Example: Option<> types. Maybe a function returns an optional string, but then you are able to improve the guarantee such that it always returns a string. With untagged unions you can just change the return type of the function from String|Null to String. No other changes necessary. For the tagged case you would have to change all(!) the call sites, which expect an Option<String>, to instead expect a String. Completely unnecessary for untagged unions.

A similar case applies to function parameters: In case of relaxed parameter requirements, changing a parameter from String to String|Null is trivial, but a change from String to Option<String> would necessitate changing all the call sites.

> From where I stand, untagged unions are useful in an extremely narrow set of circumstances. Tagged unions, on the other hand, are incredibly useful in a wide variety of applications.

Any real world example?