hyperlinks in variable's value - links to libraries

[Drew Adams]

Between April and June 2005, the code in help-fns.el that defines
`describe-variable' had this line commented out, with this comment:

;; Hyperlinks in variable's value are quite frequently
;; inappropriate e.g C-h v <RET> features <RET>
;; (help-xref-on-pp from (point))

mea culpa - I think I might have been the one to report the bug that the
links in the value of `features' were inappropriate. What I said, however,
was that they should instead be links to the libraries, not links to
something else that happened to have the same name as a library (which was
the "inappropriate" part).

In any case, I disagree with the "fix" of commenting out this line - at the
least, I would like such links to be an option.

In fact, I have a tiny library that does only this: add links to libraries.
This is the entire library (below). I'd suggest it or the equivalent as a
patch. However, adding such links can be slow, so I would like to see an
option for adding links to libraries.

Although it can be slow, it is very useful: if you're checking `features',
for instance (though this is not the only place such links would appear),
you want to know not only if a given library has been loaded, but you (I, at
least) sometimes want to then open one of those libraries. IOW, looking at
`features' should be an entry point to accessing the features listed.

Needless to say, if `describe-variable' is not changed back by uncommenting
that line, I'll also need to add a redefinition of `describe-variable' to my
little library...

---8<--------------------

;; REPLACES ORIGINAL IN `help-mode.el'.
;; Buttonizes names of libraries also.
;; To see the effect, try `C-h v features', and click on a library name.
;;
(defun help-xref-on-pp (from to)
  "Add xrefs for symbols in `pp's output between FROM and TO."
  (if (> (- to from) 5000) nil
    (with-syntax-table emacs-lisp-mode-syntax-table
      (save-excursion
        (save-restriction
          (narrow-to-region from to)
          (goto-char (point-min))
          (condition-case nil
              (while (not (eobp))
                (cond
                 ((looking-at "\"") (forward-sexp 1))
                 ((looking-at "#<") (search-forward ">" nil 'move))
                 ((looking-at "\\(\\(\\sw\\|\\s_\\)+\\)")
                  (let* ((sym (intern-soft (match-string 1)))
                         (type (cond ((fboundp sym) 'help-function)
                                     ((or (memq sym '(t nil))
                                          (keywordp sym))
                                      nil)
                                     ((and sym (boundp sym))
                                      'help-variable)
                                     ((and sym (locate-library (symbol-name
sym)))
                                      'help-library))))
                    (when type (help-xref-button 1 type sym)))
                  (goto-char (match-end 1)))
                 (t (forward-char 1))))
            (error nil)))))))

(define-button-type 'help-library
  :supertype 'help-xref
  'help-function #'(lambda (x) (find-library (symbol-name x)))
  'help-echo (purecopy "mouse-2, RET: find this library"))