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)
Mutation
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:
let myValue = ref(5);
Usage
You can get the actual value of a ref
box through accessing its contents
field:
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:
myValue.contents = 6;
We provide a syntax sugar for this:
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 ref
s allocations are optimized away.
Tip & Tricks
Before reaching for ref
, know that you can achieve lightweight, local "mutations" through overriding let bindings.