[Answered] Disjunctions in types: Why is this "something that the type system cannot and should not support"?

[xwu (Xiaodi Wu)]

Using the search function on these forums for the specific quoted text will lead you to several discussions on the topic. See, for example:

Adding Either type to the Standard Library

The commonly rejected change, as I understand it, is specifically about union-esque types (logical OR), which is slightly different from anonymous sum types (closer to logical XOR if you want to extend that analogy). Indeed, the note explaining why it won’t be accepted says that it is “something the type system cannot and should not support,” but the type system clearly can (and should!) support anonymous sum types—they are just enums with less characters.