[maattdd]

Inheritance (the subtyping part of it) is considered the OOP way to write sum type.

sum Expr { Int; Add(Int,Int) }

VS

class Expr { }

class Add extends Expr { Expr left; Expr right; }

[OvbiousError]

You should probably look up what a sum type is, it has nothing to do with summations. Your example doesn't contain a sum type. A C++ example:

    std:variant<int,std::string> sum_type_instance = 5;

[maattdd]

My sum type example is exactly this (but I didn't use C++ std::variant<> syntax to not confuse the reader).

The most common example of a sum type is the "Expression problem" - please read some literature before commenting on a topic.

(Btw, it's called sum type for a reason: summation. The cardinality of the sum type is the sum of the cardinality of its variants)

[Maxatar]

You seem to have a very narrow C++ view of what sum types are. Sum types are related to the expression problem, and the given example is the canonical instance of the expression problem.

https://en.wikipedia.org/wiki/Expression_problem

[searealist]

Sum types are not “anything that can encode an expression”. They have a precise definition. They are closed and a synonym for a tagged union.

[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.

[Maxatar]

Please use quotes to refer to a statement that someone has actually made. Doing otherwise is an indicator that you are making a strawman argument.

[searealist]

Today you learned what a sum type is. Just admit your ignorance and move on with your life. Maybe bookmark this: https://en.wikipedia.org/wiki/Tagged_union

[Maxatar]

Looks like you are very easily triggered on this topic. Quite amusing to be honest :)

[gpderetta]

The expression problem enters the room.

[searealist]

That's not C++.

[maattdd]

Any reader who comment here hopefully has enough knowledge to understand the implied C++.

struct Add {} std::variant<Add, int> Expr;

OR

class Expr {} class Add : Expr {}

[searealist]

I dont think you have ever seriously used c++.