I have a question about Emacs Lisp. What is the difference between setq and setq-default?
Tutorials say setq takes effect in the local buffer while setq-default affects all buffers.
For example, if I wrote (setq a-var a-vars-value) in init.el, I found after starting Emacs and opening a new buffer, the a-var is also there and its value is a-vars-value. I thought it was not supposed to be there. It seems there is no difference between setq and setq-default.
Is there something wrong with my understanding?
For example:
-
I wrote (setq hello 123) in the init.el file, and I run emacs abuffer in the shell, then I input "hello C-x C-e", it shows "123". The same happens when I run this in all new buffers.
-
I wrote (setq tab-width 4) in the init.el file. When I run tab-width C-x C-e, it shows "8" (Current mode is 'Text'). However, when I use (setq-default tab-width 4), it show "4". I can't explain this phenomenon.
Best Answer
Some variables in Emacs are "buffer-local", meaning that each buffer is allowed to have a separate value for that variable that overrides the global default.
tab-width
is a good example of a buffer-local variable.If a variable is buffer-local, then
setq
sets its local value in the current buffer andsetq-default
sets the global default value.If a variable is not buffer-local, then
setq
andsetq-default
do the same thing.In your case 2,
(setq tab-width 4)
set the buffer-local value oftab-width
to 4 in the current buffer, leaving the global default value oftab-width
still at 8, so when you evaluatedtab-width
in a different buffer that had no local value, you saw that 8. Then, when you set the default value to 4, that buffer picked it up, since it still had no local value.