Comment by TacticalCoder

Amazing to see Jane Street uses Emacs. And property-based testing too.

> 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.