bug#13687: /srv/bzr/emacs/trunk r111878: * lisp/replace.el (read-regexp): Let-bind `default' to the first

[Juri Linkov]

> So the question is: should the default value in the caller
> `highlight-regexp' be changed from `(car regexp-history)'
> to code that gets the tag at point?  You could propose
> such a change, but since it changes the long-standing behavior,
> expect some disagreement (not from me :-)

There are surprisingly many users who prefer the previous item from the
history instead of the tag at point as the default value of `occur',
`highlight-regexp', `rgrep'.  They got `(car regexp-history)' hard-coded
into core Emacs for `occur' and `highlight-regexp', but not for `rgrep',
so they raise the questions how to do the same for `rgrep', and get such
horribly ugly solutions as this one:

http://stackoverflow.com/questions/15161592/make-emacs-rgrep-default-to-last-search-term-rather-than-word-at-point

This situation suggests that the default values should be customizable.
Possible variants:

1. Put a special value on the command's symbol like:
   (put 'highlight-regexp 'default 'history)
   (put 'highlight-regexp 'default 'tag-at-point)
   checked on `this-command' in minibuffer-reading functions.
   Cons: Not easy to use.

2. Add a new defcustom like:
   (defcustom minibuffer-defaults '((highlight-regexp-default . tag-at-point)
                                    (rgrep . history)
                                    (occur . tag-at-point)
                                    (how-many . history)
                                    ...))
   Cons: Too large list of commands for one option.

3. In the DEFAULT arg of minibuffer-reading calls
   specify a function that returns default values:

   (defun highlight-regexp (regexp &optional face)
     (interactive
      (list
       (read-regexp "Regexp to highlight" 'highlight-regexp-default)
       ...

   (defun highlight-regexp-default ()
     (car regexp-history))

   where users can override it with another function:

   (defun highlight-regexp-default ()
     (let* ((tagf (or find-tag-default-function
                             (get major-mode 'find-tag-default-function)
                             'find-tag-default))
                   (tag (funcall tagf)))
              (cond ((not tag) "")
                    ((eq tagf 'find-tag-default)
                     (format "\\_<%s\\_>" (regexp-quote tag)))
                    (t (regexp-quote tag)))))

   Pros: Flexible and easy to configure.