How do I get this done in constant mem?

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mf-hcafe-15c311f0c＠etc-network.de

9 Oct 2009 9 Oct '09

8:05 p.m.

Hi all, I think there is something about my use of the IO monad that bites me, but I am bored of staring at the code, so here you g. The code goes through a list of records and collects the maximum in each record position. -- test.hs import Random import System.Environment (getArgs) import System.IO (putStr) samples :: Int -> Int -> IO [[Double]] samples i j = sequence . replicate i . sequence . replicate j $ randomRIO (0, 1000 ** 3) maxima :: [[Double]] -> [Double] maxima samples@(_:_) = foldr (\ x y -> map (uncurry max) $ zip x y) (head samples) (tail samples) main = do args <- getArgs x <- samples (read (head args)) 5 putStr . (++ "\n") . show $ maxima x I would expect this to take constant memory (foldr as well as foldl), but this is what happens: $ ghc -prof --make -O9 -o test test.hs [1 of 1] Compiling Main ( test.hs, test.o ) Linking test ... $ ./test 100 +RTS -p [9.881155955344708e8,9.910336352165401e8,9.71000686630374e8,9.968532576451201e8,9.996200333115692e8] $ grep 'total alloc' test.prof total alloc = 744,180 bytes (excludes profiling overheads) $ ./test 10000 +RTS -p [9.996199711457872e8,9.998928358545277e8,9.99960283632381e8,9.999707142123885e8,9.998952151508758e8] $ grep 'total alloc' test.prof total alloc = 64,777,692 bytes (excludes profiling overheads) $ ./test 1000000 +RTS -p Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it. $ so... does sequence somehow force the entire list of monads into evaluation before the head of the result list can be used? what can i do to implement this in constant memory? thanks! matthias

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Luke Palmer

9 Oct 9 Oct

11:48 p.m.

On Fri, Oct 9, 2009 at 2:05 PM, wrote:

...

Hi all,

I think there is something about my use of the IO monad that bites me, but I am bored of staring at the code, so here you g. The code goes through a list of records and collects the maximum in each record position.

-- test.hs import Random import System.Environment (getArgs) import System.IO (putStr)

samples :: Int -> Int -> IO [[Double]] samples i j = sequence . replicate i . sequence . replicate j $ randomRIO (0, 1000 ** 3)

Yes, you should not do this in IO. That requires the entire computation to finish before the result can be used. This computation should be pure and lazy. It is possible, using split (and I believe not without it, unless you use mkStdGen), to make a 2D list of randoms where the random generation matches exactly the structure of the list. splits :: (RandomGen g) => Int -> g -> [g] splits 0 _ = [] splits n g = let (g1,g2) = split g in g1 : splits (n-1) g2 samples :: (RandomGen g) => Int -> Int -> g -> [[Double]] samples i j gen = map row (splits i gen) where row g = take j (randomRs (0, 10^9) g) In fact, we could omit all these counts and make an infinite 2D list, which you can cull in the client code. splits :: (RandomGen g) => g -> [g] splits g = let (g1,g2) = split g in g1 : splits g2 samples :: (RandomGen g) => g -> [[Double]] samples = map row . splits where row = randomRs (0, 10^9) I find the latter to be more straightforward and obvious. Maintaining the laziness here is a fairly subtle thing, so study, perturb, try to write it yourself in different ways, etc.

...

maxima :: [[Double]] -> [Double] maxima samples@(_:_) = foldr (\ x y -> map (uncurry max) $ zip x y) (head samples) (tail samples)

FWIW, This function has a beautiful alternate definition: maxima :: [[Double]] -> [Double] maxima = map maximum . transpose

...

main = do args <- getArgs x <- samples (read (head args)) 5 putStr . (++ "\n") . show $ maxima x

I would expect this to take constant memory (foldr as well as foldl), but this is what happens:

$ ghc -prof --make -O9 -o test test.hs [1 of 1] Compiling Main ( test.hs, test.o ) Linking test ... $ ./test 100 +RTS -p [9.881155955344708e8,9.910336352165401e8,9.71000686630374e8,9.968532576451201e8,9.996200333115692e8] $ grep 'total alloc' test.prof total alloc = 744,180 bytes (excludes profiling overheads) $ ./test 10000 +RTS -p [9.996199711457872e8,9.998928358545277e8,9.99960283632381e8,9.999707142123885e8,9.998952151508758e8] $ grep 'total alloc' test.prof total alloc = 64,777,692 bytes (excludes profiling overheads) $ ./test 1000000 +RTS -p Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it. $

so...

does sequence somehow force the entire list of monads into evaluation before the head of the result list can be used?

Yep. IO is completely strict; in some sense the same as "call by value" (don't take the analogy too far). Rule of thumb: keep your distance from it ;-)

Thomas Hartman

10 Oct 10 Oct

4:33 p.m.

...

Yes, you should not do this in IO. That requires the entire computation to finish before the result can be used.

Not really the entire computation though... whnf, no? main = do let thunks :: IO [Int] thunks = (sequence . replicate (10^6) $ (randomRIO (0,10^9))) putStrLn . show . head =<< thunks -- prints putStrLn . show . last =<< thunks -- overflows In the case of [[num]] from the top post, I belive that would be the first complete list. 2009/10/9 Luke Palmer :

...

On Fri, Oct 9, 2009 at 2:05 PM, wrote:

...
Hi all,

I think there is something about my use of the IO monad that bites me, but I am bored of staring at the code, so here you g. The code goes through a list of records and collects the maximum in each record position.

-- test.hs import Random import System.Environment (getArgs) import System.IO (putStr)

samples :: Int -> Int -> IO [[Double]] samples i j = sequence . replicate i . sequence . replicate j $ randomRIO (0, 1000 ** 3)

Yes, you should not do this in IO. That requires the entire computation to finish before the result can be used. This computation should be pure and lazy.

It is possible, using split (and I believe not without it, unless you use mkStdGen), to make a 2D list of randoms where the random generation matches exactly the structure of the list.

splits :: (RandomGen g) => Int -> g -> [g] splits 0 _ = [] splits n g = let (g1,g2) = split g in g1 : splits (n-1) g2

samples :: (RandomGen g) => Int -> Int -> g -> [[Double]] samples i j gen = map row (splits i gen) where row g = take j (randomRs (0, 10^9) g)

In fact, we could omit all these counts and make an infinite 2D list, which you can cull in the client code.

splits :: (RandomGen g) => g -> [g] splits g = let (g1,g2) = split g in g1 : splits g2

samples :: (RandomGen g) => g -> [[Double]] samples = map row . splits where row = randomRs (0, 10^9)

I find the latter to be more straightforward and obvious. Maintaining the laziness here is a fairly subtle thing, so study, perturb, try to write it yourself in different ways, etc.

...
maxima :: [[Double]] -> [Double] maxima samples@(_:_) = foldr (\ x y -> map (uncurry max) $ zip x y) (head samples) (tail samples)

FWIW, This function has a beautiful alternate definition:

maxima :: [[Double]] -> [Double] maxima = map maximum . transpose

...
main = do args <- getArgs x <- samples (read (head args)) 5 putStr . (++ "\n") . show $ maxima x

I would expect this to take constant memory (foldr as well as foldl), but this is what happens:

$ ghc -prof --make -O9 -o test test.hs [1 of 1] Compiling Main ( test.hs, test.o ) Linking test ... $ ./test 100 +RTS -p [9.881155955344708e8,9.910336352165401e8,9.71000686630374e8,9.968532576451201e8,9.996200333115692e8] $ grep 'total alloc' test.prof total alloc = 744,180 bytes (excludes profiling overheads) $ ./test 10000 +RTS -p [9.996199711457872e8,9.998928358545277e8,9.99960283632381e8,9.999707142123885e8,9.998952151508758e8] $ grep 'total alloc' test.prof total alloc = 64,777,692 bytes (excludes profiling overheads) $ ./test 1000000 +RTS -p Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it. $

so...

does sequence somehow force the entire list of monads into evaluation before the head of the result list can be used?

