Industry is just starting to come around to it, but I've never been more happy programming than when using a strongly-typed language with sum types. What most people fail to understand is that fighting the type checker is almost always a feature, not a bug. It is training you to write code in a way that it understands, which forces your thinking to be less sloppy, even on non-happy paths.
Sum types enable much higher levels of expressivity of what valid states are while still being statically analyzable. Any new PL lacking them, IMO, is making a huge unforced error. They don't apply for every situation, but they do handle a large amount of day-to-day programming concerns.
Side note: OO is oft-maligned in OCaml, but I really appreciate that they included it anyway. I much prefer languages that give you a set of tools to use in whatever situation you find yourself in.
Sum types enable much higher levels of expressivity of what valid states are while still being statically analyzable. Any new PL lacking them, IMO, is making a huge unforced error. They don't apply for every situation, but they do handle a large amount of day-to-day programming concerns.
Side note: OO is oft-maligned in OCaml, but I really appreciate that they included it anyway. I much prefer languages that give you a set of tools to use in whatever situation you find yourself in.