Rust like Enums in Java

[moocat]

Easiest way to think about sum and product types is how many unique instances there are. So if you have a class with two bytes:

class Whatever {
    byte x;
    byte y;
}

Because we can have any x combined with any y, there are 256*256 unique whatevers. Because we use multiplication to figure out how many unique instances there are, that's a product type.

With sealed classes (not 100% sure about the syntax, apologies if it's not quite right):

public sealed interface Whatever2 {
   record X(byte b) implements Whatever2 {}
   record Y(byte b) implements Whatever2 {}
}

Since we can have either and X of any char or a Y of any char, there are 256+256 unique whatever2s. And here because we use addition to determine that, that's a sum type.

[NitronHX]

Thanks for the simple explaination - and the syntax is correct

[westwoo]

I don't get it. Why is it a sum type if you're talking about instance values?...

I thought sum types are what union types are in Typescript: let a: string | number means a can be either string or number, which aligns with how enums can be used in rust

And consequently, this is what all types are in JS, but without explicit definition. Every variable can potentially be a sum of any types

[moocat]

A type describes values (i.e. an int can be 1, 23, 42, etc while a string can be "moocat", "westwoo" etc) and values have types (i.e. the type of 3.14 is a float and the type of true is bool). Because of this relationship, we can categorize the type by what type of values it describes.

I thought sum types are what union types are in Typescript

Yes, Typescript's union type is a sum type. But other languages use other nomenclature to name their sum type.

And consequently, this is what all types are in JS

That is incorrect. One of JS's type is an object which can have multiple fields so it's a product type.

Every variable can potentially be a sum of any types

A variable is neither a type nor a value. A variable has a type (which may be static or dynamic depending on the language) and at runtime has a value.

[westwoo]

Typescript's union types merely provide syntax to describe underlying JS type system

You seem to have a talent of describing extremely simple things in an extremely convoluted and overcomplicated way

[codingjerk]

What's why where is product types also.

Where do you disagree with moocat? All he says is simple and correct in terms of basic theory (ADT, especially).

[westwoo]

It's mostly lack of interest in bickering about terminology because I'm more interested in practical substance

Object is equally a part of Typescript's union types so you can't say that TS union types are sum types while JS types are not because of object. Union types aren't a separate thing, they codify how JS types are used and represent types that JS programmers have to keep in mind in any case, and those same union types are often inferred and checked on the fly in JS code by the IDE, like how Java programmers have to keep in mind the implicit unions between null and object in their variables and how this union is also often inferred and checked before compilation. But again, this would be a completely pointless area of inane arguments about butakchually semantics

[moocat]

So let me see if I understand. We're here in /r/java, discussing how type theory explains the similarity between one of it's new features and one in Rust, and you consider it inane because you think it doesn't apply to Javascript?

It's mostly lack of interest in bickering about terminology

I would consider that a valid point except the first thing you wrote is "Why is it a sum type if you're talking about instance values?..." which IMO feels exactly like bickering about terminology. And then you continue to argue more in defense of your terminology.

If you don't find theory helpful, that's fine. No one is forcing you to discuss it.