String Split in Scheme

gistfile1.scm

;;; str-split : Apr 2006 Doug Hoyte, hcsw.org.
;;; ----
;;; Splits a string 'str into a list of strings
;;; that were separated by the delimiter character 'ch
;;; ----
;;; Efficient as possible given that we can't count on
;;; 'str being an immutable string.

(define (str-split str ch)
  (let ((len (string-length str)))
    (letrec
      ((split
        (lambda (a b)
          (cond
            ((>= b len) (if (= a b) '() (cons (substring str a b) '())))
              ((char=? ch (string-ref str b)) (if (= a b)
                (split (+ 1 a) (+ 1 b))
                  (cons (substring str a b) (split b b))))
                (else (split a (+ 1 b)))))))
                  (split 0 0))))

Thanks. The search engine AI gave me all sorts of implementations that didn't work, and I couldn't fix them.