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Re: How to use a symbol and its value to create alist?


From: Pascal J. Bourguignon
Subject: Re: How to use a symbol and its value to create alist?
Date: Wed, 12 Aug 2015 09:03:12 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

Navy Cheng <address@hidden> writes:

>> >> I need to push some global variable to a "stack" and pop them later. If
>> >> I don't do like this, the global variables will be changed by program
>> >
>> >     (defvar a 1)
>> >     (defvar b 2)
>> >     (defvar cc 3)
>> >
>> >     (defun do-something ()
>> >       (print (list 'before a b cc))
>> >       (setf a 0 b 0 cc 0)
>> >       (print (list 'after a b cc)))
>> >
>> >     (progn
>> >       (let ((a a)
>> >             (b b)
>> >             (cc cc))
>> >          (do-something))
>> >       (list 'finally a b cc))
>> >     prints:
>> >
>> >     (before 1 2 3)
>> >
>> >     (after 0 0 0)
>> >     --> (finally 1 2 3)
>> 
>> If you have a lot of global variables you want to preserve like this, or
>> in a lot of places, you can write a macro:
>> 
>>     (defmacro with-saved-variables (variables &rest body)
>>       `(let ,(mapcar (lambda (var) (list var var)) variables) 
>>          ,@body))
>> 
>>      (progn
>>        (with-saved-variables (a b cc)
>>           (do-something))
>>        (list 'finally a b cc))
>>     prints:
>>     (before 1 2 3)
>> 
>>     (after 0 0 0)
>>     --> (finally 1 2 3)
>> 
> Thank you for your answers. As I'm not familar with macros in elisp, I think 
> your
> first answer is good for me.
>
> By the way, you define global variable by (defval). But I always use
> (setq). What's the difference bewteen (defval) and (setq)? And with
> one is recommand to define a global variable?

Use emacs!  C-h f defvar RET  C-h f setq RET

It's defvar, not defval.

The difference is that defvar DEFINES a variable, ie, it CREATES it,
while SETQ doesn't create or define it,  it just modifies an EXISTING
variable.  

Also, if the variable already exists, defvar doesn't do anything, so you
can easily reload your lisp files without resetting all the variables.


-- 
__Pascal Bourguignon__                 http://www.informatimago.com/
“The factory of the future will have only two employees, a man and a
dog. The man will be there to feed the dog. The dog will be there to
keep the man from touching the equipment.” -- Carl Bass CEO Autodesk


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