"One of your test API keys has been filled into the examples, so you can test out any example right away." Stripe, I'm impressed.
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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
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)?
"Example Driven Development" using Glamorous and Pharo Smalltalk: https://medium.com/feenk/an-example-of-example-driven-development-4dea0d995920
Tests returning values and composing is a really interesting model. It establishes structure and shows which test failure is the most 'fundamental'.