Web Development with ReasonML

Using the option Type

In JavaScript, we could have solved the problem of bad input for shirtSizeOfString by returning null or undefined, but that leads to all manner of difficulties. What we really want is a way to handle invalid data in a type-safe manner.

ReasonML solves the problem with a built-in variant data type named option. If you were to write it yourself, it would look like this:

type option('a) =
  | Some('a)
  | None;

This is a parametric data type (see ​What’s a Parametric Data Type?​ for details) where ’a is a type variable. The leading single quote is required for type variables.

Joe asks:

What’s a Parametric Data ...