Add Making Things Mutable as a reason til

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/).
 
-_690 TILs and counting..._
+_691 TILs and counting..._
 
 ---
 
@@ -511,6 +511,7 @@ _690 TILs and counting..._
 - [Generate Starter Reason Projects](reason/generate-starter-reason-projects.md)
 - [Helping The Compiler Help Us With Variants](reason/helping-the-compiler-help-us-with-variants.md)
 - [Inline Component Styles With Reason React](reason/inline-component-styles-with-reason-react.md)
+- [Making Things Mutable](reason/making-things-mutable.md)
 - [Multi-Argument Functions As Syntactic Sugar](reason/multi-argument-functions-as-syntactic-sugar.md)
 - [Pattern Match On Exceptions](reason/pattern-match-on-exceptions.md)
 - [Quickly Bootstrap A React App Using Reason](reason/quickly-bootstrap-a-react-app-using-reason.md)

reason/making-things-mutable.md

@@ -0,0 +1,31 @@
+# Making Things Mutable
+
+In [ReasonML](https://reasonml.github.io/en/), things that we create with
+`let` are immutable -- which means that we can't change them.
+
+```reason
+let num = 5;
+```
+
+Once `num` is _bound_ to `5` it is stuck with that value for the duration
+of it's scope.
+
+ReasonML doesn't completely restrict us to immutability though. The
+`ref` construct allows us to bind a variable to a sort of box that holds a
+value. We can then look in the box and change what is in the box.
+
+```reason
+let num = ref(5); /* put 5 in the box */
+
+Js.log(num^); /* use `^` to look in the box */
+
+num := 3; /* remove 5, put 3 in the box */
+```
+
+We use `ref` to bind our variable to a box with some initial value. The `:=`
+assignment operator allows us to change what's in the box. Anytime we want
+to refer to what's in the box, we postfix our variable with `^`.
+
+Also of note: while `list` instances are not mutable, `array` instances are.
+
+[source](https://reasonml.github.io/docs/en/mutation)