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<!doctype html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no">
<title>reveal.js</title>
<link rel="stylesheet" href="css/reset.css">
<link rel="stylesheet" href="css/reveal.css">
<link rel="stylesheet" href="css/theme/black.css">
<!-- Theme used for syntax highlighting of code -->
<link rel="stylesheet" href="lib/css/monokai.css">
<!-- Printing and PDF exports -->
<script>
var link = document.createElement( 'link' );
link.rel = 'stylesheet';
link.type = 'text/css';
link.href = window.location.search.match( /print-pdf/gi ) ? 'css/print/pdf.css' : 'css/print/paper.css';
document.getElementsByTagName( 'head' )[0].appendChild( link );
</script>
<style>
.h-layout {
display: flex;
height: 600px;
}
.h-layout > * {
flex-grow: 1;
}
.editor, editor {
width: 90%;
min-height: 400px;
/* overflow: hidden; */
/* height: 100%; */
}
body {
background: #232327;
background: -moz-radial-gradient(center, circle cover, #373a4c 0%, #232327 100%);
background: -webkit-gradient(radial, center center, 0px, center center, 100%, color-stop(0%, #373a4c), color-stop(100%, #232327));
background: -webkit-radial-gradient(center, circle cover, #373a4c 0%, #232327 100%);
background: -o-radial-gradient(center, circle cover, #373a4c 0%, #232327 100%);
background: -ms-radial-gradient(center, circle cover, #373a4c 0%, #232327 100%);
background: radial-gradient(center, circle cover, #373a4c 0%, #232327 100%);
}
</style>
</head>
<body>
<div class="reveal has-">
<div class="slides">
<!--
Refs
https://gist.github.com/acutmore/9d2ce837f019608f26ff54e0b1c23d6e (4-bit VM)
https://github.com/pirix-gh/medium/blob/master/types-curry-ramda/src/index.ts good TS kung-fu
https://github.com/Microsoft/TypeScript/issues/14833 TypeScripts Type System is Turing Complete
-->
<!-- Section: Intro -->
<section>
<p>This is an interactive presentation. Press "s" to open the speaker view.</p>
<aside class="notes">
Because this presentation contains live coding, it doesn't make any sense unless you have the speaker notes handy ;)<br>
<br>
Snippets of code will be provided here and you can copy-paste them in the editor when needed.
</aside>
</section>
<section>
<aside class="notes">
Start here
</aside>
</section>
<section>
<div style="display: flex; flex-direction: column;">
<img src="./imgs/thumbnail.png" alt="" style="display: block; width: 60%; margin: auto;">
<h2 style="display: block; margin-top: 40px;">10 Things You Did Not Know TypeScript Could Do</h2>
</div>
<aside class="notes">
Thank you for coming to this talk entitled "10 Things you did not know TS could do".<br>
And thank you to those who bit on the clickbait by the way.<br>
</aside>
</section>
<section>
<div class="h-layout">
<div>
<img src="./imgs/loved.png" alt="" style="display: block">
</div>
<div>
<img src="./imgs/wanted.png" alt="" style="display: block">
</div>
</div>
<aside class="notes">
Today we'll talk about TypeScript.
According to a popular developer survey, TS is the 3rd most loved language and 4th most wanted.
(StackOverflow 2019 developer survey)
</aside>
</section>
<section>
<div class="h-layout">
<div style="text-align: left; padding-top: 20px; padding-left: 100px">
<img src="./imgs/me.png" alt="" style="border: none; width: 200px; background-color: transparent; box-shadow: none">
<p>Hadrien Milano</p>
<p>Software Engineer, UI</p>
<div style="float: left">@</div>
<img src="./imgs/appdynamics.png" alt="" style="width: 200px; margin: auto; border: none">
</div>
<div>
<img class="fragment" src="./imgs/dashboard.png" alt="" style="display: block; margin: auto; border: none; width: 700px; margin-top: 100px">
</div>
</div>
<aside class="notes">
My name is Hadrien Milano, I work at AppDynamics as a UI dev. We build fancy dashboards like this one.<br>
<br>
While I like pretty UIs like this one, the thing that really drives me is pretty code.<br>
And this explains why I love TypeScript: an attempt to bring order into the chaos that is JavaScript.
