Despite most of my professional programming being in Go nowadays, I am extremely sympathetic to the Haskell/FP way of thinking about things, and trying to make invalid state unrepresentable.
However, having made the mistake a few times now of trying to use "unsigned ints" for things that I want to assert are never negative, I've learned the hard way not to do that. The problem is, there's always some bug you have in the program that will drive the uint "negative", wrapping over to a huge number. Unfortunately, since you get no warning when that happens, it tends to take longer to show up, and without a clear cutoff to say "ah, this is clearly invalid" it can be hard to even program detection code, whereas checking for "less than 0" can be unambiguous.
I'd loooove it if I could easily, cheaply, and universally (i.e., not just in Go, but across all my programming languages) turn on a behavior that says "if this int or uint under or overflows, throw an exception instead of trying to do whatever stupid thing you're going to do". I get the sense this is heading down the road where in 20 or 30 years, this is just going to be common sense. However, we are very early on the road yet from what I can see. Whenever I bring it up, even today, even in this sort of context, it still is generally negatively received. (But not uniformly. A few other people agree. I think momentum is with me. I'm not sure my programming career will live long enough to see it, though.)