Yep. IO is completely strict; in some sense the same as "call by value" (don't take the analogy too far). Rule of thumb: keep your distance from it ;-) _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Thomas Hartman

5:07 p.m.

also, looking at the following, it does seem to me that it is sequence that is too strict, and not IO that is to blame, as the Maybe monad has the same behavior: t5IO, t6IO :: IO Int t5Maybe, t6Maybe :: Maybe Int t5 = return . head =<< sequence [return 1, undefined] t6 = return . head =<< return [1,undefined] t5IO = t5 t5Maybe = t5 t6IO = t6 t6Maybe = t6 *Main> t5IO *** Exception: Prelude.undefined *Main> t5Maybe *** Exception: Prelude.undefined *Main> t6IO 1 *Main> t6Maybe Just 1 2009/10/10 Thomas Hartman :

...

...
Yes, you should not do this in IO. That requires the entire computation to finish before the result can be used.

Not really the entire computation though... whnf, no?

main = do let thunks :: IO [Int] thunks = (sequence . replicate (10^6) $ (randomRIO (0,10^9))) putStrLn . show . head =<< thunks -- prints putStrLn . show . last =<< thunks -- overflows

In the case of [[num]] from the top post, I belive that would be the first complete list.

2009/10/9 Luke Palmer :

...
On Fri, Oct 9, 2009 at 2:05 PM, wrote:

...
Hi all,

I think there is something about my use of the IO monad that bites me, but I am bored of staring at the code, so here you g. The code goes through a list of records and collects the maximum in each record position.

-- test.hs import Random import System.Environment (getArgs) import System.IO (putStr)

samples :: Int -> Int -> IO [[Double]] samples i j = sequence . replicate i . sequence . replicate j $ randomRIO (0, 1000 ** 3)

Yes, you should not do this in IO. That requires the entire computation to finish before the result can be used. This computation should be pure and lazy.

It is possible, using split (and I believe not without it, unless you use mkStdGen), to make a 2D list of randoms where the random generation matches exactly the structure of the list.

splits :: (RandomGen g) => Int -> g -> [g] splits 0 _ = [] splits n g = let (g1,g2) = split g in g1 : splits (n-1) g2

samples :: (RandomGen g) => Int -> Int -> g -> [[Double]] samples i j gen = map row (splits i gen) where row g = take j (randomRs (0, 10^9) g)

In fact, we could omit all these counts and make an infinite 2D list, which you can cull in the client code.

splits :: (RandomGen g) => g -> [g] splits g = let (g1,g2) = split g in g1 : splits g2

samples :: (RandomGen g) => g -> [[Double]] samples = map row . splits where row = randomRs (0, 10^9)

I find the latter to be more straightforward and obvious. Maintaining the laziness here is a fairly subtle thing, so study, perturb, try to write it yourself in different ways, etc.

...
maxima :: [[Double]] -> [Double] maxima samples@(_:_) = foldr (\ x y -> map (uncurry max) $ zip x y) (head samples) (tail samples)

FWIW, This function has a beautiful alternate definition:

maxima :: [[Double]] -> [Double] maxima = map maximum . transpose

...
main = do args <- getArgs x <- samples (read (head args)) 5 putStr . (++ "\n") . show $ maxima x

I would expect this to take constant memory (foldr as well as foldl), but this is what happens:

$ ghc -prof --make -O9 -o test test.hs [1 of 1] Compiling Main ( test.hs, test.o ) Linking test ... $ ./test 100 +RTS -p [9.881155955344708e8,9.910336352165401e8,9.71000686630374e8,9.968532576451201e8,9.996200333115692e8] $ grep 'total alloc' test.prof total alloc = 744,180 bytes (excludes profiling overheads) $ ./test 10000 +RTS -p [9.996199711457872e8,9.998928358545277e8,9.99960283632381e8,9.999707142123885e8,9.998952151508758e8] $ grep 'total alloc' test.prof total alloc = 64,777,692 bytes (excludes profiling overheads) $ ./test 1000000 +RTS -p Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it. $

so...

does sequence somehow force the entire list of monads into evaluation before the head of the result list can be used?

