This talk will explore inference from the perspective of protocols and generics and is based off a series of blog posts I've written(foxinswift.com) on the topic. In the first part of my talk casting number types through inference. I'll then show you struct serialization example demonstrating inferring a type through a mapping function. My last example will take you through inferring an associatedtype on a barebones promise implementation and we'll use it to in the context of making a network request. To finish things off I'll briefly speak on what's new in swift generics and some limitations of those features.

1. <Generics:
Inference>

2. <Generics: Inference>
→ Rich Fox @RGfox
→ I work at Propeller
→ I am a contributer to

3. Generics and the Art of Inferences
→ What are generics
→ What is inference
→ Goal of Talk

4. Ex 1. Casting Number Types

5. protocol NumberConvertible {
init(_ value: Int)
init(_ value: Float)
init(_ value: Double)
}
extension Double: NumberConvertible {}
extension Float: NumberConvertible {}
extension Int: NumberConvertible {}
Int,Float,Double: NumberConvertible

6. extension NumberConvertible {
func convert<T: NumberConvertible>() -> T {
switch self {
case let x as Float:
return T(x)
case let x as Int:
return T(x)
case let x as Double:
return T(x)
default:
fatalError("NumberConvertible convert failed!")
}
}
}

7. Cast by Inference!
let number = 5.5
let a: Float = number.convert()
let b: Int = number.convert()
let c: Double = number.convert()
let aa = number.convert() + Float(2)
let bb = number.convert() + Int(2)
let cc = number.convert() + Double(2)

8. Ex 2. Encoding/Decoding Structs
(Map and Inferences)

9. protocol Serializable {
init(construct: [String: Any])
func destruct() -> [String: Any]
}
extension Optional {
func unbox<T>() -> T? {
return self as? T
}
func unboxArray<T>() -> [T] {
return unbox() ?? []
}
}

10. struct SortItem {
let name: String
let subSorts: [SortItem]
}
extension SortItem: Serializable {
func destruct() -> [String: Any] {
var construct = [String: Any]()
construct["name"] = name
construct["subSorts"] = subSorts.map { $0.destruct() }
return construct
}
init(construct: [String: Any]) {
name = construct["name"].unbox() ?? ""
let sorts = construct["subSorts"]
.unboxArray()
!.map(SortItem.init)
}
}

11. struct SortItem {
let name: String
let subSorts: [SortItem]
}
extension SortItem: Serializable {
func destruct() -> [String: Any] {
var construct = [String: Any]()
construct["name"] = name
construct["subSorts"] = subSorts.map { $0.destruct() }
return construct
}
init(construct: [String: Any]) {
name = construct["name"].unbox() ?? ""
let sorts = construct["subSorts"]
.unboxArray()
!.map(SortItem.init(construct:))
}
}

12. struct SortItem {
let name: String
let subSorts: [SortItem]
//Default Value = Incognito
init(subSorts: [SortItem] = [], name: String) {
self.subSorts = subSorts
self.name = name
}
}
extension SortItem: Serializable {
func destruct() -> [String: Any] {
var construct = [String: Any]()
construct["name"] = name
construct["subSorts"] = subSorts.map { $0.destruct() }
return construct
}
init(construct: [String: Any]) {
name = construct["name"].unbox() ?? ""
let sorts = construct["subSorts"]
.unboxArray()
!.map(SortItem.init)
}
}

13. struct SortItem {
let name: String
let subSorts: [SortItem]
//Default Value = Incognito
init(subSorts: [SortItem] = [], name: String) {
self.subSorts = subSorts
self.name = name
}
}
extension SortItem: Serializable {
func destruct() -> [String: Any] {
var construct = [String: Any]()
construct["name"] = name
construct["subSorts"] = subSorts.map { $0.destruct() }
return construct
}
init(construct: [String: Any]) {
name = construct["name"].unbox() ?? ""
let sorts = construct["subSorts"]
.unboxArray()
.map(SortItem.init)
subSorts = sorts
}
}

14. Return type of unboxArray() inferred
through .map!
let sorts = construct["subSorts"]
.unboxArray()
.map(SortItem.init(construct:))

15. Ex 3: Promises/Networking

16. Result/Promise
→ Concise implementation
→ Result Enum
→ Promise Class w/AssociateType

17. enum Result<T> {
case error(BasicError),
some(T)
private func fire(target: Promise<T>) {
switch self {
case .error(let err):
target.failedAction?(err)
case .some(let val):
target.completeAction?(val)
}
}
}

