Re: [Emacs-diffs] trunk r113822: Add a test for decompressing gzipped data

["Stephen J. Turnbull" <stephen@xemacs.org>]

Juanma Barranquero writes:

 > In other words, portable elisp code should really do
 > 
 > (and (fboundp 'zlib-available-p) (zlib-available-p))
 > 
 > to check for zlib support. Perhaps it would be worth to encapsulate
 > that check into (yet another) predicate...

XEmacs has an if-fboundp macro which does the above, but it's ugly.
I prefer extending the #'featurep mechanism to optionally do such
checks.  Eg,

;;; There's Fprovide(symbol) and Fput(Fintern("symbol"),
;;;                                   Fintern("availability-predicate"),
;;;                                   availability_predicate_DEFUN_symbol))
;;; conditionally compiled on --with-symbol in the startup code.
(defun feature-available-p (symbol)
  (and (featurep symbol)
       ;; I'm a ba-a-ad boy.
       (funcall (get symbol 'availability-predicate (lambda ())))))

Contributed to XEmacs under my agreement with the FSF.  Ie, now
copyright 2013 FSF.  There may be better ways of spelling all this, of
course.