Today I learnt that Emacs has a notion of 'permanent' variables! https://www.gnu.org/software/emacs/manual/html_node/elisp/Creating-Buffer_002dLocal.html
These are variables that persist even when you change major mode (which generally resets buffer-local variables).
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Today I learnt that Emacs 28 shipped a context-menu-mode! https://oylenshpeegul.gitlab.io/blog/posts/20230129/
This seems quintessentially Emacs: deeply hackable, but building UI features in an order very different to the mainstream.
TIL Tcl has a notion of 'safe interpreters', a mode where you can run untrusted code in a sandbox: https://www.tcl.tk/man/tcl8.4/TclCmd/safe.htm
Not many programming languages have this, but it's way safer to include in the implementation than try to build as a userland library.
Claude asked me a question today: was I looking for an Emacs plugin (because I was talking about elisp) or a Rust program (because I have configured Rust preferences)?
I'm really impressed, it's rare to see LLMs ask follow-up questions.
(I wanted Emacs in this case.)