Yep. IO is completely strict; in some sense the same as "call by value" (don't take the analogy too far). Rule of thumb: keep your distance from it ;-) _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

mf-hcafe-15c311f0c＠etc-network.de

8:14 p.m.

On Sat, Oct 10, 2009 at 09:33:52AM -0700, Thomas Hartman wrote:

...

To: Luke Palmer Cc: mf-hcafe-15c311f0c@etc-network.de, haskell-cafe@haskell.org From: Thomas Hartman Date: Sat, 10 Oct 2009 09:33:52 -0700 Subject: Re: [Haskell-cafe] How do I get this done in constant mem?

...
Yes, you should not do this in IO. That requires the entire computation to finish before the result can be used.

Not really the entire computation though... whnf, no?

In that example, yes. But readFile takes the entire file into a strict String before it gives you the first Char, right? (Sorry again for my misleading code "simplification".)

...

main = do let thunks :: IO [Int] thunks = (sequence . replicate (10^6) $ (randomRIO (0,10^9))) putStrLn . show . head =<< thunks -- prints putStrLn . show . last =<< thunks -- overflows

Meaning that the entire list needs to be kept? Is there a reason (other than "it's easier to implement and it's legal" :-) why the elements that have been traversed by "last" can't be garbage collected? -m

Daniel Fischer

9:11 p.m.

Am Samstag 10 Oktober 2009 22:14:38 schrieb mf-hcafe-15c311f0c@etc-network.de:

...

On Sat, Oct 10, 2009 at 09:33:52AM -0700, Thomas Hartman wrote:

...
To: Luke Palmer Cc: mf-hcafe-15c311f0c@etc-network.de, haskell-cafe@haskell.org From: Thomas Hartman Date: Sat, 10 Oct 2009 09:33:52 -0700 Subject: Re: [Haskell-cafe] How do I get this done in constant mem?

...
Yes, you should not do this in IO. That requires the entire computation to finish before the result can be used.

Not really the entire computation though... whnf, no?

In that example, yes. But readFile takes the entire file into a strict String before it gives you the first Char, right? (Sorry again for my misleading code "simplification".)

No, readFile reads the file lazily.

...

...
main = do let thunks :: IO [Int] thunks = (sequence . replicate (10^6) $ (randomRIO (0,10^9))) putStrLn . show . head =<< thunks -- prints putStrLn . show . last =<< thunks -- overflows

Meaning that the entire list needs to be kept? Is there a reason (other than "it's easier to implement and it's legal" :-) why the elements that have been traversed by "last" can't be garbage collected?

The problem is that the randomRIO isn't done before it's needed. When you ask for the last element of the generated list, you have a stack of nearly one million calls to randomRIO to get it, that overflows the stack. If you insert a stricter version of sequence: {-# LANGUAGE BangPatterns #-} sequence' :: Monad m => [m a] -> m [a] {-# INLINE sequence' #-} sequence' ms = foldr k (return []) ms where k m m' = do { !x <- m; xs <- m'; return (x:xs) } -- ^^^^^^^^^^^ evaluate x now! main = do let thunks = sequence' . replicate (10^6) $ randomRIO (0,10^9) ... it doesn't overflow the stack. But both, sequence and sequence' must construct the entire list, so they use quite a bit of memory. You can keep the memory usage low by using unsafeInterleaveIO.

...

-m

mf-hcafe-15c311f0c＠etc-network.de

11 Oct 11 Oct

11:18 a.m.

On Sat, Oct 10, 2009 at 11:11:24PM +0200, Daniel Fischer wrote:

...

