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 <ier...@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.
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