Comment by cubefox
16 hours ago
> You cannot go around claiming that 'technically' unions are not unions
I never said this. I said that there are two different kinds of unions with quite different definitions and behaviors.
> A tagged union is so called because it is a union (overlapping memory) with a field (a tag) containing a discriminator (aka discriminated union). An untagged union is a union without this tag.
That alone doesn't sufficiently describe the "unions" in set theoretic type systems like TypeScript. As I said, these unions are not disjoint and don't involve the special constructor that tagged unions/discriminated unions/sum types in algebraic type systems do. They also have logical implications like A implying A|B. Or A|A being equivalent to A, which isn't the case for discriminated/tagged unions. Maybe they are implemented similarly under the hood, but that doesn't negate the difference.
Edit: It's probably fair to say that "untagged unions" in languages like TypeScript, Ceylon or Scala 3 (set theoretic type systems) are different from "untagged unions" in C. It only adds to the confusion...
> Maybe they are implemented similarly under the hood
It's hard to know what to say to this. Tagged unions are not tagged unions, even if they are literal tagged unions? What?
I reiterate what I said in my previous comment, you're not using the ordinary definition of the term union, and this is causing confusion. A union may or may not be a "union" as understood within various academic type theories, that really depends on how any given theory defines that word, which can be any way it wants. But a union is a CS concept with a clearly understood meaning, and when used without added context to suggest it is to be interpreted in some theoretical way, it is understood in that ordinary way.
OP's article is clearly using 'union' to mean tagged unions - he even shows off their implementation, with a tag. The author assumes that his audience will understand what he's talking about when he uses the word union, and it's not causing anyone trouble in this comments section. The fact that alternative definitions within various theoretical paradigms is very nice, bless their hearts, but not really relevant.
You may prefer other definitions to the usual CS definition, that's certainly your prerogative, but - again - that's hardly grounds for taking an article and comment section that's using the commonplace meaning, and appearing to lecture others for failing to adhere to your idiosyncratic standards for what a union must be.
See my edit above. There is the C terminology, and the TypeScript et al terminology, and it's the same "untagged" terminology for two different things. In any case, both these kinds of "untagged" unions are different from tagged/discriminated unions. So just calling them "unions" is ambiguous at best and confusing at worst.
But no one is actually confused. You yourself understand what the author meant, from your comments. Everyone here understands what he meant.
It's neither ambiguous nor confusing to use the word union in CS. The only person who's making it so is you, by introducing semi-unrelated concepts from set theory that happen to have the same name as the established CS concept.
Why stop there? Maybe the author meant the Union, as in the United States? Itself quite ambiguous - does he mean the United States of Mexico, or the United States of the Ionian Islands? Is C# getting Corfu? Corfu dot net? :P
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