I can already do functional programming like map/reduce in C# tho. Not sure what the LISP argument is. Spolsky was saying there's a perf benefit in there somewhere but I'm not seeing how unions give me that.
1. Argue from ignorance. Never try unions in any other programming languages and completely disallow their use in C# codebases that you participate in.
2. Try them out and adopt an informed opinion.
You may even choose to remain in ignorance until someone wastes their own time trying to convince you. But it isn't my job or desire to teach someone who won't put in the effort to learn for themselves.
My primary concern with this pattern versus exceptions is calling code can simply discard the resulting problem.
Except with a Result type the fact that an exception can occur and should be handled in the first place is explicit.
The problem if anything is that you MUST say something about the error case, despite the common scenario being “pass it forward” — the same reason exception do this by default. Which is also why rust for example special cases Result with the ? operator to do exactly that