Web Development with ReasonML

Creating Variant Data Types

Here’s where ReasonML’s type system begins to show some of its power. Let’s say we want a data type to represent shirt sizes: Small, Medium, Large, and XLarge (extra-large). We could use an alias for the string type, but it wouldn’t keep us from doing things like this:

datatypes/src/StringSizes.re

type shirtSize = string;

let mySize = "Medium";
let otherSize = "Large";
let wrongSize = "M";

ReasonML lets us create a data type that allows only valid values with a data type constructor, which, as its name implies, tells us how to construct a value of that particular data type. This is called a variant data type, as we’re specifying the various values the data type can have:

datatypes/src/ShirtSizes.re ...