Subject: dir-locals: superdir (or parentdir) support

From: Jambunathan K <[email-masked]>
Date: Tue, 03 Sep 2013 09:09:15 +0530

Introductory word:

I am having a private library that implements a unified search
interface.  It will use `cscope' as search engine while visiting *.[ch]
files and `rgrep' as search engine while visiting *.el files.  

The search as it stands will be on the whole of search tree.

It is "specified" in as .dir-locals.el as below.

~/src/emacs/trunk/.dir-locals.el:

    ((c-mode
      (search-dir . "~/src/emacs/trunk/")
      (search-engine . cscope))
     (emacs-lisp-mode
      (search-dir . "~/src/emacs/trunk/lisp")
      (search-engine . rgrep)))

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Problem:

If I am visiting an Org file (say trunk/lisp/org/org.el), the recursive
search *on whole lisp tree* happens just fine.

But if I am visiting a Gnus file (say trunk/lisp/org/org.el), the
recursive search fails.

----------------------------------------------------------------

Diagnosis:

The reason is this because Gnus has it's own .dir-locals.el for lisp
files.

~/src/emacs/trunk/lisp/gnus/.dir-locals.el:

    ((emacs-lisp-mode . ((show-trailing-whitespace . t))))

----------------------------------------------------------------

What I need:

So to realize my use case, I want that the "emacs-lisp-mode" settings
from Gnus tree be merged with that specified from trunk.

----------------------------------------------------------------

One possible way to achieve it:

Introduce a `superdir' or a `parentdir' keyword, to traverse till the
root.

~/src/emacs/trunk/lisp/gnus/.dir-locals.el:

    ((emacs-lisp-mode . ((show-trailing-whitespace . t)
			 (superdir . t) ; <=====================
			 )))

Compare `superdir' to the already existing `subdir'.  From 

(info "(emacs) Directory Variables")

    ((c-mode . ((c-file-style . "BSD")
                (subdirs . nil) ; <=====================
                )))

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Closing note:

Particularly useful for "submodules" which are part of a "sumo" project.