bug#30078: 27.0.50; Use lexical-binding for M-:

[Luis Gerhorst]

> On 12. Jan 2018, at 22:11, Michael Heerdegen <michael_heerdegen@web.de> wrote:
> 
> Ah, so you doubt that lexical-binding once should just be the default.
> Then I think we should discuss about this goal.

Here are a few possible pros of lexical binding:

1) Make Elisp more approachable to newcomers. Lexical binding is the default in 
most programming languages and thus more people are familiar with it. This 
makes it easier for people to understand existing code since they don‘t have to 
get used to implicit dynamic binding first, which the code may take advantage 
of.

2) Make existing code more maintainable. I was involved in porting the package 
emacs-eclim to lexical binding. The process shed light on some very dark parts 
of the codebase nobody had touched in a while since they were written in very 
bad style and thus hard to understand. Using lexical binding forced us to 
simplify those parts.

3) This is a minor one: I‘ve heard that lexical binding allows for more 
optimizations. I don‘t know to which extend this actually happens and whether 
it is noticeable to the user.

I believe 1) and 2) will significantly help improving the general quality of 
Elisp code and thus of Emacs in general.

Here‘s the big con:

If lexical binding should become the default one day, you would have to give 
everyone a reasonable amount of time (multiple years?) to adapt for it and port 
their code or set lexical binding to nil in their files.

One idea would be to make elisp-mode somehow encourage users to enable lexical 
binding in the files they are editing. Any ideas how this can be done without 
annoying users? Ideas:
- Prompting every time elisp-mode is enabled in a file that does not have a 
lexical binding header.
- I know that certain functions insert a apropriate copyright notice into 
el-files, do / can those also set lexical binding to t?

Best regards,
Luis