The Rust Programming Language — Pattern Matching — Enums

enum or more elaborately known as Enumerations, are a group of possible set of values that can be assigned to a variable. Defining a set of values reduces chances of errors of accidentally assigning a wrong value.

Let’s say we want to restrict the types of colours a user can chose from Red,` Green, Yellow. Rust allows us to encode these possibilities as enums. Please check below snippet.

enum Color {
    Red,
    Yellow,
    Green
}

fn main() {
    let go = Color::Green;
    let caution = Color::Yellow;
    let stop = Color::Red;
}

Observations:

1. You need to use enum keyword to define an enum
2. You need to give it a name, in our case Color.
3. In order to access values to be assigned to a variable, we need to access them using the namespace with double-colons ::. The namespace is the name of enum that we game which is Color. For e.g. Color::Green, Color::Yellow or Color::Red.

In this case, Color is known as identifier and Red, Yellow and Green are known as variants. The identifier Color can be used as a type of variable. For e.g. refer to below snippet, the type of go, caution and stop is Color:

enum Color {
    Red,
    Yellow,
    Green
}

fn main() {
    let go: Color = Color::Green;
    let caution: Color = Color::Yellow;
    let stop: Color = Color::Red;
}

An enum can contain a tuple as a variant as well as a struct.

Refer to below snippet for an example of tuple as a variant in an enum:

enum Color {
    Red,
    Yellow,
    Green,
    Custom(u8, u8, u8)
}

fn main() {
    let go: Color = Color::Green;
    let caution: Color = Color::Yellow;
    let stop: Color = Color::Red;
    let brown = Color::Custom(128, 64, 0);
}

Observations:

1. You can define a Custom tuple as a variant in the enum.
2. Custom(u8, u8, u8) is a type added in the Color enum.
3. You can assign a Custom(u8, u8, u8) value to a variable brown using the namespace concept.

Below is an example of how to use structs as a variants in the enum.

enum Color {
    Red,
    Yellow,
    Green,
    Custom { red: u8, green: u8, blue: u8 }
}

fn main() {
    let go: Color = Color::Green;
    let caution: Color = Color::Yellow;
    let stop: Color = Color::Red;
    let purple = Color::Custom { red: 128, green: 0, blue: 128 };
}

Observations:

1. Just like tuples we can define a struct variant
2. Custom { red: u8, green: u8, blue: u8 }

I hope you liked this articles on how to create enum in Rust. Please leave your feedback in the comments section.

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