Collecting IO outputs into list

Question

How can I issue multiple calls to SDL.pollEvent :: IO Event until the output is SDL.NoEvent and collect all the results into a list?

In imperative terms something like this:

events = []
event = SDL.pollEvent
while ( event != SDL.NoEvent ) {
        events.add( event )
        event = SDL.pollEvent
}

Answer 1

James Cook was so kind to extend monad-loops with this function:

unfoldWhileM  :: Monad  m => (a -> Bool) -> m a -> m [a]

used with SDL:

events <- unfoldWhileM (/= SDL.NoEvent) SDL.pollEvent

Answer 2

You could use something like:

takeWhileM :: (a -> Bool) -> IO a -> IO [a]
takeWhileM p act = do
  x <- act
  if p x
    then do
      xs <- takeWhileM p act
      return (x : xs)
    else
      return []

Instead of:

do
  xs <- takeWhileM p act
  return (x : xs)

you can also use:

liftM (x:) (takeWhileM p act) yielding:

takeWhileM :: (a -> Bool) -> IO a -> IO [a]
takeWhileM p act = do
  x <- act
  if p x
    then liftM (x:) (takeWhileM p act)
    else return []

Then you can use: takeWhileM (/=SDL.NoEvent) SDL.pollEvent