</aside>
</section>
<section>
<div style="position: absolute; margin-left: 420px">
<img class="fragment" src="./imgs/pavement.jpg" alt="" style="display: block; margin: auto; border: none; width: 500px; margin-top: 40px">
</div>
<div style="position: absolute; margin-left: 410px">
<img class="fragment" src="./imgs/cookie.jpg" alt="" style="display: block; margin: auto; border: none; width: 500px; margin-top: 80px">
</div>
<div style="position: absolute; margin-left: 414px">
<img class="fragment" src="./imgs/coke.jpg" alt="" style="display: block; margin: auto; border: none; width: 600px; margin-top: 140px">
</div>
<div style="position: absolute; margin-left: 414px">
<img class="fragment" src="./imgs/simple.png" alt="" style="display: block; margin: auto; border: none; width: 600px; margin-top: 140px">
</div>
<div style="position: absolute; margin-left: 404px">
<img class="fragment" src="./imgs/complicated.png" alt="" style="display: block; margin: auto; border: none; width: 630px; margin-top: 120px">
</div>
<div style="position: absolute; margin-left: 380px">
<img class="fragment" src="./imgs/refinery.jpg" alt="" style="display: block; margin: auto; border: none; width: 700px; margin-top: 100px">
</div>
<div style="position: absolute; margin-left: 100px">
<img class="fragment" src="./imgs/github-pr.png" alt="" style="display: block; margin: auto; border: none; width: 1100px; margin-top: 300px">
</div>
<div style="position: absolute; margin-left: 420px">
<img class="fragment" src="./imgs/fuckyou.png" alt="" style="display: block; margin: auto; border: none; width: 600px; margin-top: 100px">
</div>
<aside class="notes">
Once in a while I'll see a piece of code which, although it works, could be improved.<br>
<br>
[3 slides]
<br>
So I set out to the task of improving it. I come up with a simple plan.<br>
<br>
And quickly I find that there is more to it than I initially thought.<br>
<br>
So I iterate and work around the different complications.<br>
<br>
After a while I end up with a monstruosity of TypeScript hacks and tricks to do just that one thing I wanted to do.<br>
I submit it for code review and then...<br>
My coworkers ask me to burn that code to hell.<br>
</aside>
</section>
<section>
<aside class="notes">
But this code is a testimony of a voyage to the edge of TypeScript.<br>
I can't let this go to waste.<br>
</aside>
</section>
<section>
<h2>Focus of this talk</h2>
<img src="./imgs/purpose.png" alt="" style="display: block; width: 60%; margin: auto;">
<aside class="notes">
And so this is what this presentation is about.<br>
To give this werid code an afterlife so you too can be transported to the edge of TypeScript, and in this process, learn more about this popular language.<br>
<br>
Obviously you will not become experts just watching this presentation, but you'll walk out with a better understanding of it.
</aside>
</section>
<!-- Section: fundamentals -->
<section>
<h2>Foundations</h2>
<p class="fragment">
<span style="color: #5b94ec">Types</span> + <span style="color: #e8d11d">JavaScript</span> =
<span style="color: #5b94ec">Type</span><span style="color: #e8d11d">Script</span>
</p>
<aside class="notes">
Before we get into the weird stuff, we need to start with the basics. Let's see how familiar this room is with TypeScript...<br>
<br>
[Poll the room]<br>
<br>
Let's quickly review the basics.
</aside>
</section>
<section>
<p>JS with types</p>
<editor data-trim>
let someNumber = 1;
someNumber = '2';
</editor>
<aside class="notes">
TS adds types to variables to maximize correctness of the code.<br>
This is valid JavaScript, but it's not valid TypeScript. TS, by default considers that variables don't change type.<br>
Notice that the error could be either in the first or the second line.<br>
Maybe I want this variable to be a number. Or I want it to be a string, or both!<br>
If I want both, I should write <pre>let someNumber: string | number = 1;</pre><br>
When I don't write any type annotation, TS infers the type from the right hand side.<br>
</aside>
</section>
<section>
<p>Type aliases</p>
<editor data-trim>
type MyString = string;
const someString: MyString = 'abc';
</editor>
<aside class="notes">
I can define a custom type as an alias for another type.
