Rust by Example

A common use for enums is to create a linked-list:

use crate::List::*;

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

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::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 => {

format!("Nil")

},

}

}

}

fn main() {

// Create an empty linked list

let mut list = List::new();

// Prepend 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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