Support for "special" file local variables

Support for "special" file local variables

[Emilio Lopes]

The following patch implements support for "special" file local variables,
in the vein of the already existent `Mode' and `Coding'.

These special variables can be a short alias for a "real" file local.  They
can also trigger some special action when the variable is set in the local
variable specifications.

My motivation was to support (for historical reasons) file variable
specifications like

   ; -*- Mode: Scheme; Syntax: Scheme; Package: vm; -*-

without polluting the name space with short-named variables like `syntax'
or `package'.

It's such a simple change that I see no obvious reason why this could not
be added to Emacs.  I'm sure others will find nice applications for this
too.

Do you find this useful?


2005-08-02  Emilio C. Lopes  <eclig@gmx.net>

	* files.el (hack-one-local-variable): support "special" file
        local variables.
	(special-local-variables-alist): new user option.


*** orig/lisp/files.el
--- mod/lisp/files.el
***************
*** 473,478 ****
--- 473,494 ----
  		 (other :tag "Query" other))
    :group 'find-file)
  
+ (defcustom special-local-variables-alist nil
+   "Alist of local variable names to be handled specially.
+ Each element looks like (VAR-NAME . NEW-VAR).
+ 
+ Whenever VAR-NAME is set to some value in the local variable
+ specifications that value is actually assigned to NEW-VAR.
+ 
+ As a special case, NEW-VAR can be a function.  In this case it
+ will be called with two arguments, the name of the variable and
+ the corresponding value, and should do whatever is necessary to
+ actually accomplish the assignment."
+   :type '(repeat (cons (symbol :tag "Original variable name")
+ 		       (choice (symbol :tag "New variable name")
+                              (function :tag "Function called to accomplish the assignment"))))
+   :group 'find-file)
+ 
  ;; Avoid losing in versions where CLASH_DETECTION is disabled.
  (or (fboundp 'lock-buffer)
      (defalias 'lock-buffer 'ignore))
***************
*** 2469,2475 ****
    "\"Set\" one variable in a local variables spec.
  A few patterns are specified so that any name which matches one
  is considered risky."
!   (cond ((eq var 'mode)
  	 (funcall (intern (concat (downcase (symbol-name val))
  				  "-mode"))))
  	((eq var 'coding)
--- 2485,2498 ----
    "\"Set\" one variable in a local variables spec.
  A few patterns are specified so that any name which matches one
  is considered risky."
!   (cond ((assq var special-local-variables-alist)
!          (let ((new-var (cdr (assq var special-local-variables-alist))))
!            (if (functionp new-var)
!                (funcall new-var var val)
!              ;; note that there's no check for recursive entries in
!              ;; `special-local-variables-alist'
!              (hack-one-local-variable new-var val))))
!         ((eq var 'mode)
  	 (funcall (intern (concat (downcase (symbol-name val))
  				  "-mode"))))
  	((eq var 'coding)

Re: Support for "special" file local variables

[Richard M. Stallman]

It seems like a good idea.  However, special-local-variables-alist
should not be customizable; it is not for users to set.
Also, it has to be marked as a risky local variable.

But this shouldn't be installed now; it should wait for after the
release.