Rust by Example

22 Enums

The enum keyword allows the creation of tagged unions, which can be used as algebraic data types (ADT).

#![feature(box_syntax)] // 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<List>), // Nil: A node that signifies the end of the linked list Nil, } // Methods can be attached to an enum impl List { // Create an empty list fn new() -> List { // `Nil` has type `List` Nil } // 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 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 => { format!("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()); }