(define (hexencode str)
  (let loop ((l   (reverse (string->list str)))
             (r   ""))
    (if (null? l)
        r
        (let ((n   (char->integer (car l))))
          (if (< n 16)
              (loop (cdr l) (string-append "0" (number->string n 16) r))
              (loop (cdr l) (string-append (number->string n 16) r)))))))

(define md5sum
  (let ((d   (digest-by-name "md5")))
    (lambda (f)
      (hexencode (file-digest d f)))))

(define sha1sum
  (let ((d   (digest-by-name "sha1")))
    (lambda (f)
      (hexencode (file-digest d f)))))

; this gives different values for the same file with different names.