Re: (copy-marker nil)

[John Mastro]

Andreas Röhler <andreas.roehler@easy-emacs.de> wrote:
> being surprised WRT behavior of copy-marker:
>
> (setq a (copy-marker nil)) -> #<marker in no buffer>
> (markerp a) -> t
> a -> #<marker in no buffer>
>
> Is taking nil by copy-marker reasonable?
>
> Wanted to check for a valid buffer position, which doesn't seem
> possible that way. While without copy-marker, the variable a would be
> set to nil, now the result evaluates to #<marker in no buffer>, which
> is a kind of t.

There are other ways to get a marker to no buffer, such as

(let ((a (point-marker)))
  (set-marker a nil))

So if you don't control the creation of the marker, it's probably a
scenario you need to take care for either way.

Explicitly testing the marker's buffer should work:

(let ((a (copy-marker nil)))
  (and (markerp a)
       (buffer-live-p (marker-buffer a))))

-- 
John