At compile time, Rust needs to know how much space a type takes up. One type whose size can’t be known at compile time is a recursive type, where a value can have as part of itself another value of the same type. Because this nesting of values could theoretically continue infinitely, Rust doesn’t know how much space a value of a recursive type needs. However, boxes have a known size, so by inserting a box in a recursive type definition, you can have recursive types.
Let’s explore the cons list, which is a data type common in functional programming languages, as an example of a recursive type. The cons list type we’ll define is straightforward except for the recursion; therefore, the concepts in the example we’ll work with will be useful any time you get into more complex situations involving recursive types.
A cons list is a data structure.
cons function (short for “construct function”) constructs a new pair from its two arguments, which usually are a single value and another pair. These pairs containing pairs form a list.
cons x onto y” informally means to construct a new container instance by putting the element x at the start of this new container, followed by the container y.
Each item in a cons list contains two elements: the value of the current item and the next item. The last item in the list contains only a value called Nil without a next item. A cons list is produced by recursively calling the cons function. The canonical name to denote the base case of the recursion is Nil.
The canonical name to denote the base case of the recursion is Nil. Note that this is not the same as the “null” or “nil” concept.
Although functional programming languages use cons lists frequently, the cons list isn’t a commonly used data structure in Rust. Most of the time when you have a list of items in Rust, Vec<T> is a better choice to use. Other, more complex recursive data types are useful in various situations, but by starting with the cons list, we can explore how boxes let us define a recursive data type without much distraction.
Nil variant stores no values, so it needs less space than the Cons variant