# [racket] static variables question

I've seen Eli's example of static variables on Stackoverflow, but I still
need help.
I've created a working sieve and now I want to wrap it in a manner that
save primes to be used later.
This is what I tried:
; wrapper function for sieve
(define (primes-from-to start end)
(let ([lastend 0] [storedlst '()])
(lambda ()
(cond [(= lastend 0) (set! storedlst (primes-to end))]
[(= lastend start) (set! storedlst (sieve (append storedlst
(interval-list start end))))]
[(< lastend end) (primes-from-to lastend end)])
; storedlst now has all the primes needed
(set! lastend end)
(filter (lambda (v) (if (and (>= v start) (<= v end)) #t #f)) storedlst)
)))
It works, but I can't get any speed advantages as I simply don't know how
to syntactically vary start and end and save lastend and storedlst.
(define a (prime-from-to 100 200)) is not useful when I want to later call
(a 50 70).
I read the manual about all the define* functions but really nothing seems
to fit, what am I missing?
Thanks,
-joe
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