[racket] idiomatic and fast

Hi,

I'm trying to come up with the most idiomatic way to generate triangle
numbers that is also fast:

(for/list ([i (in-range 2 22)]) (for/sum ([j (in-range i)]) j))

works but is slow because it regenerates the sums each time

(define s 0) (for/list ([i (in-range 1 21)]) (set! s (+ s i)) s)

is fast, but ugly.

How do I keep the sum in an idiomatic fashion?

Thanks,
-Joe
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