Add Making Things Mutable as a reason til

reason/making-things-mutable.md

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+# 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)