My Miller-Rabin implementation (see http://codereview.stackexchange.com/a/129991/216 for context)

miller-rabin.rkt

#lang racket
(define (expmod base exp m)
  (let recur ([exp exp])
    (cond [(zero? exp) 1]
          [(even? exp) (let* ([prev (recur (quotient exp 2))]
                              [result (modulo (sqr prev) m)])
                         (if (and (< 1 prev (sub1 m))
                                  (= result 1))
                             0
                             result))]
          [else (modulo (* base (recur (sub1 exp))) m)])))

(define (composite? n [trials 100])
  (for/first ([i (in-range trials)]
              #:unless (= (expmod (random 2 (min (sub1 n) 4294967087))
                                  (sub1 n) n) 1))
    #t))

BTW, the (min (sub1 n) 4294967087), as opposed to just (sub1 n), is to prevent random from issuing a domain error.