Re: [Haskell-cafe] Strictness leak
Jeff Polakow [EMAIL PROTECTED] writes:
Besides anything else, sequence will diverge on an infinite list.
Argh, of course. Thanks!
It is necessary to compute all of the computations in the list before
returning
any of the pure resulting list.
Replacing sequence with sequence', given as:
sequence' ms = foldr k (return []) ms
where
k m m' = do { x - m; xs - unsafeInterleaveIO m'; return (x:xs) }
seems to solve it.
-k
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[Haskell-cafe] Strictness leak
Some time ago, I posted this 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)
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.
I'm not sure why this happens, can anybody else see it?
-k
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Re: [Haskell-cafe] Strictness leak
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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Re: [Haskell-cafe] Strictness leak
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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are not the intended recipient (or have received this e-mail in error)
please notify the sender immediately and destroy this e-mail. Any
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e-mail is strictly forbidden.
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Re: [Haskell-cafe] Strictness leak
I forgot to send this reponse to haskell-cafe earlier... Hello, You mean for the IO monad, right? Sorry. I meant divergence is unavoidable for any strict Monad, such as IO. However, sequence will always compute over the entire list; if the resulting computation itself is lazy then the result can be inspected lazily. take 10 $ execWriter $ sequence $ repeat $ tell ([3]::[Int]) This is a good example. Note that the computation of sequence itself is infinite. snd $ runWriter $ sequence (repeat $ tell [3]) = return . take 10 will result in an infinite list, but fst $ runWriter $ sequence (repeat $ tell [3]) = return . take 10 will return a 10 element list. -Jeff --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
