Re: Why is lexical-binding's global value ignored?

[abq@bitrot.link]

On 2023-01-28 23:54, Eli Zaretskii wrote:
> Wasn't that already explained?  What happens if you say
> 
>   (setq-default lexical-binding t)
> 
> and then load a Lisp file that expects dynamic-binding by default?

Then it breaks! As I wrote in my response to tomas: “In order to break 
anything, you would have to do so explicitly, via (setq-default 
lexical-binding t).”

Of course, nobody actually does that. Therefore, it would be safe to 
honor the global value in Emacs 29.

And the reason nobody sets the global value is that currently it would 
be pointless, because it isn't honored in Emacs 24 through 28.

There's no danger of anybody accidentally setting it with plain setq, 
since it automatically becomes buffer local when set. You have to 
purposefully shoot yourself in the foot with setq-default.


> IOW, the lack of lexical-binding: cookie in a file is interpreted as
> having an explicit "lexical-binding: nil" in that file, for the
> reasons Tomas explained.

But of course, dynamic binding isn't always used intentionally. It's 
also often used in cases where the programmer gave no consideration to 
the difference (and often, unconsciously intended lexical binding). 
Since the longstanding, widespread computer science consensus is to 
program using lexical binding except in special cases where dynamic 
binding is intentionally chosen, it's sensible to facilitate a bias in 
that direction, at least optionally.

If you use lots of historical code written with no consideration of the 
difference, one way to test the code would be to sprinkle 
lexical-binding: t across the tops of all your files, and keep track of 
which ones have it set because they actually expect it vs. which ones 
have it set just so you can test them. Which, ironically, is a bit like 
using a purely functional language with no dynamic binding or global 
variables, so you're forced to modify all your function signatures and 
calls to explicitly pass global state as an argument.

Or, a more sensible way to test your historical code would be to simply 
do (setq-default lexical-binding t) in your init file. But you can't do 
that in Emacs 24 through 28, because the global value is ignored.

If it were honored in Emacs 29, then a news item could be added, 
suggesting everybody try (setq-default lexical-binding t) in the init 
file, to test all the packages they use, then add explicit 
lexical-binding: nil (or defvar) in the cases where dynamic binding is 
intentional. People who think this is nonsense can harmlessly ignore the 
suggestion.