Node:fwd-para between paragraphs, Next:fwd-para within paragraph, Previous:fwd-para while, Up:forward-paragraph

Between paragraphs

First, let us look at the inner while loop. This loop handles the case when point is between paragraphs; it uses three functions that are new to us: prog1, eobp and looking-at.

• prog1 is similar to the progn special form, except that prog1 evaluates its arguments in sequence and then returns the value of its first argument as the value of the whole expression. (progn returns the value of its last argument as the value of the expression.) The second and subsequent arguments to prog1 are evaluated only for their side effects.
• eobp is an abbreviation of End Of Buffer P and is a function that returns true if point is at the end of the buffer.
• looking-at is a function that returns true if the text following point matches the regular expression passed looking-at as its argument.

The while loop we are studying looks like this:

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

This is a while loop with no body! The true-or-false-test of the loop is the expression:

(prog1 (and (not (eobp))
            (looking-at paragraph-separate))
       (forward-line 1))

The first argument to the prog1 is the and expression. It has within in it a test of whether point is at the end of the buffer and also a test of whether the pattern following point matches the regular expression for separating paragraphs.

If the cursor is not at the end of the buffer and if the characters following the cursor mark the separation between two paragraphs, then the and expression is true. After evaluating the and expression, the Lisp interpreter evaluates the second argument to prog1, which is forward-line. This moves point forward one line. The value returned by the prog1 however, is the value of its first argument, so the while loop continues so long as point is not at the end of the buffer and is between paragraphs. When, finally, point is moved to a paragraph, the and expression tests false. Note however, that the forward-line command is carried out anyhow. This means that when point is moved from between paragraphs to a paragraph, it is left at the beginning of the second line of the paragraph.