> For the tagged case you would have to change all(!) the call sites
Yeah, that's exactly why I want a tagged union; so when I make a change, the compiler tells me where I need to go to do updates to my system, instead of manually hunting around for all the sites.
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The only time an untagged union is appropriate is when the tag accounts for an appreciable amount of memory in a system that churns through a shit-ton of data, and has a soft or hard realtime performance constraint. Other than that, there's just no reason to not use a tagged union, except "I'm lazy and don't want to", which, sometimes, is also a valid reason. But it'll probably come back to bite you, if it stays in there too long.
> > For the tagged case you would have to change all(!) the call sites
> Yeah, that's exactly why I want a tagged union; so when I make a change, the compiler tells me where I need to go to do updates to my system, instead of manually hunting around for all the sites.
You don't have to do anything manually. There is nothing to do. Changing the return type of a function from String|Null to String is completely safe, the compiler knows that, so you don't have to do any "manual hunting" at call sites.
C doesn't support any untagged unions (or intersections) in the modern sense. In a set-theoretic type system, if you want to call a method of Foo, and the type of your variable is Foo|Bar|Baz, you have to do a type check for Bar and Baz first, otherwise the compiler won't compile.
If I have an untagged union in <language_of_your_choice>, and I'm iterating over an array of elements of type `Foo|Bar|Baz`, and I have to do a dynamic cast before accessing the element (runtime typecheck) .. I believe that must actually be a tagged union under the hood, whether or not you call it a tagged union or not... right? ie. How would the program possibly know at runtime what the type of a heterogeneous set of elements is without a tag value to tell it?
I believe that, by the description provided, most languages that you're talking about must actually represent 'untagged unions' as tagged unions under the hood. See my sibling comment. I'm curious
Yeah, that's exactly why I want a tagged union; so when I make a change, the compiler tells me where I need to go to do updates to my system, instead of manually hunting around for all the sites.
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The only time an untagged union is appropriate is when the tag accounts for an appreciable amount of memory in a system that churns through a shit-ton of data, and has a soft or hard realtime performance constraint. Other than that, there's just no reason to not use a tagged union, except "I'm lazy and don't want to", which, sometimes, is also a valid reason. But it'll probably come back to bite you, if it stays in there too long.