Um …duh? Get out a calculator. Consult a reference, etc. Otherwise compute the result, and ensure you've done that correctly, ideally as independent of the code under test as possible. A lot of even mathematical stuff has "test vectors"; e.g., the SHA algorithms.

> Here’s how you’d do it with an expect test:

  printf "%d" (fibonacci 15);
  [%expect {||}]

…you're kidding me. This is "fix the current state of the function — whether correct or not — as the expected output."

Yeah… no kidding that's easier.

We gloss over errors — "some things just looked incorrect" — well, but how do you know that any differently than fib(10)?

I love determinism and plain old data.

https://github.com/pointfreeco/swift-snapshot-testing

What if writing tests was a joyful experience? - https://news.ycombinator.com/item?id=34350749 - Jan 2023 (122 comments)

> you don’t just get a build failure telling you that you want 610 instead of a blank string

So I had to scratch my head a bit because I was thinking: "Wait, the whole point is that you don't know whether what you're testing is correct or not, so how can you rely on that as input to your tests!?".

But even though I didn't understand everything they do yet I do see at least a big case where it makes lots of sense. And it happens to be a case where a lot of people see the benefits of test: before refactoring.

> What does fibonacci(15) equal? If you already know, terrific—but what are you meant to do if you don’t?

Yeah a common one is reuse a function in the same language which you believe is correct (you probably haven't proven it to be correct). Another typical one is you reuse a similar function from another language (once again, it's probably not been proven it is correct). But if two implementation differ, you know you have an issue.

> let d = create_marketdata_processor () in > ( Do some preprocessing to define the symbol with id=1 as "APPL" )

Typo. It's AAPL, not APPL. It's correctly used as AAPL later on.

FWIW writing tests better become a joyful experience for we're going to need a lot* of these with all our AI generated code.

It feels ... strangely empowering.