</aside>
</section>
<section>
<p>Object types</p>
<editor data-trim>
type User = {
age: number;
name: string;
isHuman: boolean;
};
</editor>
<aside class="notes">
Define a variable:
<pre>const user: User = {
age: 5,
isHuman: false,
name: 'wall-e'
}</pre>
TS helps with auto-completion.<br>
TS has a built-in concept for these object types: an interface.<br>
<pre>interface User { ... }</pre>
</aside>
</section>
<!-- Section: Intersection, Union, Conditional -->
<section>
<h2>Operators</h2>
<aside class="notes">
With what we've seen so far, you already know 60% of what you'll ever need in TS.<br><br>
But you are going to need operators to combine types and start doing interesting stuff.<br>
</aside>
</section>
<section>
<p>Intersection (mixins)</p>
<editor data-trim>
<hidden>
declare function getUser(): any;
</hidden>
type User = { username: string; }
type Admin = { admin: true }
type Moderator = { moderator: true }
let john: User & Admin & Moderator = getUser()
</editor>
<aside class="notes">
John is a user AND an admin AND a Moderator.<br>
If we inspect the type of john, we can see it has the properties of all types.<br>
</aside>
</section>
<section>
<p>Trick #1<br>Safe html strings.</p>
<editor data-trim style="height: 300px;">
<hidden>
declare function escapeHtmlString(s: string): string;
declare function getUserInput(): string;
</hidden>
type SafeString = string & { __isEscaped: true };
function sanitize(raw: string): SafeString {
return escapeHtmlString(raw) as SafeString;
}
function render(template: string, ...args: SafeString[]) { /* ... */ }
</editor>
<aside class="notes">
And this leads us to our first trick!<br>
As you know protecting against XSS is hard. It's easy to miss a spot, but one breach is enough to compromise your app's security.<br>
<br>
__isEscaped is a "pseudo type" or "virtual type". It doesn't exist at runtime.<br>
<pre>
const userName = sanitize(getUserInput());
render("Hello %name%", userName);
</pre>
</aside>
</section>
<section>
<p>Trick #1½</p>
<editor data-trim style="height: 300px;">
<hidden>
declare function escapeHtmlString(s: string): string;
declare function getUserInput(): string;
type SafeString = string & { __isEscaped: true };
function sanitize(raw: string): SafeString {
return escapeHtmlString(raw) as SafeString;
}
</hidden>
function html<T extends string[]>(body: TemplateStringsArray, ...args: string[]): string {
let builder = '';
// ...
return builder;
}
const userName = getUserInput();
html`Hello ${userName}!`; // "Hello John!"
</editor>
<aside class="notes"><pre>
This is a JS tagged template. In TS they are type-checked! <br>
Replace argument string[] with SafeString[] .
</pre></aside>
</section>
<section>
<p>Union</p>
<editor data-trim>
<hidden>
type User = { handle: string; };
type Admin = { admin: true };
type Moderator = { modo: true };
declare function getUser(): any;
</hidden>
function anyone(who: Admin | User) { /* ... */ }
let john: Admin = getUser();
anyone(john);
</editor>
<aside class="notes">
This functions accepts either an admin or a user.<br>
<br>
[Change the type of john to be User, this still works]
</aside>
</section>
<section>
<div class="h-layout">
<div style="min-width: 400px;">
<p style="margin-top: 40%">Trick #2<br>Type narrowing</p>
</div>
<editor data-trim>
<hidden>
</hidden>
interface HumanUser {
kind: 'human';
firstname: string;
lastname: string;
}
interface BotUser {
kind: 'bot';
handle: string;
}
type AnyUser = HumanUser | BotUser;
function greet(user: AnyUser): string {
}
</editor>
</div>
<aside class="notes">
This union is discriminated by the member "kind".<br>
<br>
In the function, the only accessible member is "kind".<br>
<br>
If I discriminate based on this value, I gain access to the other members!<br>
<pre>
if (user.kind === 'human') {
return `Hello ${user.firstname} ${user.lastname}!`;
} else {
return `0101000110 ${user.handle}`;
}</pre>
</aside>
</section>
<section>
<div class="h-layout">
<div style="min-width: 500px;">
<p style="margin-top: 50%">Trick #2½<br>Exhaustive matching (without return type)</p>
<p class="fragment">Bonus trick<br>Type-level error messages</p>
</div>
<editor data-trim>
<hidden>
interface HumanUser {
type: 'human';
firstname: string;
lastname: string;
}
interface BotUser {
type: 'bot';
handle: string;
}
interface AlienUser {
type: 'alien',
codename: string
}
type AnyUser = HumanUser | BotUser | AlienUser;
declare function postMessage(s: string): void;
</hidden>
function greet(user: AnyUser): void {
switch (user.type) {
case 'human':
postMessage(`Hello ${user.firstname} ${user.lastname}!`);
break;
case 'bot':
postMessage(`0101000110 ${user.handle}`);
break;
// Am I missing something?
