Append pair to list in racket/scheme

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As shown below, I have the load-mobile-plan function which iterate the text and construct the nth and n+1 lines(n = odd) into a pair, it then append it to a list.

(define temp-pair '())
(define mobile-plan '())

(define load-mobile-plan
  (lambda (file)
    (let [(line (read-line file))]
      (unless (eof-object? line)
        (set! temp-pair (cons line (read-line file)))
        (set! mobile-plan (cons temp-pair mobile-plan))
        (load-mobile-plan file)))))

(call-with-input-file "xxx.txt" load-mobile-plan)

My questions:
-Is this an appropriate method in functional programming? As i heard that function in functional programming should not have side-effects and depends on its own input variables only.
-If so, how to modify my codes to achieve same goal?

Posted 29-Oct-12 22:35pm

tuolesi

Updated 30-Oct-12 3:29am

v2

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tuolesi · Accepted Answer · 2012-11-03T23:28:00

OK after few days of "investigation" into functional programming, i manage to code the following, which do the same job as my codes in question:

(define load-mobile-plan
  (lambda (file)
    (let [(mobile-plan '())]
      (read-file file mobile-plan))))

(define read-file
  (lambda (file mobile-plan)
    (let [(line (read-line file))]
      (if [eof-object? line]mobile-plan
          (let* [(temp-pair (cons (string-normalize-spaces line)
                                  (string-normalize-spaces (read-line file))))
                 (mobile-plan (cons temp-pair mobile-plan))]
            (read-file file mobile-plan))))))

(call-with-input-file "xxx.txt" load-mobile-plan)