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 function,
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.
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