(define large-num (string-append
 "73167176531330624919225119674426574742355349194934"
  "96983520312774506326239578318016984801869478851843"
  "85861560789112949495459501737958331952853208805511"
  "12540698747158523863050715693290963295227443043557"
  "66896648950445244523161731856403098711121722383113"
  "62229893423380308135336276614282806444486645238749"
  "30358907296290491560440772390713810515859307960866"
  "70172427121883998797908792274921901699720888093776"
  "65727333001053367881220235421809751254540594752243"
  "52584907711670556013604839586446706324415722155397"
  "53697817977846174064955149290862569321978468622482"
  "83972241375657056057490261407972968652414535100474"
  "82166370484403199890008895243450658541227588666881"
  "16427171479924442928230863465674813919123162824586"
  "17866458359124566529476545682848912883142607690042"
  "24219022671055626321111109370544217506941658960408"
  "07198403850962455444362981230987879927244284909188"
  "84580156166097919133875499200524063689912560717606"
  "05886116467109405077541002256983155200055935729725"
  "71636269561882670428252483600823257530420752963450"))

(define width 13)

(define (char->number c)
  (if (char-numeric? c)
     (- (char->integer c) (char->integer #\0))
     -1))

(define (substring->numbers subs)
  (map char->number (string->list subs)))

(define (iter i largest)
  (if (>= (+ i width) (string-length large-num))
    largest
    (let* ((sub (substring large-num i (+ i width)))
           (set (substring->numbers sub))
           (product (apply * set)))
        (if (> product (car largest))
            (iter (+ i 1) (cons product set))
            (iter (+ i 1) largest)))))

(display (iter 0 '(-1 . '())))