> For the complete documentation index, see [llms.txt](https://ploegert.gitbook.io/til/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ploegert.gitbook.io/til/programmy/ml/reason/making-things-mutable.md).

# 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)