}
}
</editor>
</div>
<aside class="notes">
TypeScript has a type which means "should never happen". It is called <pre>never</pre>.<br>
<br>
Declare this function and use it in the <pre>default</pre> block.
<pre>
function isSwitchExhaustive(a: never) {
throw new Error('Unexpected case');
}
</pre>
<br>
For type-level error messages, change function parameter to:
<pre>a: "ERROR: You are missing a case in this switch"</pre>
<br>
Why does this work?<br>
Because unions can always be written as <pre>A | B | ... | never</pre>.<br>
Never is what is left when you've removed everything from a union.
</aside>
</section>
<section>
<div class="h-layout">
<div style="min-width: 500px;">
<p style="margin-top: 50%">Trick #3<br>Exhaustive matching</p>
</div>
<editor data-trim>
<hidden>
interface HumanUser {
type: 'human';
firstname: string;
lastname: string;
}
interface BotUser {
type: 'bot';
handle: string;
}
</hidden>
interface AlienUser {
type: 'alien',
codename: string
}
type AnyUser = HumanUser | BotUser | AlienUser;
function greet(user: AnyUser): string {
switch (user.type) {
case 'human':
return `Hello ${user.firstname} ${user.lastname}!`;
case 'bot':
return `0101000110 ${user.handle}`;
}
}
</editor>
</div>
<aside class="notes">
Can you spot the mistake in this code snippet?
...missing a case for 'alien' type.
</aside>
</section>
<!-- <section>
What type is left when there is nothing left?
<pre><code data-trim>
type SomeUnion = A | B | C; // | never;
</code></pre>
</section>
<section>
<p>What type is left when there is nothing left?</p>
<editor data-trim style="height: 300px">
<hidden>
interface HumanUser {
type: 'human';
firstname: string;
lastname: string;
}
interface BotUser {
type: 'bot';
handle: string;
}
interface AlienUser {
type: 'alien',
codename: string
}
</hidden>
type AnyUser = HumanUser | BotUser | AlienUser | never;
const myNever: never = 123;
function blackHole(a: number): never {
throw new Error('Oh no!');
}
</editor>
<aside class="notes"><pre>
function notBlackHole(a: number): never {
if (a === 2) {
throw new Error('Oh no!');
}
}
</pre></aside>
</section> -->
<section>
<p>Conditional types</p>
<pre><code data-trim>
A extends B ? Yes : No;
</code></pre>
<aside class="notes">
The last tool we need in our toolbox before we can get serious.
</aside>
</section>
<section>
<p>Conditional types</p>
<editor data-trim>
type IsNumber<T> = T extends number ? 'A number' : 'Not a number';
type A = IsNumber<2>;
type B = IsNumber<'hello'>;
</editor>
<aside class="notes"></aside>
</section>
<section>
<p>Conditional type inference</p>
<editor data-trim>
type MyTuple = ['one', 'two'];
type T = MyTuple extends [any, infer Second] ? Second : never;
</editor>
<img class="fragment" src="./imgs/ReturnType.png" />
<aside class="notes">
We capture a type in the matching pattern, and then use it<br>
Note that the same thing could be done with
<pre>type T = MyTuple[1]</pre>
However, this can not be done without conditional type inference:
<br>
See this definition from the standard library extracts the return type of a function.
