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()
  );

The billion dollar mistake

By James Morcom

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The billion dollar mistake

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