;;; Procedure:
;;;   sum-of-file
;;; Parameters:
;;;   file-name, a string that names a file.
;;; Purpose:
;;;   Sums the values in the given file.
;;; Produces:
;;;   sum, a number.
;;; Preconditions:
;;;   file-name names a file. [Unverified]
;;;   That file contains only numbers. [Verified]
;;; Postconditions:
;;;   Returns a number.
;;;   That number is the sum of all the numbers in the file.
;;;   Does not affect the file.
(define sum-of-file
  (lambda (file-name)
    (let* ((source (open-input-file file-name))
           (result (sum-of-file-kernel file-name source)))
       (close-input-port source) 
       result)))
       
;;; Helper:
;;;   sum-of-file-kernel
;;; Notes:
;;;   A lot like sum-of-file, except that it reads the values from
;;;     an open input port rather than a file name.  (The file name
;;;     is also passed in so that it can be used for error messages.)
;;;   Does not verify that the input port is open.
;;;   Crashes (with an error message) if the file contains
;;;     non-numbers.  In that case, it still closes the input port.
(define sum-of-file-kernel
  ; A helper to a helper.  Used only when we need to crash and burn.
  (let ((failure (lambda (file-name source)
                   (close-input-port source)
                   (error "sum-of-file" 
                          (string-append "The file "
                                         file-name
                                         " contains a non-number.")))))
    (lambda (file-name source)
      ; Read a value from the port.
      (let ((nextval (read source)))
        (cond 
          ; Are we at the end of the file?  Then stop and return 0 for
          ; "no numbers read".  Here, we're taking advantage of 0 being
          ; the arithmetic identity.
          ((eof-object? nextval) 0)
          ; Have we just read a number?  If so, add it to the sum of the
          ; remaining numbers.
          ((number? nextval) (+ nextval 
                                (sum-of-file-kernel file-name source)))
          ; Hmmm ... something has gone horribly wrong.
          (else (failure file-name source)))))))