Add Data Structures With Self-Referential Types as a reasonml til

reason/data-structures-self-referential-types.md

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+# 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] => [] */
+```