In ReasonML, things that we create with
let are immutable -- which means that we can't change them.
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.
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.