[Haskell-beginners] Array update and `seq`

[Chul-Woong Yang]

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 <wangbj at gmail.com>:
> 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 <cwyang at aranetworks.com>
> 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
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>
>
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