> Yeah, I meant "trivial now", of course. But that's been in Emacs for
> what? Five years?
There's no released Emacs with defvaralias, so it's still recent.
>> Among the local patches I use, I've expanded this `constant' field to
>> 2 bits, so it can be either hard-read-only or soft-read-only (or writable,
>> of course). The soft-read-only state is used by defconst so that variables
>> defined by defconst cannot be setq but can be changed via defconst
>> (important when you reload a new version of a package where a defconst was
>> changed).
> Do you allow un-const'ing things?
No. But I could make `defvar' un-const a defconst if that's necessary.
> If so, special care should be taken with `enable-multibyte-characters',
> nil, t and perhaps a few others...
These are hard-read-only so they can't be redefined with defconst
or un-const'd.
>> Making defconst variables into constants introduces a few occasional
>> problems with some packages, of course, but nothing too hard to fix.
> And in fact I'd bet most of these problems are, if not bugs, at least
> bad style... :)
Yes, most. But for some the fix is not too elegant either.
Typical problems:
- The const-value can't be conveniently constructed at top-level:
(defvar foo nil)
(defvar bar nil)
(let ((blabla))
(setq foo (toto blabla))
(setq bar (toto blabla)))
Removing the defvars and replacing the setq with defconst is correct, but
the byte-compiler is a bit dumb and don't realize that foo and bar are
both unconditionally defined, so it may warn of unknown variables.
- name clashes. E.g.
(defconst e 2.718281828459045)
...
(let ((b (match-beginning 0))
(e (match-end 0)))
...
- there are also other cases where a const-var is not quite constant: some
part of the code (or some advice, or hook) wants to temporarily let-bind
it to some other value.
> Could you please post your code here? I think it would be a great
> feature (after 22.1).
I doubt it'll ever make its way into Emacs: it introduces compatibility
problems without bringing any clear benefit.
Stefan
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