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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