for package:verset

for is traverse with its arguments flipped. For a version that ignores the results see for_.

forM is mapM with its arguments flipped. For a version that ignores the results see forM_.

forM_ is mapM_ with its arguments flipped. For a version that doesn't ignore the results see forM. forM_ is just like for_, but specialised to monadic actions.

for_ is traverse_ with its arguments flipped. For a version that doesn't ignore the results see for. This is forM_ generalised to Applicative actions. for_ is just like forM_, but generalised to Applicative actions.

Examples

Basic usage:

>>> for_ [1..4] print
1
2
3
4

Repeat an action indefinitely.

Examples

A common use of forever is to process input from network sockets, Handles, and channels (e.g. MVar and Chan). For example, here is how we might implement an echo server, using forever both to listen for client connections on a network socket and to echo client input on client connection handles:

echoServer :: Socket -> IO ()
echoServer socket = forever $ do
client <- accept socket
forkFinally (echo client) (\_ -> hClose client)
where
echo :: Handle -> IO ()
echo client = forever $
hGetLine client >>= hPutStrLn client

Note that "forever" isn't necessarily non-terminating. If the action is in a MonadPlus and short-circuits after some number of iterations. then forever actually returns mzero, effectively short-circuiting its caller.

Fork a thread and call the supplied function when the thread is about to terminate, with an exception or a returned value. The function is called with asynchronous exceptions masked.

forkFinally action and_then =
mask $ \restore ->
forkIO $ try (restore action) >>= and_then

This function is useful for informing the parent when a child terminates, for example.

Creates a new thread to run the IO computation passed as the first argument, and returns the ThreadId of the newly created thread. The new thread will be a lightweight, unbound thread. Foreign calls made by this thread are not guaranteed to be made by any particular OS thread; if you need foreign calls to be made by a particular OS thread, then use forkOS instead. The new thread inherits the masked state of the parent (see mask). The newly created thread has an exception handler that discards the exceptions BlockedIndefinitelyOnMVar, BlockedIndefinitelyOnSTM, and ThreadKilled, and passes all other exceptions to the uncaught exception handler. WARNING: Exceptions in the new thread will not be rethrown in the thread that created it. This means that you might be completely unaware of the problem if/when this happens. You may want to use the async library instead.

bifor is bitraverse with the structure as the first argument. For a version that ignores the results, see bifor_.

Examples

Basic usage:

>>> bifor (Left []) listToMaybe (find even)
Nothing

>>> bifor (Left [1, 2, 3]) listToMaybe (find even)
Just (Left 1)

>>> bifor (Right [4, 5]) listToMaybe (find even)
Just (Right 4)

>>> bifor ([1, 2, 3], [4, 5]) listToMaybe (find even)
Just (1,4)

>>> bifor ([], [4, 5]) listToMaybe (find even)
Nothing

As bitraverse_, but with the structure as the primary argument. For a version that doesn't ignore the results, see bifor.

Examples

Basic usage:

>>> bifor_ ("Hello", True) print (print . show)
"Hello"
"True"

>>> bifor_ (Right True) print (print . show)
"True"

>>> bifor_ (Left "Hello") print (print . show)
"Hello"