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
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