Swift 3 generics / associated types limitations with protocols

Question

I have a generic protocol:

protocol SectionType {
    associatedtype I: Equatable
    associatedtype T: Equatable
    var info: I? { get }
    var items: [T] { get }
}

and an Array extension for it:

/// Offers additional method(s) to process SectionType list.
extension Array where Element: SectionType {
    /// Dummy comparision method. Implementation is not that relevant but just to employ Equatable.
    /// - Parameter to: Another array of sections.
    /// - Returns: True if first info and first elemements in both arrays exist and both are equal.
    func dummyCompare(with otherArray: [Element]) -> Bool {
        guard
            let first = self.first,
            let firstOther = otherArray.first,
            let firstElement = first.items.first,
            let firstOtherElement = firstOther.items.first,
            let firstInfo = first.info, let firstOtherInfo = firstOther.info
            else { return false }
        return firstInfo == firstOtherInfo && firstElement == firstOtherElement
    }
}

No problem when using with concrete implementations:

Example 1: Works with specific implementation with build-in String type:

Declaration:

struct StringSection: SectionType {
    let info: String?
    let items: [String]
}

Usage:

let stringSection1 = StringSection(info: "Info 1", items: ["Item 1", "Item 2"])
let stringSection2 = StringSection(info: "Info 2", items: ["Item 3", "Item 4"])
let stringSections1 = [stringSection1, stringSection2]
let stringSections2 = [stringSection2, stringSection1]
var areEqual = stringSections1.dummyCompare(with: stringSections2)
print("String section 1 equals 2?: \(areEqual)")

Example 2 - Works with specific implementation with custom types:

Declarations:

protocol SectionInfoType {
    var title: String { get }
}

/// BTW: This is just Swift's stragne way of implementing Equatable protocol for your type:
func == (lhs: SectionInfoType, rhs: SectionInfoType) -> Bool {
    return lhs.title == rhs.title
}

struct SpecificSectionInfo: SectionInfoType, Equatable {
    let title: String
    static func == (lhs: SpecificSectionInfo, rhs: SpecificSectionInfo) -> Bool {
        return lhs.title == rhs.title
    }
}

protocol SectionItemType {
    var text: String { get }
}

/// BTW: This is just Swift's stragne way of implementing Equatable protocol for your type:
func == (lhs: SectionItemType, rhs: SectionItemType) -> Bool {
    return lhs.text == rhs.text
}

struct SpecificSectionItem: SectionItemType, Equatable {
    let text: String
    static func == (lhs: SpecificSectionItem, rhs: SpecificSectionItem) -> Bool {
        return lhs.text == rhs.text
    }
}

struct SpecificSection: SectionType {
    let info: SpecificSectionInfo?
    let items: [SpecificSectionItem]
}

Usage:

let specInfo1 = SpecificSectionInfo(title: "Info 1")
let specItem1 = SpecificSectionItem(text: "Specific item 1")
let specItem2 = SpecificSectionItem(text: "Specific item 2")
let specInfo2 = SpecificSectionInfo(title: "Info 2")
let specItem3 = SpecificSectionItem(text: "Specific item 3")
let specItem4 = SpecificSectionItem(text: "Specific item 4")
let specSection1 = SpecificSection(info: specInfo1, items: [specItem1, specItem2])
let specSection2 = SpecificSection(info: specInfo2, items: [specItem3, specItem4])
let specSections1 = [specSection1, specSection2]
let specSections2 = [specSection2, specSection1]
let areEqual = specSections1.dummyCompare(with: specSections2)
print("Specific section 1 equals 2?: \(areEqual)")

So far, so good, everything works and compiles. But ... I have at least 2 problems with this approach:

Problem 1:

Just from 2 examples above, one can see that this approach needs 'specific' implementation of a SectionType protocol for every combination of info and items type. This seems not so efficient (tremendous amount of code for every each implementation) nor generic.

What I need is a more generalised embodiment of SectionType protocol where types for info and items need to be protocols (provided from external APIs as protocols).

A perfect example (but does not compile):

struct Section: SectionType {
    typealias I = SectionInfoType
    typealias T = SectionItemType
    let info: I?
    let items: [T]
}

Problem 2:

I need to be able to pass it over to other protocol oriented API, f.ex.: as an argument to function like:

func consume(section: SectionType<SectionInfoType, SectionItemType>) {}

But above function declaration produces: Cannot specialize non-generic type 'SectionType' with Xcode proposing a fix: 'Delete <SectionInfoType, SectionItemType>' resulting in this:

func consume(section: SectionType) {}

This does not compile neither and I can not find a way to make it work.

Is it possible to do or is that Swift 3 limitation (I am using Swift 3.1 )?

I created Git repository demonstrating those issues. Feel free to collaborate:

https://github.com/lukaszmargielewski/swift3-learning-generics

Answer 1

If I understand correctly problem 2 you want your a function that consumes a generic variable that is also specialised, adhering to protocol SectionType . Try:

func consume<Section: SectionType>(section: Section) {
    let foo = section.info
    let foobar = section.items
}