To: haskell-cafe@haskell.org From: Daniel Fischer Date: Sat, 10 Oct 2009 23:11:24 +0200 Subject: Re: [Haskell-cafe] How do I get this done in constant mem?

Am Samstag 10 Oktober 2009 22:14:38 schrieb mf-hcafe-15c311f0c@etc-network.de:

...
On Sat, Oct 10, 2009 at 09:33:52AM -0700, Thomas Hartman wrote:

...
To: Luke Palmer Cc: mf-hcafe-15c311f0c@etc-network.de, haskell-cafe@haskell.org From: Thomas Hartman Date: Sat, 10 Oct 2009 09:33:52 -0700 Subject: Re: [Haskell-cafe] How do I get this done in constant mem?

...
Yes, you should not do this in IO. That requires the entire computation to finish before the result can be used.

Not really the entire computation though... whnf, no?

In that example, yes. But readFile takes the entire file into a strict String before it gives you the first Char, right? (Sorry again for my misleading code "simplification".)

No, readFile reads the file lazily.

hm? oh, you are right, now that i fixed all the other problems in my code readFile isn't a problem any more either... (-: (but then how does it know when to close the handle? gotta go read the code i guess.) thanks! -m

Ben Franksen

12:17 p.m.

mf-hcafe-15c311f0c@etc-network.de wrote:

...

On Sat, Oct 10, 2009 at 11:11:24PM +0200, Daniel Fischer wrote:

...
No, readFile reads the file lazily.

hm? oh, you are right, now that i fixed all the other problems in my code readFile isn't a problem any more either... (-:

(but then how does it know when to close the handle? gotta go read the code i guess.)

It is somewhat documented, see http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-IO.html or http://www.haskell.org/onlinelibrary/io.html, section 21.2.2 Semi-Closed Handles: "[...] A semi-closed handle becomes closed: * if hClose is applied to it; * if an I/O error occurs when reading an item from the handle; * or once the entire contents of the handle has been read." It is not stated here that the file /immediately/ gets closed after the last byte has been read. Does that mean implementations are free to postpone closing (e.g. until the next GC cycle)? Cheers Ben

mf-hcafe-15c311f0c＠etc-network.de

10 Oct 10 Oct

8:06 p.m.

On Fri, Oct 09, 2009 at 05:48:15PM -0600, Luke Palmer wrote:

...

To: mf-hcafe-15c311f0c@etc-network.de Cc: From: Luke Palmer Date: Fri, 9 Oct 2009 17:48:15 -0600 Subject: Re: [Haskell-cafe] How do I get this done in constant mem?

On Fri, Oct 9, 2009 at 2:05 PM, wrote:

...
Hi all,

I think there is something about my use of the IO monad that bites me, but I am bored of staring at the code, so here you g. The code goes through a list of records and collects the maximum in each record position.

-- test.hs import Random import System.Environment (getArgs) import System.IO (putStr)

samples :: Int -> Int -> IO [[Double]] samples i j = sequence . replicate i . sequence . replicate j $ randomRIO (0, 1000 ** 3)

Yes, you should not do this in IO. That requires the entire computation to finish before the result can be used. This computation should be pure and lazy.

Yeah. I also got an excellent reason via private mail why sequence has to be strict: sequence [Maybe 3, Maybe 4, Nothing] = Nothing sequence [Maybe 3, Maybe 4] = Just [3, 4]

...

...
maxima :: [[Double]] -> [Double] maxima samples@(_:_) = foldr (\ x y -> map (uncurry max) $ zip x y) (head samples) (tail samples)

