seq only force to WHNF (https://wiki.haskell.org/Weak_head_normal_form), while deepseq could force elements in the data structure (hence the name) list1 :: [Int] list1 = [undefined] list1' = length (list1 `seq` list1) -- OK list1'' = length (list1 `deepseq` list1) -- error I'd expect something similar happened in IArray. On Tue, Apr 26, 2016 at 11:36 PM Chul-Woong Yang wrote:

Thank you for your answer. I'll look into MArray.

However, I want to know whether `seq` forces evaluation of array update (//) or not. My prior experimen shows that `deepseq` forces evaluation while `seq` does not force. Is it right? And, is there no way to force evaluation of array update with `seq`?

2016-04-27 15:12 GMT+09:00 Baojun Wang :

...

Immutable array update creates a new array for each call to update (//), thus if you need frequent update, it is recommended to use MArray (such as ST or IO variant) instead.

i.e: the loop can be rewrite as:

update_ u x = readArray u x >>= \old -> writeArray u x (1+old) array100 = runSTUArray $ do stu <- newArray (0, 99) 0 :: ST s (STUArray s Int Int) mapM_ (update_ stu) (concat . replicate 5000 $ [0..99]) return stu

- baojun

On Tue, Apr 26, 2016 at 4:53 PM Chul-Woong Yang wrote:

...

Hi, all

When I fold a list to update Data.Array, memory usage is very high. Consider following source, which counts occurence of each element in a list (1):

import Data.Array import Data.List import Control.DeepSeq go :: [Int] -> [Int] go = elems . foldl' update (array (0,99) [(i,0) | i <- [0..99]]) where update acc x = acc // [(x, acc ! x + 1)] main = putStrLn . unwords . map show . go . concat . replicate 5000 $ [0..99]

Above program uses about 350MB at heap. Memory usage is same if I try to force strictness in array update with `seq` (2) :

where update acc x = let v = acc ! x + 1 a' = acc // [(x,v `seq` v)] in a' `seq` a'

However, when I use `deepseq`, memory usage is gone (around 35Kbyte) (3):

where update acc x = let v = acc ! x + 1 a' = acc // [(x,v `seq` v)] in a' `deepseq` a'

What's the missing part in (2)? At (2), evaluation of updated array a' is forced and the value of array cell is also evaluated forcefully with `seq`.

Any help would be appreciated deeply. -- Regards, Chul-Woong Yang _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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