xplr/docs/en/src/sum-type.md

Sum Type

This section isn't specific to xplr. However, since xplr configuration makes heavy use of this particular data type, even though it isn't available in most of the mainstream programming languages (yet), making it a wild or unfamiliar concept for many, it's worth doing a quick introduction here.

If you're already familiar with Sum Type / Tagged Union (e.g. Rust's enum), you can skip ahead.

While reading this doc, you'll come across some data types like Layout, Color, Message etc. that says something like "x is a sum type that can be any of the following", and then you'll see a list of strings and/or lua tables just below.

Yes, they are actually sum types, i.e. they can be any of the given set of tagged variants listed there.

Notice the word "be". Unlike classes or structs (aka product types), they don't "have" values, they "are" (i.e. "be") the value, or rather, one of the possible set of values.

Also notice the word "tagged". Unlike the single variant null, or the dual variant boolean types, the variants of sum types are tagged (i.e. named), and may further have, or be, value or values of any data type.

A simple example of a sum type is an enum. Many programming languages have them, but only a few modern programming languages allow nesting other types into a sum type.

enum Result {
    Ok,
    Err,
}

Here, Result can be one of two the possible values: Ok and Err (just like boolean, but tagged).

We'd document it here as:

Result is a sum type that can be one of the following:

• "Ok"
• "Err"

But some languages (like Rust, Haskell, Elm etc.) go even further, allowing us to associate each branch of the enum with further nested types like:

enum Result {
    Ok(bool),
    Err(String),
}

We'd document it here as:

Result is a sum type that can be one of the following:

• { Ok = bool }
• { Err = "string" }

Here, Result still has only two possibilities, but unlike the first example, each possibility here has further set of possible value(s).

And there you go. This is exactly what sum types are - glorified enums that can have nested types in each branch.