</aside>
</section>
<section>
<p>Let's get serious!</p>
</section>
<!-- Section: recursive types, working with tuples, Rest.ts -->
<section>
<p>HTTP API definition</p>
<editor data-trim style="min-height: 400px">
<hidden>
declare function createClient<T>(api: T): { [k in keyof T]: (params?: any) => Promise<any>};
declare function createServer<T>(api: T, server: { [k in keyof T]?: (req: { params: any }) => void});
</hidden>
// Shared
const api = {
getAllCats: 'GET /api/cats',
getCatByName: 'GET /api/cats/:name'
};
// Client
const client = createClient(api);
// Server
const server = createServer(api, {});
</editor>
<p class="fragment"><small>Not super TypeScript friendly...</small></p>
<aside class="notes">
You have a web application with a front-end and a back-end component.<br>
You want to share the API definition between the front and back to maximize the safety of this link.<br>
This would be a typical way to do it. A shared API definition, then helpers to create client and server implementations of that API.<br>
<br>
But this naive approach is not safe. Take this code:<br>
<pre>client.getCatByName({
name: 'Olinka'
})</pre>
The parameter "name" could as well be "id" and this would compile, but fail at runtime...<br>
<br>
For the server implementation<br>
<pre>{
getAllCats() {},
getCatByName(req) {
}
}</pre>
req.params isn't type-checked either...<br>
<br>
Problem: There is no way to link the parameter names to their usage.
</aside>
</section>
<section>
<p>What if...</p>
<editor data-trim style="min-height: 400px">
<hidden>
interface Endpoint<Params extends string> {
params: { [k in Params]: string; };
}
interface Api {
[k: string]: Endpoint<any>;
}
type HttpClient<T extends Api> = {
[k in keyof T]: (params: T[k]['params']) => void;
}
interface HttpRequest<T extends Endpoint<any>> {
params: T['params'];
}
type HttpServer<T extends Api> = {
[k in keyof T]?: (req: HttpRequest<T[k]>) => void;
}
declare function createClient<T extends Api>(def: T): HttpClient<T>;
declare function createServer<T extends Api>(def: T, serv: HttpServer<T>): void;
declare function GET(str: TemplateStringsArray): Endpoint<never>;
declare function GET<A extends string>(str: TemplateStringsArray, a: A): Endpoint<A>;
declare function GET<A extends string, B extends string>(str: TemplateStringsArray, a: A, b: B): Endpoint<A | B>;
</hidden>
const api = {
getAllCats: GET `/api/cats`,
getCatByName: GET `/api/cats/${'name'}`
};
const client = createClient(api);
const server = createServer(api, { });
</editor>
<aside class="notes">
Call endpoint with name.<br>
Change parameter, notice type error.<br>
Add second parameter<br>
</aside>
</section>
<section>
<p>Is this... magic?</p>
<editor data-trim style="min-height: 250px">
interface Endpoint<Params extends string> {
params: { [k in Params]: string; }
}
declare function GET<A extends string>(str: TemplateStringsArray, a: A): Endpoint<A>
</editor>
<p>No, it's TypeScript! (trick #5)</p>
<aside class="notes">
GET is a template tag. You can use it like this:<br>
<pre>const endpoint = GET `/cats/${'name'}/`</pre><br>
And now the string literal is captured in the type of <pre>endpoint</pre>!
<br>
But this doesn't work when you add a second parameter...<br>
<pre>const endpoint = GET `/cats/${'owner'}/${'name'}/`</pre>
</aside>
</section>
<section>
<img src="./imgs/runtypes.png" alt="" class="fragment" style="position: absolute; display: block; width: 400px; margin-left: 400px;">
<img src="./imgs/rxjs.png" alt="" class="fragment" style="position: absolute; display: block; width: 1000px; margin-left: 100px; margin-top: 100px">
<div style="position: relative">
</div>
<p>Hmmm...</p>
<editor data-trim style="min-height: 300px">
<hidden>
interface Endpoint<Params extends string> {
params: { [k in Params]: string; };
}
</hidden>
declare function GET<A extends string>(str: TemplateStringsArray, a: A): Endpoint<A>;
declare function GET<A extends string, B extends string>(str: TemplateStringsArray, a: A, b: B): Endpoint<A | B>;
declare function GET<A extends string, B extends string, C extends string>(str: TemplateStringsArray, a: A, b: B, c: C): Endpoint<A | B | C>;
declare function GET<A extends string, B extends string, C extends string, D extends string>(str: TemplateStringsArray, a: A, b: B, c: C, d: D): Endpoint<A | B | C | D>;
</editor>
<aside class="notes">
An easy solution is to declare a version of the function for many different number of arguments.<br>
But this is not DRY, and imposes a hard limit on how many arguments can be accepted, even though the underlying function can accept any number of arguments!<br>
<br>
But this seems to be the industry standard at the moment...
