One neat way of thinking of assertions vs unit tests: declarative vs imperative:
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Has anyone built a great solution to 'run all my unit tests automatically'?
It's straightforward to write a while loop in bash, but handling timeouts, syntax errors etc well is hard.
Running on save would be good, although I wonder if you could run fast tests on each keystroke.
I'm experimenting with live-evaluating tests in my programming language project.
It's relatively fiddly to hook up a UI for this, but it saves a precious keystroke to run the tests! I'm hoping that it results in more, better tests due to the convenience.
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.