Web Development with ReasonML

Creating Variant Data Types with Parameters

Shirt sizes don’t end with extra-large. There are double, triple, and even quadruple extra-large, usually abbreviated as XXL, XXXL, and XXXXL. Parameterized types let us specify a parameter for the constructor. In our case, we want the parameter to tell us how many Xs are on the shirt size. Here’s a parameterized version of the shirt size constructor (it has the same name, but it’s in a separate file):

datatypes/src/ParamShirtSizes.re

type shirtSize =
  | Small
  | Medium
  | Large
  | XLarge(int);

The last line says that to construct an XLarge variant, you need to provide an integer, which we’ll use to tell how many “extras” we want:

datatypes/src/ParamShirtSizes.re

let mySize ...