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.

Mutate Let-binding

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:

Reason (Old Syntax)ML (Older Syntax)JS Output
let myValue = ref(5);


You can get the actual value of a ref box through accessing its contents field:

Reason (Old Syntax)ML (Older Syntax)JS Output
let five = myValue.contents; // 5

We provide a syntax shortcut for myValue.contents: myValue^. Though we no longer encourage it.

Assign a new value to myValue like so:

Reason (Old Syntax)ML (Older Syntax)JS Output
myValue.contents = 6;

We provide a syntax sugar for this:

Reason (Old Syntax)ML (Older Syntax)JS Output
myValue := 6;

Note that the previous binding five stays 5, since it got the underlying item on the ref box, not the ref itself.

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.