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I wrote my shell prompt in Zig years ago in part because I was interested to use its async/await to run all the git calls in parallel for the git status. My prompt is still fast despite never having parallelized things -- slightly slower now after adding Jujutsu status -- but I'm looking forward to getting to do the thing I originally wanted and have my super fast shell prompt.

To speak to the Zig feature: as a junior I kept bugging the seniors about unit testing and how you were supposed to test things that did IO. An explanation of "functional core imperative shell" would have been helpful, but their answer was: "wrap everything in your own classes, pass them everywhere, and provide mocks for testing". This is effectively what Zig is doing at a language level.

It always seemed wrong to me to have to wrap your language's system libraries so that you could use them the "right way" that is testable. It actually turns out that all languages until Zig have simply done it wrong, and IO should be a parameter you pass to any code that needs it to interact with the outside world.