetry v",7)
"Flipped choice to True to avoid infinite loop"
"onCommit Start"
("onCommit v",42)
("result","foo","retries",1)
"testUnlift"
"onRetry Start"
("onRetry v",7)
"Flipped choice to True to avoid infinite loop"
"onCommit Start"
("onCommit v",42)
("result","foo","retries",2)
"bye world"