</aside>
</section>
<section>
<p>Problem:</p>
<pre><code data-trim>
let Flatten = [A, B, C, ...] -> A | B | C | ...
</code></pre>
<aside class="notes">
How to flatten a tuple?
</aside>
</section>
<section>
<p>First attempt</p>
<editor data-trim style="height: 100px;">
type Flatten<T extends any[]> =
</editor>
<div class="fragment">
<p>sub-problems:</p>
<ul>
<li>"Rest" of a tuple</li>
<li>Recursive types</li>
</ul>
</div>
<aside class="notes">
Naively, one would start with a conditional like this:
<pre>type Flatten<T extends any[]> = T extends [infer Frist, ...infer Rest] ? First | Flatten<T[Rest]> : T;</pre>
<br>
How do I get the rest of a tuple?<br>
How do I make a recursive type?
</aside>
</section>
<section>
<p>Rest of a tuple</p>
<editor data-trim style="min-height: 400px;">
type Tail<T extends any[]> = ;
type test = Tail<['a', 'b', 'c']>
</editor>
<aside class="notes">
Well known trick: Use function varargs.
<pre>type Tail<T extends any[]> = ((...args: T) => void) extends (head: any, ...tail: infer Tail) => void ? Tail : never;</pre>
</aside>
</section>
<section>
<p>Tuple utilities (trick #4)</p>
<editor data-trim style="min-height: 400px;">
type List<Data extends any[]> = (...tail: Data) => void;
type Push<El, list extends List<any>> = list extends List<infer Data> ? (head: El, ...tail: Data) => void : never;
type Pop<Head, Tail extends any[]> = (h: Head, ...tail: Tail) => void;
type Tuple<L extends List<any>> = L extends List<infer T> ? T : never;
</editor>
<aside class="notes">
Armed with this varargs exploit, we can devise a type-level list library.<br>
This is how you use it:<br>
<br>
Create a list:<br>
<pre>type MyList = List<['a', 'b', 'c']>;</pre>
Prepend an element to the list:<br>
<pre>type MyList2 = Push<'z', MyList>;</pre>
Remove an element from the list:<br>
<pre>type MyList3 = MyList2 extends Pop<any, infer Tail> ? Tail : never;</pre>
Get the first element:<br>
<pre>type MyHead = MyList2 extends Pop<infer Head, any> ? Head : never;</pre>
<br>
The usage of pop is a little convoluted, but you'll see why it works like this in a minute...
</aside>
</section>
<section>
<p>Tuple flattening problems</p>
<ul>
<li><span style="color:red;text-decoration:line-through"><span style="color: white">"Rest" of a tuple</span></span></li>
<li>Recursive types</li>
</ul>
</section>
<section>
<p>Recursive types</p><editor data-trim style="min-height: 400px;">
<hidden>
type List<DATA extends any[]> = (...tail: DATA) => void;
type Push<El, list extends List<any>> = list extends List<infer Data> ? (head: El, ...tail: Data) => void : never;
</hidden>
type Pop<Head, Tail extends any[]> = (h: Head, ...tail: Tail) => void;
type Flatten<T extends List<any>> = T extends Pop<infer Head, infer Tail>
? Head | Flatten<List<Tail>>
: never
type A = Flatten<List<['a', 'b', 'c']>>
</editor>
<aside class="notes">
Split the type into two cases: edge case and general case.
