The billion dollar mistake

Tony Hoare

1965 - Invented "null"
2009 - Called it his "billion dollar mistake"

function getFerrari() {}

function getFerrari(): Ferrari {}

function getFerrari(): Ferrari {}

const myCar = getFerrari();

function getFerrari(): Ferrari {
  return null;
}

const myCar = getFerrari();

function getFerrari(): Ferrari {
  return null;
}

const myCar = getFerrari();
// ...
const key = myCar.getKey();

TypeError: Cannot read property 'getKey' of null

function getFerrari(): Ferrari {
  return null;
}

const myCar = getFerrari();
// ...
const key: Key;
if (myCar === null)
  key = null;
else
  key = myCar.getKey();

function getFerrari(): Ferrari {
  return null;
}

const myCar = getFerrari();
// ...
const key = myCar && myCar.getKey();

The problem with null

Can hide in any form

Any value could be null

Hard to debug

The Alternative

function getFerrari(): Ferrari {
  return null;
}

function getFerrari(): Ferrari {
  return new F40();
}

function getFerrari(): Ferrari {
  
                return null;
}

function getFerrari(): Maybe<Ferrari> {
  return null;
}

New Rules

Cannot use null
If a value can be null, use Maybe type

Arrays

function getFerraris(): Array<Ferrari> {
  // ...
}

function getFerraris(): Array<Ferrari> {
  // ...
}

const myFerraris = getFerraris();

function getFerraris(): Array<Ferrari> {
  // ...
}

const myFerraris = getFerraris();
const key = myFerraris.getKey();

function getFerraris(): Array<Ferrari> {
  // ...
}

const myFerraris = getFerraris();
const key = myFerraris.getKey();

function getFerraris(): Array<Ferrari> {
  // ...
}

const myFerraris = getFerraris();
const keys = 
  myFerraris.map(ferrari => ferrari.getKey());

function getFerraris(): Array<Ferrari> {
  // ...
}

const myFerraris = getFerraris();
const keys: Array<Key> = 
  myFerraris.map(ferrari => ferrari.getKey());

Promises

function requestFerrari(): Promise<Ferrari> {
  // ...
}

function requestFerrari(): Promise<Ferrari> {
  // ...
}

const promiseOfFerrari = requestFerrari();

function requestFerrari(): Promise<Ferrari> {
  // ...
}

const promiseOfFerrari = requestFerrari();
const key = promiseOfFerrari.getKey();

function requestFerrari(): Promise<Ferrari> {
  // ...
}

const promiseOfFerrari = requestFerrari();
const key = promiseOfFerrari.getKey();

function requestFerrari(): Promise<Ferrari> {
  // ...
}

const promiseOfFerrari = requestFerrari();
const promiseOfKey = 
  promiseOfFerrari.then(
    ferrari => ferrari.getKey()
  );

function requestFerrari(): Promise<Ferrari> {
  // ...
}

const promiseOfFerrari = requestFerrari();
const promiseOfKey: Promise<Key> = 
  promiseOfFerrari.then(
    ferrari => ferrari.getKey()
  );