Generalize APIs with parameter packs

1:13 - radians function

func radians(from degrees: Double) -> Double

1:26 - Array type

struct Array<Element>

1:48 - radians function and Array type with concrete expressions

func radians(from degrees: Double) -> Double
radians(from: 180)

struct Array<Element>
Array<Int>

2:04 - generic query

func query<Payload>(_ item: Request<Payload>) -> Payload

2:22 - variadic query

func query(_ item: Request...)

3:16 - generic query

func query<Payload>(_ item: Request<Payload>) -> Payload

3:23 - two query overloads

func query<Payload>(
    _ item: Request<Payload>
) -> Payload

func query<Payload1, Payload2>(
    _ item1: Request<Payload1>,
    _ item2: Request<Payload2>
) -> (Payload1, Payload2)

3:28 - three query overloads

func query<Payload>(
    _ item: Request<Payload>
) -> Payload

func query<Payload1, Payload2>(
    _ item1: Request<Payload1>,
    _ item2: Request<Payload2>
) -> (Payload1, Payload2)

func query<Payload1, Payload2, Payload3>(
    _ item1: Request<Payload1>,
    _ item2: Request<Payload2>,
    _ item3: Request<Payload3>
) -> (Payload1, Payload2, Payload3)

3:31 - four query overloads with extra argument error

func query<Payload>(
    _ item: Request<Payload>
) -> Payload

func query<Payload1, Payload2>(
    _ item1: Request<Payload1>,
    _ item2: Request<Payload2>
) -> (Payload1, Payload2)

func query<Payload1, Payload2, Payload3>(
    _ item1: Request<Payload1>,
    _ item2: Request<Payload2>,
    _ item3: Request<Payload3>
) -> (Payload1, Payload2, Payload3)

func query<Payload1, Payload2, Payload3, Payload4>(
    _ item1: Request<Payload1>,
    _ item2: Request<Payload2>,
    _ item3: Request<Payload3>,
    _ item4: Request<Payload4>
) -> (Payload1, Payload2, Payload3, Payload4)

let _ = query(r1, r2, r3, r4, r5)

5:52 - for-in loop over requests

for request in requests {
    evaluate(request)
}

8:41 - four query overloads

func query<Payload>(
    _ item: Request<Payload>
) -> Payload

func query<Payload1, Payload2>(
    _ item1: Request<Payload1>,
    _ item2: Request<Payload2>
) -> (Payload1, Payload2)

func query<Payload1, Payload2, Payload3>(
    _ item1: Request<Payload1>,
    _ item2: Request<Payload2>,
    _ item3: Request<Payload3>
) -> (Payload1, Payload2, Payload3)

func query<Payload1, Payload2, Payload3, Payload4>(
    _ item1: Request<Payload1>,
    _ item2: Request<Payload2>,
    _ item3: Request<Payload3>,
    _ item4: Request<Payload4>
) -> (Payload1, Payload2, Payload3, Payload4)

9:37 - parameter pack query interface

func query<each Payload>(_ item: repeat Request<each Payload>) -> (repeat each Payload)

10:01 - parameter pack query with single argument call

func query<each Payload>(_ item: repeat Request<each Payload>) -> (repeat each Payload)

let result = query(Request<Int>())

10:08 - parameter pack query with single and triple argument calls

func query<each Payload>(_ item: repeat Request<each Payload>) -> (repeat each Payload)

let result = query(Request<Int>())

let results = query(Request<Int>(), Request<String>(), Request<Bool>())

10:15 - parameter pack query with triple argument call

func query<each Payload>(_ item: repeat Request<each Payload>) -> (repeat each Payload)

let results = query(Request<Int>(), Request<String>(), Request<Bool>())

10:56 - parameter pack query interface

func query<each Payload>(
    _ item: repeat Request<each Payload>
) -> (repeat each Payload)

11:03 - parameter pack query interface with conformance

func query<each Payload: Equatable>(
  _ item: repeat Request<each Payload>
) -> (repeat each Payload)

11:17 - parameter pack query interface with where clause

func query<each Payload>(
    _ item: repeat Request<each Payload>
) -> (repeat each Payload)
    where repeat each Payload: Equatable

11:44 - parameter pack query interface with minimum parameter count

func query<FirstPayload, each Payload>(
    _ first: Request<FirstPayload>, _ item: repeat Request<each Payload>
) -> (FirstPayload, repeat each Payload) 
    where FirstPayload: Equatable, repeat each Payload: Equatable

13:42 - parameter pack query implementation

struct Request<Payload> {
    func evaluate() -> Payload
}

func query<each Payload>(_ item: repeat Request<each Payload>) -> (repeat each Payload) {
    return (repeat (each item).evaluate())
}

16:04 - parameter pack query implementation with different input and output types

protocol RequestProtocol {
    associatedtype Input
    associatedtype Output
    func evaluate(_ input: Input) -> Output
}

struct Evaluator<each Request: RequestProtocol> {
    var item: (repeat each Request)

    func query(_ input: repeat (each Request).Input) -> (repeat (each Request).Output) {
        return (repeat (each item).evaluate(each input))
    }
}

17:05 - parameter pack query implementation with control flow break

protocol RequestProtocol {
    associatedtype Input
    associatedtype Output
    func evaluate(_ input: Input) throws -> Output
}

struct Evaluator<each Request: RequestProtocol> {
    var item: (repeat each Request)

    func query(_ input: repeat (each Request).Input) -> (repeat (each Request).Output)? {
        do {
            return (repeat try (each item).evaluate(each input))
        } catch {
            return nil
        }
    }
}