I'm sorry, but I don't fully understand what do you want from putback. If putback'ed action does io and then returns x, it's trivial: putback x io = io >> return x If you want putback'ed action to return x on its first call and do io on second, third... calls, then you need to put somewhere a mark indicating that action was called. You can use IORefs. import Data.IORef import System.IO.Unsafe putback x io = do m <- readIORef r if m then io else (writeIORef r True >> return x) where r = unsafePerformIO $ newIORef False Sergey Mironov <ierton@gmail.com> писал(а) в своём письме Sun, 15 May 2011 17:33:51 +0300:
Hi Cafe. I wonder if it is possible to write a IO putback function with following interface
putback :: a -> IO a -> IO a putback x io = ???
where io is some action like reading from file or socket. I want putback to build new action which will return x on first call, and continue executing io after that.
Thanks in advance! Sergey.