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Programming in Emacs Lisp

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The forward motion while loop

The second part of the body of the let* deals with forward motion. It is a while loop that repeats itself so long as the value of arg is greater than zero. In the most common use of the function, the value of the argument is 1, so the body of the while loop is evaluated exactly once, and the cursor moves forward one paragraph.

This part handles three situations: when point is between paragraphs, when point is within a paragraph and there is a fill prefix, and when point is within a paragraph and there is no fill prefix.

The while loop looks like this:

 
(while (> arg 0)
  (beginning-of-line)

  ;; between paragraphs
  (while (prog1 (and (not (eobp))
                     (looking-at paragraph-separate))
           (forward-line 1)))

  ;; within paragraphs, with a fill prefix
  (if fill-prefix-regexp
      ;; There is a fill prefix; it overrides paragraph-start.
      (while (and (not (eobp))
                  (not (looking-at paragraph-separate))
                  (looking-at fill-prefix-regexp))
        (forward-line 1))

    ;; within paragraphs, no fill prefix
    (if (re-search-forward paragraph-start nil t)
        (goto-char (match-beginning 0))
      (goto-char (point-max))))

  (setq arg (1- arg)))

We can see immediately that this is a decrementing counter while loop, using the expression (setq arg (1- arg)) as the decrementer.

The body of the loop consists of three expressions:

 
;; between paragraphs
(beginning-of-line)
(while
    body-of-while)

;; within paragraphs, with fill prefix
(if true-or-false-test
    then-part

;; within paragraphs, no fill prefix
  else-part

When the Emacs Lisp interpreter evaluates the body of the while loop, the first thing it does is evaluate the (beginning-of-line) expression and move point to the beginning of the line. Then there is an inner while loop. This while loop is designed to move the cursor out of the blank space between paragraphs, if it should happen to be there. Finally, there is an if expression that actually moves point to the end of the paragraph.


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