You are currently looking at the v6.0 - v8.2 docs (Reason v3.6 syntax edition). You can find the latest manual page here.
(These docs are equivalent to the old BuckleScript docs before the ReScript rebrand)
ReScript has great traditional imperative & mutative programming capabilities. You should use these features sparingly, but sometimes they allow your code to be more performant and written in a more familiar pattern.
Let-bindings are immutable, but you can wrap it with a
ref, exposed as a record with a single mutable field in the standard library:
let myValue = ref(5);
You can get the actual value of a
ref box through accessing its
let five = myValue.contents; // 5
We provide a syntax shortcut for
myValue^. Though we no longer encourage it.
Assign a new value to
myValue like so:
myValue.contents = 6;
We provide a syntax sugar for this:
myValue := 6;
Note that the previous binding
5, since it got the underlying item on the
ref box, not the
Note: you might see in the JS output tabs above that
ref allocates an object. Worry not; local, non-exported
refs allocations are optimized away.
Tip & Tricks
Before reaching for
ref, know that you can achieve lightweight, local "mutations" through overriding let bindings.