> Why don't you spell it out?

The earlier comment already spelled it out. Hence why it wished you luck as we knew you would not be able to deliver. Yet right on cue you hilariously tried anyway.

I take that you haven't actually read anything in this discussion? If you have read it, you haven't understood it. Slow down, comprehend before posting. It seems you've become so fixated on telling us how sum types work – something we understood decades ago – that you have missed the forest for the trees.

> Why do I want to spend time writing generic test code that executes off of fuzzed input?

Remember when you mistakenly wrote (p2 - p1 / t2 - t1)? That's why. A complete type system would negate the need for testing there, but if all you have is sum types... There is no difference between a fuzz test and any other test other than the tooling will feed it 'arbitrary' inputs. It is not like they take more time to write or something. Testing is testing is testing.

In fact, it took me like ten seconds to write the fuzz test for your function when I tried it out earlier. This time argument is disconnected from reality. You have 10 seconds to spare to ensure correctness, especially when you admitted to writing a bunch of useless tests instead. One good test would have gone further and saved you time.

> I think you're kind of not getting it.

Let's change that. Consider your code:

v, err := getFile(fileName)
doSomething(v)

Presumably your requirements dictate that your code must do something when getFile fails, so you are obviously going to write a test for that scenario. How do you envision that potential crash condition evading the test? The CPU steps through until getFile fails, then checks if you are running a test and if it so it invents a valid file handle, but if it notices you are in production it returns a corrupt file handle and then crashes? That doesn't make any sense. What does make sense to you that explains how your code will pass tests but fail in production?

If what you are struggling to say is that you have no such requirement to deal with the failure so it is unspecified behaviour, and thus you didn't feel the need to write a test for it, all you can do with sum types, to keep the compiler happy, is purposefully panic. It is unspecified behaviour. Anything else but crashing would be nonsensical – something that is true even if it were written in Rust. Is the source of the panic significant?

> Seriously like the feature of golang is to allow you to unintentionally crash the program and you think that's a good thing?

It doesn't matter in practice. When the program is going to do the wrong thing, it makes little difference how it goes about doing it wrong. But if you really have to choose, crashing is better than silently corrupting data, yes. The latter is far more scary.

However, better is to ensure that your program doesn't do the wrong thing in the first place. If you have a complete type system, that's you best bet. But that isn't realistic. No language you are going to encounter in the real world has that. Meaning, prepare to learn how to write tests. They aren't easy to do right, but you're going to have to do it anyway because without a complete type system you have no other choice.

Anything else is running on hopes and prayers. If you honestly believe that hope and prayer is sufficient, then why are you so worried about crashing?