<pre>
type Flatten<T extends List<any>> = {
rec: T extends Pop<infer Head, infer Tail> ? Head | Flatten<List<Tail>> : never
end: never
};
</pre>
Here only one iteration is performed. Most of the list is still uncomputed. I can access the partial result by doing:
<pre>type A = Flatten<List<['a', 'b', 'c']>>['rec']</pre>
<br>
But this needs to happen on each level of the recursion, automatically.<br>
<br>
I can try<br>
<pre>type Flatten = { ... }['rec']</pre><br>
But now the type is detected as a circular reference again!<br>
And it makes sense: This recursion would be infinite, because I am never invoking the end case.<br>
I need an exit condition...<br>
<pre>
type Flatten<T extends List<any>> = {
tail: T extends Pop<infer Head, infer Tail> ? Head | Flatten<List<Tail>> : never;
end: never;
} [
T extends Pop<unknown, any> ? 'end' : 'tail'
];
</pre>
The conditional type confuses the cycle detection algorithm!<br>
<br>
As seen here: https://github.com/Microsoft/TypeScript/issues/14833
</aside>
</section>
<section>
<p>Trick #5: Recursive type</p>
<editor data-trim style="min-height: 400px;">
<hidden>
type List<DATA extends any[]> = (...tail: DATA) => void;
type Push<El, list extends List<any>> = list extends List<infer Data> ? (head: El, ...tail: Data) => void : never;
type Pop<Head, Tail extends any[]> = (h: Head, ...tail: Tail) => void;
</hidden>
type Flatten<T extends List<any>> = {
tail: T extends Pop<infer Head, infer Tail> ? Head | Flatten<List<Tail>> : never;
end: never;
} [
T extends Pop<unknown, any> ? 'end' : 'tail'
];
</editor>
<aside class="notes"></aside>
</section>
<section>
<p>Putting it all together</p>
<editor data-trim style="min-height: 500px;">
<hidden>
interface Endpoint<Params extends string> {
params: { [k in Params]: string; };
}
interface Api {
[k: string]: Endpoint<any>;
}
type HttpClient<T extends Api> = {
[k in keyof T]: (params: T[k]['params']) => void;
}
interface HttpRequest<T extends Endpoint<any>> {
params: T['params'];
}
type HttpServer<T extends Api> = {
[k in keyof T]?: (req: HttpRequest<T[k]>) => void;
}
declare function createClient<T extends Api>(def: T): HttpClient<T>;
declare function createServer<T extends Api>(def: T, serv: HttpServer<T>): void;
type List<DATA extends any[]> = (...tail: DATA) => void;
type Push<El, list extends List<any>> = list extends List<infer Data> ? (head: El, ...tail: Data) => void : never;
type Pop<Head, Tail extends any[]> = (h: Head, ...tail: Tail) => void;
type Flatten<T extends any[]> = FlattenRec<List<T>, 't'>['t'];
interface FlattenRec<T extends List<any>, k extends 't'> {
t: T extends Pop<infer Head, infer Tail> ?
unknown extends Head ? never : Head | FlattenRec<List<Tail>, k>[k] : never;
}
</hidden>
declare function GET<Args extends string[]>(str: TemplateStringsArray, ...args: Args): Endpoint<Flatten<Args>>;
const api = {
getAllCats: GET `/api/cats`,
getCatByName: GET `/api/cats/${'name'}`
};
const client = createClient(api);
const server = createServer(api, {});
</editor>
</section>
<section>
<img class="fragment" src="./imgs/i-love-it.jpg" alt="" style="display: block; width: 60%; margin: auto;">
<p class="fragment">More at <a href="https://github.com/hmil/rest.ts">https://github.com/hmil/rest.ts</a></p>
<aside class="notes">
We won by combining a variable-arguments hack and exploiting a limitation of the circular reference detection code!
</aside>
</section>
<!-- Section: Turing completeness? Lambda calculus -->
<section>
<p>TypeScript can do a lot...</p>
<p class="fragment">But can it do everything?</p>
<aside class="notes">
We can manipulate objects, tuples, etc...
But what about numbers? Math? String manipulation? What about other algorithms?
</aside>
</section>
<section>
<p>What is <em>everything</em>?</p>
<aside class="notes">
We must first define what _everything_ actually is.<br>
<br>
</aside>
</section>
<section>
<img src="./imgs/turing.jpg" alt="" style="display: inline-block; width: 400px">
<img src="./imgs/church.jpg" alt="" style="display: inline-block; width: 400px">
<aside class="notes">
Luckily for us, these guys in black and white figured it out already.<br>
<br>
In the 1930s, the question of computability was a hot topic (unlike today, when it's butchered at the end of JavaScript meetups).<br>
<br>
Alan Turing and Alonzo Church were studying models of computation in the early days...
