> You never know when you have enough coverage to rule out NPEs or any other bug. And to get confidence about lack of NPEs you want to have coverage of all lines involving dereferences, which means you need near 100% test coverage to have a reasonable level of confidence.
This is not true. In the example I spoke about above, if I take a CacheClient, and a ServiceXClient when my type is being constructed, assign them to local fields and then never modify that field again, then I don't need to exercise every dereference of these fields, just one. And again, I don't test that my code handles NPEs, I test that my code does what it is supposed to, and in the process of doing that, NPEs get flushed out.
> Generally types are far more concise and guarantee more than tests. I find 5 lines of types more readable than dozens or hundreds of lines of tests.
I think you are viewing this through red-black-tree colored glasses. Specifically, you believe that a lot of code has mathematical constraints the way that example did. To me, this is an extremely remote possibility. I think if you tried to encode even the smallest real-world example of this, say a service implementing a URL shortener, you would run into a wall.
> Your code is much safer simply by construction, so I am not sure what you mean here.
I should have said more reliable.
> I do think non-nullability aids reliability, but that having sum types, proper pattern matching and parametric polymorphism aids it even more. And Go lacks all of these.
Truly, it baffles me that people still harp on the reliability aspect. It is quite likely that every piece of software you use day-to-day is written in a language with nullability, without pattern matching, and no sum types. Most of that software probably doesn't even have memory-safety (gasp!). Probably every website you visit is in the same sorry state. I'm sorry, but your arguments would be far more convincing if the world written in these languages were a buggy, constantly crashing hell. It's not.