How to set tags-table-list as a per-directory local variable?

Question

I have two tags tables that I would like to be searched/visited when I'm working on one of my R packages. I am able to set them globally with the following code in my .emacs config file:

(setq tags-table-list '("/R/Source/R-devel-SVN/TAGS" "/gitRepos/DADS/TAGS"))

But I cannot for the life of me figure out how to get the equivalent effect by directives placed in the project's .dir-locals.el. I've tried each the following (one at a time, of course), and many others, without success:

(nil (tags-table-list ("/R/Source/R-devel-SVN/" "/gitRepos/DADS/"))
((nil (tags-table-list ("/R/Source/R-devel-SVN/" "/gitRepos/DADS/")))
(nil (tags-table-list ("/R/Source/R-devel-SVN/TAGS" "/gitRepos/DADS/TAGS"))
(nil (tags-table-list '("/R/Source/R-devel-SVN/" "/gitRepos/DADS/"))
(nil (tags-table-list . ('("/R/Source/R-devel-SVN/" "/gitRepos/DADS/"))))
(nil . ((tags-table-list . (("/R/Source/R-devel-SVN/" "/gitRepos/DADS/")))))

Perhaps relevant is that setting the following variable does work as expected, which makes me suspect that I'm just missing the right incantation for setting tags-table-list -- not that the mechanism underlying .dir-locals.el is broken:

((nil (tags-file-name . "/gitRepos/DADS/TAGS")))

How can I set the value of tags-table-list using the notation required by .dir-locals.el?

Answer 1

All your attempts to set tags-table-list are errorneous.

All cases except the second one do not define a mapping of modes to alists of settings and are therefore wrong.

Only the second case remains to be discussed. In the following I add the missing closing parenthesis ):

((nil (tags-table-list ("/R/Source/R-devel-SVN/" "/gitRepos/DADS/"))))

Nevertheless, also this setting for tags-table-list is wrong. The elements of the alist mapping variables to values are cons, so it is better to write the setting using dotted-pair notation:

((nil (tags-table-list . (("/R/Source/R-devel-SVN/" "/gitRepos/DADS/")))))

This shows that you assign a list of a list to tags-table-list. But you should assign a list of directory names. One simple solution is adding a dot behind tags-table-list:

((nil (tags-table-list . ("/R/Source/R-devel-SVN/" "/gitRepos/DADS/"))))

I have just tested it with

emacs-version: GNU Emacs 26.2 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.22.30) of 2019-04-12

Contents of file ~/tmp/.dir-locals.el:

((nil (tags-table-list . ("~/tmp/DADS/"))))

Contents of file ~/tmp/DADS//TAGS:

^L
../test/test.c,14
int main(^?1,0

Contents of file ~/tmp/test/test.c:

int main() {
    return 0; /* \f$\sqrt{x^2+y^2}\f$. */
}

The command tags-search called with interactive argument main works like expected when the buffer for file ~/tmp/test/test.c is active and point is at the start of that file. It places point at the end of main in file ~/tmp/test/test.c.