alt Hacker News> 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.