a question about `,' in backquote

a question about `,' in backquote

[Masatake YAMATO]

I found a following line in the definition of `tags-search':

   (setq tags-loop-scan `(re-search-forward ',regexp nil t)

Could you tell me why "'" is needed before "," ?
Is there any strong reason?

I think it is easier to read next line:

   (setq tags-loop-scan `(re-search-forward ,regexp nil t)


;;;###autoload
(defun tags-search (regexp &optional file-list-form)
  "Search through all files listed in tags table for match for REGEXP.
Stops when a match is found.
To continue searching for next match, use command \\[tags-loop-continue].

See documentation of variable `tags-file-name'."
  (interactive "sTags search (regexp): ")
  (if (and (equal regexp "")
	   (eq (car tags-loop-scan) 're-search-forward)
	   (null tags-loop-operate))
      ;; Continue last tags-search as if by M-,.
      (tags-loop-continue nil)
    (setq tags-loop-scan `(re-search-forward ',regexp nil t)
	  tags-loop-operate nil)
    (tags-loop-continue (or file-list-form t))))

Re: a question about `,' in backquote

[Andreas Schwab]

Masatake YAMATO <jet@gyve.org> writes:

> I found a following line in the definition of `tags-search':
>
>    (setq tags-loop-scan `(re-search-forward ',regexp nil t)
>
> Could you tell me why "'" is needed before "," ?

Because the form is passed to eval, which evaluates the function arguments
before applying them.  Thus without the quote it would effectively be
doing (eval regexp), which is not what is wanted.  Of course, the only
valid type of this argument is a string which is self-evaluating, but you
don't want to evaluate arbitrary values in the error case.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
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"And now for something completely different."

Re: a question about `,' in backquote

[Masatake YAMATO]

> [...]  Of course, the only
> valid type of this argument is a string which is self-evaluating, but you
> don't want to evaluate arbitrary values in the error case.

This is the point which I'd like to know.
I have to revise my private elisp library:-(

Thank you.

Masatake YAMATO