You mean for the IO monad, right? take 10 $ execWriter $ sequence $ repeat $ tell ([3]::[Int]) / Emil On 10/30/2007 02:04 PM, Jeff Polakow wrote:
Hello,
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)
It wraps a list with IO operations, so that progress can be reported while evaluating the list elements. Unfortunately, there seems to be a "stricness leak" here - and consequently, it does not work on an infinite list.
Besides anything else, sequence will diverge on an infinite list. This can be seen directly from the type:
sequence :: Monad m => [m a] -> m [a]
It is necessary to compute all of the computations in the list before returning any of the pure resulting list.
-Jeff
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