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

[abq@bitrot.link]

Richard Stallman wrote:
> A year or two after that. all packages people use will explicitly
> specify lexical or not.  At that point, we could safely change the
> default, if we want to.

Yes, and a great way to change the default would be specifically:
First, start honoring the global value of the lexical-binding variable. 
At first, keep it nil by default, as it is now, so no behavior actually 
changes, and nothing breaks. But announce that the default will change 
in a future version. Of course, buffer local lexical-binding continues 
to work as it does now.

Second, brave users can optionally setq-default lexical-binding t in the 
init file to get the new behavior now, and report breakage they discover 
in packages they use. Users who want the old behavior, or don't care 
about the issue and have no desire to learn about it, can ignore it, and 
nothing breaks. The default remains nil.

Third, as announced long in advance, change the default to t in a future 
version, and announce that users can setq-default lexical-binding nil in 
the init file if they still want the old behavior.

The key here is honoring the global value of lexical-binding. This makes 
the transition as smooth as possible, and makes it trivially easy for 
users to opt out even after the transition is complete and the default 
is t.

The current problem is just that Emacs 24 through 28 ignores the global 
value. When loading a file, Emacs pretends that the global value is 
always nil even when it's actually t. (But of course, this entire 
discussion is irrelevant whenever lexical-binding is set buffer 
locally.)

Honoring the global value is arguably just a bug fix.


Or did you have another mechanism in mind for changing the default? 
Another possibility would be to still ignore the global value, but start 
pretending that it's always t even when it's actually nil. But surely 
nobody is advocating that Emacs ever do that. It makes much more sense 
to simply honor the global value (which could start right now, with no 
harm), then change the global default to t sometime in the future.

There is a potential problem: Eli worries that “many people will not 
understand the disastrous consequences” of doing (setq-default 
lexical-binding t), and therefore he decided to delay honoring the 
global value. But a prominent warning DO NOT TOUCH THIS GLOBAL VARIABLE 
UNLESS YOU'RE AN EXPERT would be just as effective for Emacs 29 as it 
would be for Emacs 30 or even 42; the delay accomplishes nothing. And 
people who don't read the news at all, and just blindly upgrade and 
continue using their init file and packages unmodified, will experience 
no breakage, because the default remains nil. No adaptation is required, 
by anybody.

There is another theoretical problem. Maybe somebody, somewhere, 
irrationally already does (setq-default lexical-binding t) in his init 
file, to no effect, and then (even more irrationally) subsequently 
relies on dynamic binding despite explicitly trying to set lexical 
binding. Any future version of Emacs that starts honoring the global 
value will break his config. But abstaining on his behalf is as 
unreasonable as total abstinence from ever adding any new global 
variable to any future version of Emacs ever again, due to the risk that 
the variable will conflict with somebody's private usage of it.

Eli's decision is final, but it's worthwhile to make the situation 
clear.