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Tuples are a ReScript-specific data structure that don't exist in JavaScript. They are:

  • immutable

  • ordered

  • fix-sized at creation time

  • heterogeneous (can contain different types of values)

Reason (Old Syntax)ML (Older Syntax)JS Output
let ageAndName = (24, "Lil' ReScript");
let my3dCoordinates = (20.0, 30.5, 100.0);

Tuples' types can be used in type annotations as well. Tuple types visually resemble tuples values.

Reason (Old Syntax)ML (Older Syntax)JS Output
let ageAndName: (int, string) = (24, "Lil' ReScript");
// a tuple type alias
type coord3d = (float, float, float);
let my3dCoordinates: coord3d = (20.0, 30.5, 100.0);

Note: there's no tuple of size 1. You'd just use the value itself.


To get a specific member of a tuple, destructure it:

Reason (Old Syntax)ML (Older Syntax)JS Output
let (_, y, _) = my3dCoordinates; // now you've retrieved y

The _ means you're ignoring the indicated members of the tuple.

Tuples aren't meant to be updated mutatively. You'd create new ones by destructuring the old ones:

Reason (Old Syntax)ML (Older Syntax)JS Output
let coordinates1 = (10, 20, 30);
let (c1x, _, _) = coordinates1;
let coordinates2 = (c1x + 50, 20, 30);

Tips & Tricks

You'd use tuples in handy situations that pass around multiple values without too much ceremony. For example, to return many values:

Reason (Old Syntax)ML (Older Syntax)JS Output
let getCenterCoordinates = () => {
  let x = doSomeOperationsHere();
  let y = doSomeMoreOperationsHere();
  (x, y)

Try to keep the usage of tuple local. For data structures that are long-living and passed around often, prefer a record, which has named fields.