I don't follow. Errors are part of your functional specification. Not even a complete type system can avoid you needing to write tests to document the error con

I don't follow. Errors are part of your functional specification. Not even a complete type system can avoid you needing to write tests to document the error conditions. We are not talking about writing tests to stand in where an incomplete type system may be lacking. Assuming you don't hate other developers, and given the tests you would write in any language, how do you foresee missing an error branch? Again, not specifically trying to test for something that a sufficiently advanced type system would catch, just your regular level documenting that demonstrates to other developers that you don't hate them. > then you can just as easily forget to add a proper test for that error condition If an error condition is easily forgotten in the documentation, then who cares about the implementation thereof? It is obviously not important. In fact, it is arguable that your program is incorrect if you do handle the condition in such a case.