You can't prove the absence of bugs, but 91% test coverage on an important system does help me sleep at night.
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I love how the CommonMark Spec has a test suite that's just a JSON array. It's really easy to test a library for compliance, and I've seen developers nerd-sniped into full compliance.
https://spec.commonmark.org/0.31.2/spec.json
Trying the nix CLI today, and I'm seriously impressed with the formatting of its --help output.
Indented warnings, italics, bullets, even adding a left border to code snippets! It's a nice reminder to take full advantage of terminal features to help the reader.
What are the most interesting upcoming scripting languages? Scripting remains an important part of the programming language ecosystem.
Optimise time to first feature. Allow mutation of the running system to experiment. Interactive inspection of data.