</aside>
</section>
<section>
<p>What is <em>everything</em>?</p>
<div style="display: flex; flex-direction: row; align-items: center">
<img src="./imgs/turing-machine.jpg" alt=""/>
<div class="fragment" style="padding: 20px; font-size: 70px;"> ≡ </div>
<img src="./imgs/lambda-calculus.png" alt="" />
</div>
<aside class="notes">
Turing came up with a theoretical machine (now dubbed the Turing machine) which could "compute stuff".<br>
Meanwhile, Church devised a mathematical language that could also "compute stuff".<br>
<br>
They both tried to formalize what their respective systems could compute, in vain...<br>
Until one day, they figured that their models, as well as a third one which for some reason isn't as popupar as the two former, were equivalent.<br>
<br>
Since then, the benchmark for how powerful a logical tool is has been "can it simulate a turing machine"? If it can, it is called "Turing complete".<br>
<br>
It could as well have been "can it simulate lambda-calculus" and "lambda-complete" - since this is equivalent - but for some reason the version that stuck in history was the Turing one.
</aside>
</section>
<section>
<h2>Is TypeScript Turing Complete?</h2>
<p class="fragment">Can it simulate a turing machine?</p>
<aside class="notes">
So, we know TS is probably not more powerful than a Turing machine because:<br>
a. It runs inside a computer which is itself no more powerful than a T. machine<br>
b. This would be the biggest scientific breakthrough of the century so far.<br>
So the question really is, can TS simulate a Turing machine?
</aside>
</section>
<section>
<p>Can we implement lambda calculus in TypeScript?</p>
<h2 class="fragment">[NSFW]</h2>
<p class="fragment">Do not try this at <span style="text-decoration: line-through">home</span> work.</p>
<aside class="notes">
We'll use lambda calculus because it is a much better fit for our task than a turing machine.
</aside>
</section>
<section>
<div style="display: flex; flex-direction: row; justify-content: space-between">
<div>
Variable:
<pre style="width: 300px; text-align: center"><code data-trim>x</code></pre>
<pre class="fragment" style="width: 300px; text-align: center"><code data-trim>var x</code></pre>
</div>
<div>
Abstraction:
<pre style="width: 400px; text-align: center"><code data-trim>λx. <something></code></pre>
<pre class="fragment" style="width: 400px; text-align: center"><code data-trim>x => <something></code></pre>
</div>
<div>
Application:
<pre style="width: 500px; text-align: center"><code data-trim><something1> <something2></code></pre>
<pre class="fragment" style="width: 500px; text-align: center"><code data-trim><something1>(<something2>)</code></pre>
</div>
</div>
<!-- <img src="./imgs/lambda1.png" alt="" style="max-width: 50%">
<pre class="fragment"><code data-trim>
λx. λy. x y
(x: any) => (y: any) => x(y)
</code></pre> -->
<aside class="notes">
Lambda calculus is extremely simple. There are only 3 constructs in total:<br>
variablesl, abstractions and applications.<br>
A variable is like a variable in any old programing language.<br>
An abstraction can be thought of as a function in JS<br>
An application is what happens when you apply an expression to another expression (which hopefully evaluates to a function)<br>
<br>
Looking at this, it's easy to see that JavaScript is turing complete. We have here a direct mapping from lambda calculus to JavaScript.<br>
But this is not what we are looking for, we want to evaluate these expressions in the type system itself!
</aside>
</section>
<section>
<p>Example</p>
<pre><code data-trim>
(λx. x) y
= y
</code></pre>
<pre class="fragment"><code data-trim>
(λx. x x) (λy. y y)
(λx=(λy. y y). x x)
= (λy. y y) (λy. y y)
</code></pre>
<aside class="notes">
Let's take a look at some examples.
</aside>
</section>
<section>
<p>Example</p>
<pre><code data-trim>
0 = λf. λx. x
1 = λf. λx. f x
2 = λf. λx. f (f x)
SUCC = λn. λf. λx. f ((n f) x)
</code></pre>
<pre class="fragment"><code data-trim>SUCC 0 = (λn. λf. λx. f ((n f) x)) 0</code></pre>