The performance of `sequence->list' came up on this thread:
http://thread.gmane.org/gmane.comp.lang.racket.user/16384
`sequence->list' uses the sequence API to make a copy of its input. This
can dominate the running time of functions that look like this:
(define (f xs)
(let ([xs (sequence->list xs)])
... simple O(n) computation using xs ...))
Also, it seems wasteful to make a copy if `xs' is already a list.
I'd like to change the definition of `sequence->list' from this:
(define (sequence->list s)
(for/list ([v s]) v))
to this, to handle the two most common sequences specially:
(define (my-sequence->list s)
(cond [(list? s) s]
[(vector? s) (vector->list s)]
[else (for/list ([v s]) v)]))
For vectors, I measure a 3x speedup. For lists, it's of course O(1).
It's a semantic change, but I can't imagine anyone relying on this fact:
(not (eq? xs (sequence->list xs)))
If someone does, [(list? s) (map values s)] would preserve that fact,
and is 3x faster in my measurements.
Thoughts?
Neil ⊥
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