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 can't "have" values, they can only "be" the value, or rather, be 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.
#![allow(unused)] fn main() { enum Result { Ok, Err, } }
Here, Result
can be one of two possible set of values: Ok
and Err
, just
like boolean
, but unlike boolean
, being tagged allows Result
to have more
than two variants if required, by changing the definition.
e.g.
#![allow(unused)] fn main() { enum Result { Ok, Err, Pending, } }
We'd document it here as:
Result is a sum type that can be one of the following:
- "Ok"
- "Err"
- "Pending"
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:
#![allow(unused)] fn main() { enum Result { Ok(bool), Err(Error), Pending, } }
Here, as we can see, unlike the first example, some of Result
's possible
variants can have further nested types associated with them. Note that Error
here can be either a sum type (e.g. enum), or a product type (e.g.
class/struct), but whatever it is, it will only exist when Result
is Err
.
We'd document it here as:
Result is a sum type that can be one of the following:
- { Ok = bool }
- { Err = Error }
- "Pending"
And then we'd go on documenting whatever Error
is.
So, there you go. This is exactly what sum types are - glorified enums that can have nested types in each branch.
If you're still confused about something, or if you found an error in this explanation, feel free to discuss together.