# Testcase: linked-list

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<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::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());
    }

### See also:

`Box` and methods