To my surprise, Emacs does not use tagged values for floats, like JS implementations. Instead, it uses structs! https://github.com/emacs-mirror/emacs/blob/53d5bd786130a937a0e691e0c470675fe9c55485/src/lisp.h#L2599-L2607
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Assertions are a surprisingly nuanced design space. In a test, if I assert `x < y`, I really want to see the values of x and y when it fails.
Do you define an API for every possible predicate (Python's assertLess, expect.js) or try to support the native syntax (c.f. pytest)?
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.
One interesting design choice in Emacs that I haven't seen in other editors: reserved shortcuts.
An Emacs extension shouldn't use F5 through F12, or Ctrl-c LETTER. This lets users configure their own shortcuts. https://www.gnu.org/software/emacs/manual/html_node/emacs/Key-Bindings.html
Does it exist elsewhere? I miss it in VS Code, where e.g. all the Fn keys are already assigned.