Chris Beggy <[EMAIL PROTECTED]> writes:
> From: Alex Schroeder <[EMAIL PROTECTED]>
>>
>> Lots of packages define missing functions in order to run on Emacs and
>> XEmacs. Other packages test wethere critical functions are bound or
>> not, and then assume they are on Emacs or XEmacs. :(
>>
>> I've had a similar problem with one package or another because BBDB
>> used to do that as well (don't know wether it still does).
>
> What's the best solution for library writers then? Do you know
> of any code examples where this is handled well?
Unfortunately, I don't have a general solution for this. All I can
offer is some rules to guide would-be elisp authors:
Always use the most specific test. Here are some examples from
unspecific (avoid) to specific:
* Testing for Emacs/XEmacs using (featurep 'xemacs).
* Testing for packages using featurep.
* Testing for variables using boundp and functions using fboundp.
Here's an example from color-theme.el where I had to test for
Emacs/XEmacs directly because the function define-key exists in both
versions:
(if color-theme-xemacs-p
(define-key map (kbd "<button2>") 'color-theme-install-at-mouse)
(define-key map (kbd "<mouse-2>") 'color-theme-install-at-mouse))
If you determine that a specific function or variable is lacking,
implementing it may be dangerous and break other things. It's better
to use your own names, and use defalias. Here's from color-theme.el,
for example:
(cond ((fboundp 'custom-face-attributes-get)
(defalias 'color-theme-face-attr-construct
'custom-face-attributes-get))
((fboundp 'face-custom-attributes-get)
(defalias 'color-theme-face-attr-construct
'face-custom-attributes-get))
(t
(defun color-theme-face-attr-construct (&rest ignore)
(error "Unable to construct face attributes"))))
An example of how things can go wrong is in my bug report from
2001-10-05. BBDB defined the function set-specifier if it was
missing. ispell tested for set-specifier to execute some XEmacs
specific code. I think that ispell was was right, because it use the
most specific test and was right in assuming that if set-specifier was
defined, other XEmacs specifier-related machinery would be present
(such as the variable has-modeline-p).
bbdb-gui.el contains the following code:
(or (fboundp 'set-specifier)
(fset 'set-specifier 'ignore))
Therefore, on my Emacs 20.7, I have set-specifier defined. This
causes a problem with ispell.el, however. In ispell-command-loop,
there is the following code:
(and (fboundp 'set-specifier) ; prevent XEmacs modeline hiding
(set-specifier has-modeline-p (cons (current-buffer) nil)))
This will result in:
Signaling: (void-variable has-modeline-p)
The problem with this code is that it defines a function which exists
in the other emacsen, unlike the example from color-theme. As you can
see, however, the choices are not always easy to make. :(
Alex.
--
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