[Haskell-cafe] implementing python-style dictionary in Haskell

Jan-Willem Maessen jmaessen at alum.mit.edu
Tue Nov 18 08:33:31 EST 2008

On Nov 18, 2008, at 7:03 AM, Bulat Ziganshin wrote:

> Hello Tillmann,
> Tuesday, November 18, 2008, 2:46:47 PM, you wrote:
>> Why should a Haskell hash table need more memory then a Python hash
>> table? I've heard that Data.HashTable is bad, so maybe writing a good
>> one could be an option.
> about Data.HashTable: it uses one huge array to store all the entries.
> the catch is that GC need to scan entire array on every (major) GC.

Actually, the scan on every major (full) GC is unavoidable.  What  
*can* be avoided is a scan on every *minor* GC that occurs after an  
update.  I forget what the exact strategy is here, but I know that one  
write used to cause the entire array to be re-scanned; what I don't  
remember is when/if the array transitions back to a state where it  
isn't being scanned by minor GC anymore.

> using array of hashtables may improve situation a lot

Yes, this would be worth trying.  Understanding the current GC  
strategy would make it easier to make the right tradeoffs here; we  
expect n insertions will touch O(n) subtables, so repeated insertion  
will make life worse if we're not careful.

-Jan-Willem Maessen

> plus check GC times for every version: +RTS -Soutfile
> -- 
> Best regards,
> Bulat                            mailto:Bulat.Ziganshin at gmail.com
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