Q5.1.10: map-extents won’t traverse all of my extents!

I tried to use map-extents to do an operation on all the extents in a region. However, it seems to quit after processing a random number of extents. Is it buggy?

No. The documentation of map-extents states that it will iterate across the extents as long as function returns nil. Unexperienced programmers often forget to return nil explicitly, which results in buggy code. For instance, the following code is supposed to delete all the extents in a buffer, and issue as many ‘fubar!’ messages.

(map-extents (lambda (ext ignore)
               (delete-extent ext)
               (message "fubar!")))

Instead, it will delete only the first extent, and stop right there – because message will return a non-nil value. The correct code is:

(map-extents (lambda (ext ignore)
               (delete-extent ext)
               (message "fubar!")
               nil))