[tialaramex]

"synonym for a tagged union"

Tagged unions are an implementation and one that's often a poor fit for the problem. Sum types are an idea from type theory, rather than an implementation detail. It's very on-brand for C++ to have standardized a poor implementation detail rather than the useful idea.

Look at how much hoop jumping was required to make std::optional<T&> work for C++ 26 and then compare how Rust's Option<&T> isn't even special, that's just naturally what happens.

[searealist]

These are all synonyms, even in type theory:

> In computer science, a tagged union, also called a variant, variant record, choice type, discriminated union, disjoint union, sum type, or coproduct, is a data structure used to hold a value that could take on several different, but fixed, types. [1]

C++ class inheritance is not a sum type for a couple reasons:

- It does not allow you to discriminate on the type.

- You can add RTTI, which allows you to discriminate, but then it is not a fixed set of types.

Obviously, Rust does sum types better than C++, but that is really irrelevant.

[1] https://en.wikipedia.org/wiki/Tagged_union

[tialaramex]

It's true, in the "Encyclopedia anyone can edit" you will find that quote.

[searealist]

Today you learned what a sum type is.

[tialaramex]

I already knew what a sum type is. Fortunately I discovered Rust, which actually has proper sum types rather than trying to make do with a tagged union and a whole lot of squinting.