[plt-scheme] Statistics for Sequences

From: Jakub Piotr Cłapa (jpc-ml at zenburn.net)
Date: Thu Sep 10 10:47:53 EDT 2009

On 09-09-10 16:30, Doug Williams wrote:
> It's interesting that if I use (in-vector ...) in the for/fold
> statements, the times for the for/fold version are about the same as for
> the (uglier) do version (with vector-refs). [This one probably would
> benefit from Matthew's performance improvements.] Actually using it
> would mean giving up the flexibility in going to sequences in the first
> place, but it means there is some hope of eventually getting the same
> performance for the sequence versions (at least for vectors).
> using in-vector in the for
> cpu time: 266 real time: 265 gc time: 0
> cpu time: 250 real time: 250 gc time: 47
> current science collection routines
> cpu time: 250 real time: 249 gc time: 0
> cpu time: 218 real time: 218 gc time: 16
> It would be nice if (for ((x some-vector)) ...) and (for ((x (in-vector
> some-vector))) ...) had similar performance. I realize that at expansion
> time the latter knows to expect a vector while the former does not and
> can generate code accordingly. But, I can dream.

AFAIU you could special case vectors (duplicating the code) if you 
expect them to be used frequently. Probably a for-like macro expanding 
into shortcuts for specified fast iterators would be nice to have. 
Something like

(for ([x (in (list vector string) lst)])

would expand to

   [(list? x) (for ([x (in-list lst)]) x)]
   [else (for ([x lst]) x)])

PS. And what about generating such special cases by evaling a 
dynamically generated lambda at runtime? I guess it would make really 
long iterations faster but the eval overhead would kill the performance 
for short ones?

Jakub Piotr Cłapa

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