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Foreign function interface (FFI)

FFI types

hsc2hs

Exceptions

Simple example

{-# LANGUAGE DeriveDataTypeable #-}

import Prelude hiding (catch)
import Control.Exception
import Data.Typeable

data MyError = MyError String deriving (Show, Typeable)
instance Exception MyError

catcher :: IO a -> IO (Maybe a)
catcher action = fmap Just action `catch` handler
    where handler (MyError msg) = do putStrLn msg; return Nothing
*Main> catcher $ readFile "/dev/null"
Just ""
*Main> catcher $ throwIO $ MyError "something bad"
something bad
Nothing

Exceptions in pure code

A few more exception functions

Monadic exceptions

Haskell threads

Example: timeout

newtype TimedOut = TimedOut UTCTime deriving (Eq, Show, Typeable)
instance Exception TimedOut

timeout :: Int -> IO a -> IO (Maybe a)
timeout usec action = do
  -- Create unique exception val (for nested timeouts):
  expired <- fmap TimedOut getCurrentTime

  ptid <- myThreadId
  let child = do threadDelay usec
                 throwTo ptid expired
      parent = do ctid <- forkIO child
                  result <- action
                  killThread ctid
                  return $ Just result
  catchJust (\e -> if e == expired then Just e else Nothing) 
            parent
            (\_ -> return Nothing)

MVars

Example: pingpong benchmark

import Control.Concurrent
import Control.Exception
import Control.Monad

pingpong :: Bool -> Int -> IO ()
pingpong v n = do
  mvc <- newEmptyMVar
  mvp <- newEmptyMVar
  let parent n | n > 0 = do when v $ putStr $ " " ++ show n
                            putMVar mvc n
                            takeMVar mvp >>= parent
             | otherwise = return ()
      child = do n <- takeMVar mvc
                 putMVar mvp (n - 1)
                 child
  tid <- forkIO child
  parent n `finally` killThread tid
  when v $ putStrLn ""
*Main> pingpong True 10
 10 9 8 7 6 5 4 3 2 1

Sidenote: benchmarking

import Criterion.Main

...

main :: IO ()
main = defaultMain [
        bench "thread switch test" mybench
       ]
    where mybench = pingpong False 10000
$ ghc -O pingpong.hs 
[1 of 1] Compiling Main             ( pingpong.hs, pingpong.o )
Linking pingpong ...
$ ./pingpong 
...
benchmarking thread switch test
mean: 3.782984 ms, lb 3.770838 ms, ub 3.798160 ms, ci 0.950
std dev: 69.27807 us, lb 55.00853 us, ub 88.83503 us, ci 0.950

OS threads

$ rm pingpong
$ ghc -threaded -O pingpong.hs 
Linking pingpong ...
$ ./pingpong
...
mean: 113.6852 ms, lb 113.5195 ms, ub 113.8770 ms, ci 0.950
std dev: 912.0979 us, lb 731.0661 us, ub 1.226794 ms, ci 0.950

Bound vs. unbound threads

What good are OS threads?

Asynchronous exceptions

Asynchronous exceptions

Masking exceptions

Masking exceptions (continued)

wrap :: IO a -> IO a          -- Fixed version of wrap
wrap action = do
  mv <- newEmptyMVar
  mask $ \unmask -> do
      tid <- forkIO $ (unmask $ action >>= putMVar mv) `catch`
             \e@(SomeException _) -> putMVar mv (throw e)
      let loop = takeMVar mv `catch` \e@(SomeException _) ->
                 throwTo tid e >> loop
      loop

The bracket function

Working with MVars

Alternate Mutex

Condition variables

data Cond = Cond Mutex (MVar [MVar ()])

cond_create :: Mutex -> IO Cond
cond_create m = do
  waiters <- newMVar []
  return $ Cond m waiters

cond_wait, cond_signal, cond_broadcast :: Cond -> IO ()
cond_wait (Cond m waiters) = do
  me <- newEmptyMVar
  modifyMVar_ waiters $ \others -> return $ others ++ [me]
  mutex_unlock m   -- note we don't care if preempted here after this
  takeMVar me `finally` mutex_lock m

cond_signal (Cond _ waiters) = modifyMVar_ waiters wakeone
    where wakeone [] = return []
          wakeone (w:ws) = putMVar w () >> return ws

cond_broadcast (Cond _ waiters) = modifyMVar_ waiters wakeall
    where wakeall ws = mapM_ (flip putMVar ()) ws >> return []

Channels

Channel implementation [simplified]

data Item a = Item a (Stream a)
type Stream a = MVar (Item a)
data Chan a = Chan (MVar (Stream a)) (MVar (Stream a))

newChan :: IO (Chan a)
newChan = do
  empty <- newEmptyMVar
  liftM2 Chan (newMVar empty) (newMVar empty)

writeChan :: Chan a -> a -> IO ()
writeChan (Chan _ w) a = do
  empty <- newEmptyMVar
  modifyMVar_ w $ \oldEmpty -> do
    putMVar oldEmpty (Item a empty)
    return empty

readChan :: Chan a -> IO a
readChan (Chan r _) =
    modifyMVar r $ \full -> do
      (Item a newFull) <- takeMVar full
      return (newFull, a)

Networking

Example: netcat

netcat :: String -> String -> IO ()
netcat host port = do
  -- Extract address from first AddrInfo in list
  AddrInfo{addrAddress = addr}:_
      <- getAddrInfo Nothing (Just host) (Just port)

  -- Create a TCP socket connected to server
  s <- socket AF_INET Stream 0
  connect s addr

  -- Convert socket to handle
  h <- socketToHandle s ReadWriteMode
  hSetBuffering h NoBuffering  -- THIS IS IMPORTANT

  -- Deal w. broken unicode
  hSetBinaryMode stdout True

  -- Copy data back and forth
  done <- newEmptyMVar
  forkIO $ (hGetContents h >>= putStr) `finally` putMVar done ()
  getContents >>= hPutStr h
  takeMVar done
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