Rust Enums and Pattern Matching

Enums (enumerations) allow you to define a type by enumerating its possible variants. Unlike enums in many other languages, Rust enums can store data directly within their variants.

Defining Enums

Variants can have different types and amounts of associated data, including strings, numeric types, structs, or even other enums.

enum Message {
    Quit,
    Move { x: i32, y: i32 },
    Write(String),
    ChangeColor(i32, i32, i32),
}

The Option Enum

Rust replaces the concept of null with the Option<T> enum, which is part of the prelude.

• Some(T): Indicates the presence of a value of type T.
• None: Indicates the absence of a value.

  The compiler requires you to handle the None case, preventing "null reference" errors at compile time.

Pattern Matching

The match Construct

match is an exhaustive control flow construct that compares a value against a series of patterns.

• Exhaustiveness: Every possible case must be handled.
• Catch-all (_): A placeholder that matches any value without binding to it.
• Binding: Patterns can bind to the data inside enum variants (e.g., Some(i) => ...).

Concise Control Flow

• if let: Syntax sugar for a match that only cares about one pattern, ignoring the rest.
• let...else: Stay on the "happy path" by binding a pattern or returning early if it doesn't match.

References

• Source: 00_Raw/the-rust-programming-language.md
• rust-moc
• rust-structs
• rust-error-handling