Creating your own union types

Suppose your app state needs some state to hold user information. If the user is logged in the state should be "Authenticated" and you also need some data about the user. Otherwise just the info that you are "SignedOut" is enough.

Modelling your Domain

Often times this scenario is modelled with a boolean and some data like this:

{
  authenticated: true,
  userData: {name: 'Peter', ... }
}

But what if you end up in some weird scenario, where authenticated is true, but the userData is missing, null or an empty object? Very confusing stuff, my head is spinning but we can probably work around that by taking great care to keep the two of them in sync.

Alright, take a breath. But what if we need to add third state for guest users? By this time it should be pretty obvious that booleans aren't very good at representing this type of logic. It's not just me, other people think that too. There are some talks about this....

So let't try again by definging a union type.

import { union } from 'funpro';

const Auth = union({
  Authenticated: 1,
  SignedOut: 0,
})

We declared a new type called Auth with two possible states. Authenticated carries one argument, while SignedOut does not have one. We can create these states by using the constructors, like this:

const auth = Auth.Authenticated({ name: 'Peter', ... });

const someOtherAuth = Auth.SignedOut();

This is great. Now Authenticated comes batteries-included with the data that it needs, while SignedOut is fine without any arguments. Now what about that third state we talked about earlier. Let's assume guest accounts are for a limited time only and have an expiration date.

const Auth = union({
  Authenticated: 1,
  SignedOut: 0,
  Guest: 1,
})

const auth = Auth.Guest(new Date('20-01-2018'))

This was easy, now we added that third state in a breeze. We even made it carry that extra data it so desparately needs.

Triple header

TODO: mention that constructors are curried

Working with the data

• pattern matching
• work on prototype
• look at Maybe and Result for examples