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)

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