> For the complete documentation index, see [llms.txt](https://ploegert.gitbook.io/til/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ploegert.gitbook.io/til/programmy/ml/reason/data-structures-with-self-referential-types.md).

# Data Structures With Self-Referential Types

[ReasonML](https://reasonml.github.io/) has a powerful type system that allows us to create types that represent all sorts of things. For data structures like [linked lists](https://en.wikipedia.org/wiki/Linked_list), 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.

```reason
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:

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