Dice coeficient in Scheme

dicecoef.scm

#!/usr/local/bin/gsi-script

(define (bigram lst)
  (cons (car lst) (cadr lst)))

(define (bigrams lst)
    (if (> (length lst) 2)
      (cons (bigram lst) (bigrams (cdr lst)))
      (cons (bigram lst) '())))

(define (match ls1 ls2)
  (let ((nmatch 0))
    (for-each
      (lambda (x)
        (if (member x ls2)
          (set! nmatch (+ nmatch 1))
          ))
      ls1)
    nmatch))

(define (dicecoef ls1 ls2)
  (let ([bls1 (bigrams ls1)]
        [bls2 (bigrams ls2)]
        )
  (/ (* 2 (match bls1 bls2)) (+ (length bls1) (length bls2)))))

(define (main . args)
  (define (usage) (display "usage: dicecoef <word> <word>"))

  (if (or (null? args) (not (= (length args) 2)))
    (usage)
    (let ((w1 (car args)) (w2 (cadr args)))
      (display (string-append
                 "Dice coeficient of " w1 " and " w2 " is: "))
      (display (exact->inexact (dicecoef
                                 (string->list w1)
                                 (string->list w2))))
      (newline))
    ))