Functional Reactive Programming without Black Magic (UIKonf 2015)

How to use Functional
Reactive Programming
without Black Magic

let me = Person(name: "Jens Ravens")
@JensRavens
GitHub: JensRavens
nerdgeschoss.de
swift.berlin

A short introduction to
functional programming, the
universe and everything.

In the beginning McIlroy
created the unix pipe. And he
saw it was good.
ls | grep *.jpg | sort

And I promise not to use the
scary M-word.
me, 12 weeks ago.

–Saunders Mac Lane
All told, a monad is just a monoid in the
category of endofunctors.
“

Monads are just Applicative Functors
“

Buy it, use it,
break it, fix it,
Trash it, change it,
mail - upgrade it.
– Daft Punk, Technologic

Buy it;
if error {
//TODO: Handle me!
} else {
use it;
if error {
//TODO: Handle me!
} else {
break it;
if error {
//TODO: Handle me!

Monad
something that defines map and bind

The optional Monad
let string: String? = "World"
let greeting = string.map{"Hello ($0)”}
//Hello World
extension Optional {
func bind (f: T->U?) -> U? {
if let result = self.map({f($0)}) {
return result
} else {
return nil
}
}
}

The optional Monad
let string: String? = "World"
let greeting = string.map{"Hello ($0)”}
//Hello World
func greetOrNil(name: String)->String? {
if name == "World" {
return "Hello World"
} else {
return nil
}
}
let greeting2 = string.bind(greetOrNil)
//Hello World

The optional Monad
func bind (f: T->U?) -> U? {
if let result = self.map({f($0)}) {
return result
} else {
return nil
}
}
}
func bind (f: T->Optional) -> Optional {
switch self {
case let .Some(value): return f(value)
case .None: return nil
}
}
}

The result Monad
public enum Result<T> {
case Success(Box<T>)
case Error(NSError)
}

The result Monad
…
public func map(f: T -> U) -> Result {
switch self {
case let .Success(v):
return .Success(Box(f(v.value)))
case let .Error(error): return .Error(error)
}
}
…
}

The result Monad
…
public func bind(f: T -> Result)
-> Result {
switch self {
case let .Success(v): return f(v.value)
case let .Error(error): return .Error(error)
}
}
…
}

Monad
Transform

func parseString(data: NSData) ->
Result<String>
func parseJson(data: NSData) ->
Result<[String: AnyObject]>
func asyncGreeting(name: String,
completion: Result<String>->Void)

public func bind(f:(T, (Result->Void))->Void)
-> (Result->Void)->Void {
return { g in
switch self {
case let .Success(v): f(v.value, g)
case let .Error(error): g(.Error(error))
}
}
}

public final class Signal<T> {
private var value: Result<T>?
private var callbacks: [Result<T> -> Void] = []
public func subscribe(f: Result<T> -> Void) {
if let value = value {
f(value)
}
callbacks.append(f)
}
public func update(value: Result<T>) {
self.value = value
self.callbacks.map{$0(value)}
}
}

public func map(f: T -> U) -> Signal {
let signal = Signal()
subscribe { result in
signal.update(result.map(f))
}
return signal
}
public func bind(f: T -> Result) -> Signal {
subscribe { result in
signal.update(result.bind(f))
}
return signal
}

public func bind(f: (T, (Result->Void))->Void)
-> Signal {
subscribe { value in
value.bind(f)(signal.update)
}
return signal
}

infix operator >>> { associativity left precedence 160 }
public func >>> <A,B> (left: Signal<A>,
right: A->Result) -> Signal {
return left.bind(right)
}
public func >>> <A,B>(left: Signal<A>,
right: (A, (Result->Void))->Void) -> Signal{
return left.bind(right)
}
public func >>> <A,B> (left: Signal<A>, right: A->B) ->
Signal {
return left.map(right)
}

ls >>> grep("*.jpg") >>> sort

public final class Thread {
public static func main<T>(a: T, completion:
T->Void) {
dispatch_async(dispatch_get_main_queue())
{
completion(a)
}
}
public static func background<T>(queue:
dispatch_queue_t)(_ a: T, _ completion: T->Void)
{
dispatch_async(queue){
completion(a)
}
}
}
ls >>> Thread.background(queue) >>> grep("*.jpg")
>>> sort >>> Thread.main

Extending UIKit to
support Signals.

var SignalHandle: UInt8 = 0
extension UITextField {
public var textSignal: Signal<String> {
let signal: Signal<String>
if let handle = objc_getAssociatedObject(self,
&SignalHandle) as? Signal<String> {
signal = handle
} else {
signal = Signal("")
NSNotificationCenter.defaultCenter().addObserver(self,
selector: Selector("textChanged:"), name:
UITextFieldTextDidChangeNotification, object: self)
objc_setAssociatedObject(self, &SignalHandle,
signal,
objc_AssociationPolicy(OBJC_ASSOCIATION_RETAIN_NONATOMIC))
}
return signal
}
public func textChanged(notification: NSNotification) {
textSignal.update(.Success(Box(self.text)))
}
}

If it’s variable, it qualiﬁes
as a Signal.

ReactiveKitten
It’s about gifs. And cats.
And gifs of cats.

userTyping >>> createURL >>> loadRequest
>>> parseData >>> mainThread >>>
displayCats
The transform, the cat
and you.

import Interstellar
private func request(path: String, completion: Result<NSData>->Void)
{
let url = NSURL(string: baseURL.stringByAppendingString(path))!
let request = NSURLRequest(URL: url)
session.dataTaskWithRequest(request){ data, response, error in
if error != nil {
completion(.Error(error))
} else if let response = response as? NSHTTPURLResponse {
if response.statusCode >= 200 && response.statusCode<300 {
completion(.Success(Box(data)))
} else {
completion(.Error(NSError(domain: "Networking", code:
response.statusCode, userInfo: nil)))
}
} else {
completion(.Error(NSError(domain: "Networking", code: 500,
userInfo: nil)))
}
}.resume()
}

private func parseJSON(data: NSData)
->Result<[String: AnyObject]> {
var error: NSError?
if let json =
NSJSONSerialization.JSONObjectWithData(data, options:
nil, error: &error) as? [String: AnyObject] {
return .Success(Box(json))
} else {
return .Error(error!)
}
}

let imageSignal = gifSignal >>> getURL >>>
Thread.background >>> loadFromCache >>>
retryFromNetwork >>> Thread.main

class ViewController: UIViewController {
var signal: Signal<[Gif]>!
let searchBar = UISearchBar()
override func viewDidLoad() {
super.viewDidLoad()
navigationItem.titleView = searchBar
signal = searchBar.textSignal >>>
Network().search() >>> Thread.main

github.com/jensravens/reactivekitten

Functors, Applicatives
and Monads in Pictures.
http://adit.io/posts/2013-04-17-
functors,_applicatives,_and_monads_in_pictures.html

The Introduction to RP
you’ve been missing.
https://gist.github.com/staltz/868e7e9bc2a7b8c1f754

Interstellar
available on Carthage
jensravens/interstellar

Thank you.
@JensRavens
github.com/jensravens/interstellar

Functional Reactive Programming without Black Magic (UIKonf 2015)

Recommended

More Related Content

What's hot (20)

Similar to Functional Reactive Programming without Black Magic (UIKonf 2015) (20)

More from Jens Ravens (7)

Recently uploaded (20)

Functional Reactive Programming without Black Magic (UIKonf 2015)