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).
'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 =  =>  =>  */