18. final class Promise<T> {
private var completeAction: (T -> Void)?
private var failedAction: (BasicError -> Void)?
func complete(action: T! -> Void) -> Promise<T> {
completeAction = action
return self
}
func failure(action: BasicError -> Void) -> Promise<T> {
failedAction = action
return self
}
var trigger: Result<T>? {
didSet { trigger?.fire(self) }
}
}
Closure keeps Promise alive - waiting for trigger

19. Usage Example:
(Networking + Generics * Inference)

20. Promise on the Network:
func getUser(url: String) -> Promise<User> {
}

21. Promise on the Network:
func getUser(url: String) -> Promise<User> {
let promise = Promise<User>()
return promise
}

22. Promise on the Network:
func getUser(url: String) -> Promise<User> {
let promise = Promise<User>()
//Async call keeping reference to ˆpromiseˆ
makeGetRequest(url) { response in
}
return promise
}

23. Promise on the Network:
func getUser(url: String) -> Promise<User> {
let promise = Promise<User>()
//Async call keeping reference to ˆpromiseˆ
makeGetRequest(url) { response in
guard let json = response else {
Promise.trigger = Result.error(.unknown)
return
}
}
return promise
}

24. Promise on the Network:
func getUser(url: String) -> Promise<User> {
let promise = Promise<User>()
//Async call keeping reference to ˆpromiseˆ
makeGetRequest(url) { response in
guard let json = response else {
Promise.trigger = Result.error(.unknown)
return
}
if let user = try? User(object: json) {
Promise.trigger = Result.some(user)
}
}
return promise
}

25. Promise on the Network:
func getUser(url: String) -> Promise<User> {
let promise = Promise<User>()
//Async call keeping reference to ˆpromiseˆ
makeGetRequest(url) { response in
guard let json = response else {
Promise.trigger = Result.error(.unknown)
return
}
if let user = try? User(object: json) {
Promise.trigger = Result.some(user)
} else {
let error = BasicError(object: json)
Promise.trigger = Result.error(error)
}
}
return promise
}

26. More Generics and Inferred Promises
func getEncodableType<T: JSONDecodable>(url: String) -> Promise<T> {
let promise = Promise<T>()
//Async call keeping reference to ˆpromiseˆ
makeGetRequest(url) { response in
guard let json = response else {
Promise.trigger = Result.error(.unknown)
return
}
if let result = try? T(object: json) {
Promise.trigger = Result.some(result)
} else {
let error = BasicError(object: json)
Promise.trigger = Result.error(error)
}
}
return promise
}

27. More Generics and Inferred Promises
func getEncodableType<T: JSONDecodable>(url: String) -> Promise<T> {
let promise = Promise<T>() //Generic instead of User
//Async call keeping reference to ˆpromiseˆ
makeGetRequest(url) { response in
guard let json = response else {
Promise.trigger = Result.error(.unknown)
return
}
if let result = try? T(object: json) { //JSONDecodable init
Promise.trigger = Result.some(result)
} else {
let error = BasicError(object: json)
Promise.trigger = Result.error(error)
}
}
return promise
}

28. var user:User?
var guest:Guest?
func fetchPeople() {
let printError: BasicError -> Void =
{"error: ($0.message)"}
NetworkService.getEncodableType("/url/User")
.complete { user = $0 }
.failure(printError)
NetworkService.getEncodableType("/url/Guest")
.complete { guest = $0 }
.failure(printError)
}

29. Promise<T> inferred by complete:(T)->Void
NetworkService.getEncodableType("/url/User")
.complete { user = $0 }

30. New in Swift 3 Generics
Generic typealias
typealias StringDictionary<T> = Dictionary<String, T>
typealias BackwardTriple<T1,T2,T3> = (T3, T2, T1)
Limited: Generic closure Crashes Playground
typealias StringDictionaryValue<T> = (Dictionary<String, T>) -> T?
let testValue: StringDictionaryValue = { return $0["test"] }

31. New in Swift 3 Generics
Generic typealias
typealias StringDictionary<T> = Dictionary<String, T>
typealias BackwardTriple<T1,T2,T3> = (T3, T2, T1)
Function Works as Expected
typealias StringDictionary<T> = (Dictionary<String, T>)
func valueForKey<T>(dict:StringDictionary<T>, key: String) -> T? {
return dict[key]
}

32. Inference Concluded

33. 1. Return Type Context
func convert<T: NumberConvertible>() -> T
2. Though map Function
.unboxArray()
.map(SortItem.init(construct:))
3. Through associatedtype Context
func complete(action: T! -> Void) ->
Promise<T>

34. Thank You
Forward Swift
@RGfox