RE: [External] : Re: Conditional binding and testing of `lexical-binding'

[Drew Adams]

> > What's the best way, or a reasonable way, 
> > of setting `lexical-binding' conditionally,
> 
> I don't know what you mean by that, 

And yet I gave details to make it clear, as
well as examples.  Which part(s) didn't you
understand?

> nor why you think it's necessary for:

If you didn't understand what "it" is, then
how is it that you think it's not necessary
for what follows?

> > so a library can be used with both Emacs
> > versions that support lexical binding
> > and versions that don't support it?

There you have it: setting it conditionally
so it can be tested, to check whether the
Emacs version being used supports lexical
binding.

> Adding the -*- lexical-binding:t -*- at the
> beginning of the file does that

Thanks for confirming that this is the best
(the only?) way to do it.

If you read my mail you'll see that I figured
out, by experimenting, that it's _sufficient_
to do that, to be able to test even with old
Emacs versions (which don't have variable
`lexical-binding').

This isn't obvious (and it's undocumented,
AFAICT).

It wasn't clear, before experimenting, what
that declaration would mean for Emacs 23 or
earlier - which behavior:

1. It implicitly creates a defvar and sets the
   value to `t'.

2. It's ignored, so the variable is undefined
   (unbound).

3. Something else.

Before experimenting, I was guessing #1.  But
it seems to be #2.

Knowing this is important, for testing the 
variable.  As I said, the test I apparently
need is this:

 (and (boundp 'lexical-binding)
      lexical-binding)

[Emacs 22 has `bound-and-true-p', but older
versions don't have it.]

If it were #1 then this wouldn't help.  The
var would be `t' for all Emacs versions, even
those for which that variable has no effect.

Please consider documenting this in the manual.

It _looks_ very much like the declaration sets
var `lexical-binding' to t, regardless of the
Emacs version.  But it seems it does not.

> (this is used by CC-mode, for example).

I don't see that in cc-mode.el for the latest
Emacs release (which is 27.2).

Even if it'll be present for Emacs 28 or later
(when released), that's no proof or even a
hint that the given library is compatible with
Emacs 23, 22,...