Before listing the full set of clauses that
are allowed, let's look at a few example loops just to get a feel
for the loop language.
(loop for buf in (buffer-list)
collect (buffer-file-name buf))
This loop iterates over all Emacs buffers,
using the list returned by buffer-list. For each
buffer buf, it calls buffer-file-name
and collects the results into a list, which is then returned from
the loop construct. The result is a list of the file
names of all the buffers in Emacs' memory. The words
for, in, and collect are
reserved words in the loop language.
(loop repeat 20 do (insert "Yowsa\n"))
This loop inserts the phrase “Yowsa” twenty times in the current buffer.
(loop until (eobp) do (munch-line) (forward-line 1))
This loop calls munch-line on
every line until the end of the buffer. If point is already at
the end of the buffer, the loop exits immediately.
(loop do (munch-line) until (eobp) do (forward-line 1))
This loop is similar to the above one, except
that munch-line is always called at least once.
(loop for x from 1 to 100
for y = (* x x)
until (>= y 729)
finally return (list x (= y 729)))
This more complicated loop searches for a
number x whose square is 729. For safety's sake it
only examines x values up to 100; dropping the
phrase ‘to
100’ would cause the loop to count upwards
with no limit. The second for clause defines
y to be the square of x within the
loop; the expression after the = sign is reevaluated
each time through the loop. The until clause gives a
condition for terminating the loop, and the finally
clause says what to do when the loop finishes. (This particular
example was written less concisely than it could have been, just
for the sake of illustration.)
Note that even though this loop contains three clauses (two
fors and an until) that would have been
enough to define loops all by themselves, it still creates a
single loop rather than some sort of triple-nested loop. You must
explicitly nest your loop constructs if you want
nested loops.