Re: reading compile.el, modifying settings

[Tassilo Horn]

Sam Steingold <sds@gnu.org> writes:

Hi Sam,

>>> compilation-error-regexp-alist is defined with defcustom, so it should
>>> be modified with custom-set-variables, not with setq.  Don't ask.
>>
>> No, that's not true.  You can setq any defcustom as if it was a defvar.
>
> Sure.  You can setq any symbol.
> However, the _effect_ of setq on a variable defined with defcustom is
> different - it is not respected by emacs on the same level as
> custom-set-variables.
> At least it was not a few years ago, when I had the same problem - my
> setting being ignored - as Paul is having now.

If that really happens, I'd consider it a bug.

> 1. some packages use symbol properties in addition to the symbol value,
>    these are modified by custom-set-variables automatically

,----
| ;; foo is not defined before
| (setq foo 17)
| ;; now load its defcustom
| (defcustom foo 111
|   "Bla bla")
| ;; still has my value plus the props of the defcustom
| (cons foo (symbol-plist 'foo))
| ==> (17 standard-value (111) custom-requests nil variable-documentation "Bla 
bla")
`----

> 2. some custom variables have fancy setters which are called by
>    custom-set-variables but not by setq (like symbol macros in CL)

Yeah, but that has to be stated in the docs:

,----[ (info "(elisp)Variable Definitions") ]
|      If you specify the `:set' keyword, to make the variable take other
|      special actions when set through the customization buffer, the
|      variable's documentation string should tell the user specifically
|      how to do the same job in hand-written Lisp code.
`----

Good examples here are all those minor mode *variables*, which can be
customized to enable a mode globally, but to enable them from lisp, you
have to use the mode *function*:

  ;; Works
  (custom-set-variables
    '(hl-line-mode t))

  ;; Does not work (i.e., doesn't enable that mode)
  (setq hl-line-mode t)
  ;; so you need to use the mode function from lisp
  (hl-line-mode 1)

> In short, the rule of thumb is: do not use setq on custom variables,
> use custom-set-variables instead.

I really don't buy that.  My emacs is highly customized in plain lisp
only, so that I can comment what I do and structure it according to my
likings.

Bye,
Tassilo