FWIW, This function has a beautiful alternate definition:

maxima :: [[Double]] -> [Double] maxima = map maximum . transpose

Beautiful indeed! But see below. To be honest, I don't really roll dice, but I am reading from a file. I just thought that randomRIO would be more concise, but now the discussion has gone totally in that direction. Sorry... (-: reading the random number code is more fun, though! Anyhow, I fixed my example to do lazy file processing where before I used readFile (which has to be strict, as I can see now). First, I generate a file with the samples, and then I read that file back (this is the phase I'm interested in, since my real data is not really random numbers). import List import Monad import Random import System.Environment import System.IO samples :: Int -> Int -> IO [[Int]] samples i j = sequence . replicate i . sequence . replicate j $ randomRIO (0, 1000 * 1000 * 1000) maxima :: [[Int]] -> [Int] maxima samples@(_:_) = foldr (\ x y -> map (uncurry max) $ zip x y) (head samples) (tail samples) lazyProcess :: ([[Int]] -> a) -> FilePath -> IO a lazyProcess f fileName = do h <- openFile fileName ReadMode v <- fmap (f . map read . lines) $ hGetContents h v `seq` hClose h return v mkSamples = do args <- getArgs x <- samples (read (head args)) 5 putStr . (++ "\n") . join . intersperse "\n" . map show $ x -- main = mkSamples -- ghc --make -O9 test.hs -o test && ./test 10000 > test.data main = lazyProcess length "test.data" >>= putStr . show lazyProcess (What would be a better name? foldSampleFile perhaps?) is where the IO happens, but the computation is located in a pure function. And yet, only those lines are read that are relevant, and GC on previous lines is allows if the pure function allows it. This program has constant memory usage. Unfortunately, if I replace the length function with implementation of maxima, it explodes again. I tried a few things, such as maxima'3 :: [[Int]] -> [Int] maxima'3 (h:t) = foldr (\ x y -> let v = map (uncurry max) $ zip x y in sum v `seq` v) h t with no luck so far. Tricky business, that! But much more curiously, if I replace maxima'3 in main with this maxima'4 :: [[Int]] -> [Int] maxima'4 = map maximum . transpose (with explicit type signature in both definitions), I get a 'no parse' error from Prelude.read. maxima'3 with the same file gives me a result. How can there be a difference if the type signatures are identical?! Probably something about "don't use Prelude.read" :-)? I have to play with this some more... matthias

Thomas Hartman

3:10 p.m.

I don't know if this counts but how about import Control.Applicative import Control.Monad import Random import Data.List main'' i j = replicateM j $ maximum' <$> (replicateM i . randomRIO $ (0,10^9)) maximum' = foldl1' max t = main'' (10^4) 5 2009/10/9 :

...

Hi all,

I think there is something about my use of the IO monad that bites me, but I am bored of staring at the code, so here you g. The code goes through a list of records and collects the maximum in each record position.

-- test.hs import Random import System.Environment (getArgs) import System.IO (putStr)

samples :: Int -> Int -> IO [[Double]] samples i j = sequence . replicate i . sequence . replicate j $ randomRIO (0, 1000 ** 3)

maxima :: [[Double]] -> [Double] maxima samples@(_:_) = foldr (\ x y -> map (uncurry max) $ zip x y) (head samples) (tail samples)

main = do args <- getArgs x <- samples (read (head args)) 5 putStr . (++ "\n") . show $ maxima x

I would expect this to take constant memory (foldr as well as foldl), but this is what happens:

$ ghc -prof --make -O9 -o test test.hs [1 of 1] Compiling Main ( test.hs, test.o ) Linking test ... $ ./test 100 +RTS -p [9.881155955344708e8,9.910336352165401e8,9.71000686630374e8,9.968532576451201e8,9.996200333115692e8] $ grep 'total alloc' test.prof total alloc = 744,180 bytes (excludes profiling overheads) $ ./test 10000 +RTS -p [9.996199711457872e8,9.998928358545277e8,9.99960283632381e8,9.999707142123885e8,9.998952151508758e8] $ grep 'total alloc' test.prof total alloc = 64,777,692 bytes (excludes profiling overheads) $ ./test 1000000 +RTS -p Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it. $

so...

does sequence somehow force the entire list of monads into evaluation before the head of the result list can be used? what can i do to implement this in constant memory?

thanks! matthias _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Ben Franksen
Daniel Fischer
Luke Palmer
mf-hcafe-15c311f0c＠etc-network.de
Thomas Hartman