enum關鍵字允許創建標籤聯合, 它可以被用作代數數據類型 (ADT)。

// Allow Cons and Nil to be referred to without namespacing
use List::{Cons, Nil};

// A linked list node, which can take on any of these two variants
enum List {
// Cons: Tuple struct that wraps an element and a pointer to the next node
Cons(u32, Box

// Nil: A node that signifies the end of the linked list

// Methods can be attached to an enum
impl List {
// Create an empty list
fn new() -> List {
// `Nil` has type `List`

// Consume a list, and return the same list with a new element at its front
fn prepend(self, elem: u32) -> List {
    // \`Cons\` also has type List
    Cons(elem, Box::new(self))

// Return the length of the list
fn len(&self) -> u32 {
    // \`self\` has to be matched, because the behavior of this method
    // depends on the variant of \`self\`
    // \`self\` has type \`&List\`, and \`\*self\` has type \`List\`, matching on a
    // concrete type \`T\` is preferred over a match on a reference \`&T\`
    match \*self {
        // Can't take ownership of the tail, because \`self\` is borrowed;
        // instead take a reference to the tail
        Cons(\_, ref tail) => 1 + tail.len(),
        // Base Case: An empty list has zero length
        Nil => 0

// Return representation of the list as a (heap allocated) string
fn stringify(&self) -> String {
    match \*self {
        Cons(head, ref tail) => {
            // \`format!\` is similar to \`print!\`, but returns a heap
            // allocated string instead of printing to the console
            format!("{}, {}", head, tail.stringify())
        Nil => {


fn main() {
// Create an empty linked list
let mut list = List::new();

// Append some elements
list = list.prepend(1);
list = list.prepend(2);
list = list.prepend(3);

// Show the final state of the list
println!("linked list has length: {}", list.len());
println!("{}", list.stringify());


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