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

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_686 TILs and counting..._
+_687 TILs and counting..._
 
 ---
 
@@ -503,6 +503,7 @@ _686 TILs and counting..._
 
 - [Break Out Of A While Loop](reason/break-out-of-a-while-loop.md)
 - [Create A Stream From An Array](reason/create-a-stream-from-an-array.md)
+- [Data Structures With Self-Referential Types](reason/data-structures-with-self-referential-types.md)
 - [Defining Variants With Constructor Arguments](reason/defining-variants-with-constructor-arguments.md)
 - [Dynamically Create A Printf String Format](reason/dynamically-create-a-printf-string-format.md)
 - [Exhaustive Pattern Matching Of List Variants](reason/exhaustive-pattern-matching-of-list-variants.md)

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

@@ -0,0 +1,29 @@
+# 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] => [] */
+```