Today I Learned

A Hashrocket project

Data Structures With Self-Referential Types

ReasonML has a powerful type system that allows us to create types that represent all sorts of things. For data structures like linked lists, we need a sort of recursive type, a type that can reference itself — a self-referential type.

Reason’s type system allows us to define types that reference themselves. Combine that with type arguments and variants — we can create a type definition to represents something like a linked list.

type linked_list('a) =
  | Empty
  | Node('a, linked_list('a));

A linked list is a chain of nodes. It can be an empty list, hence the first part of the variant. Otherwise, it is a node that has some data and then points to another linked list (chain of nodes).

The 'a part is a type argument. When creating a linked list, we can decide what type the 'a will be. Here is an int-based linked list:

let my_list: linked_list(int) = Node(25, Node(27, Empty));
/* my_list = [25] => [27] => [] */