What is Cons()?

Question

The Rust tutorial example of a linked list is:

enum List {
    Cons(u32, Box<List>),
    Nil
}

What exactly is the Cons() struct? (It is a struct right?) I can't find any documentation on this anywhere.

Answer 1

Cons does not have a special meaning in Rust. It is only the name that the author of the tutorial chose to call that variant of the enum. The same List could have been defined as:

enum List {
    Pair(u32, Box<List>),
    Nil
}

The name cons comes from the Lisp family of programming languages where pairs (nodes of linked lists) are used as the fundamental building blocks of data structures. Here is one way to create the 1,2,3 list in Common Lisp:

(cons 1 (cons 2 (cons 3 nil)))

cons is a shorthand of construct by which Lisp programmers mean to allocate memory. Programs that allocate lots of memory are said to be consing too much.

Sources

• cons - wikipedia
• Marijn Haverbeke's presentation on JS performance