I've done something similar, I think. Often, I want to output some kind of progress indicator, just to show that the program is working. Typically, the program works by lazily evaluating a list (lines from an input file, say); each element of the list is wrapped with an IO action that outputs the status when evaluated -- which typically happens lazily from pure code.
countIO :: String -> String -> Int -> [a] -> IO [a] countIO msg post step xs = sequence $ map unsafeInterleaveIO ((blank >> outmsg (0::Int) >> c):cs) where (c:cs) = ct 0 xs output = hPutStr stderr blank = output ('\r':take 70 (repeat ' ')) outmsg x = output ('\r':msg++show x) >> hFlush stderr ct s ys = let (a,b) = splitAt (step-1) ys next = s+step in case b of [b1] -> map return a ++ [outmsg (s+step) >> hPutStr stderr post >> return b1] [] -> map return (init a) ++ [outmsg (s+length a) >> hPutStr stderr post >> return (last a)] _ -> map return a ++ [outmsg s >> return (head b)] ++ ct next (tail b)
-k -- If I haven't seen further, it is by standing in the footprints of giants