Sieve of Eratosthenes

primes.rkt

#lang racket
(define (primes-less-than n)
  (define size (sub1 (quotient n 2)))
  (define table (build-vector size (lambda (i) (+ i i 3))))
  (for ((x (in-vector table 0 (integer-sqrt size)))
        #:when x
        (y (in-range (quotient (- (* x x) 3) 2) size x)))
    (vector-set! table y #f))
  (cons 2 (filter values (vector->list table))))