DynamicKePathLiteralFunctionExpressions.swift

/// - seealso: https://www.hackingwithswift.com/articles/212/whats-new-in-swift-5-2

import Foundation

protocol OptionalType {
    associatedtype Wrapped
    var value: Wrapped? { get }
}

extension Optional: OptionalType {
    var value: Wrapped? { self }
}

@dynamicMemberLookup
struct Map<Collection: Swift.Collection> {
    let collection: Collection
    subscript<Value>(dynamicMember keyPath: KeyPath<Collection.Element, Value>) -> [Value] {
        collection.map { $0[keyPath: keyPath] }
    }
}

@dynamicMemberLookup
struct Filter<Collection: Swift.Collection> {
    let collection: Collection
    subscript(dynamicMember keyPath: KeyPath<Collection.Element, Bool>) -> [Collection.Element] {
        collection.filter { $0[keyPath: keyPath] }
    }
}

@dynamicMemberLookup
struct CompactMap<Collection: Swift.Collection> {
    let collection: Collection
    subscript<Value>(dynamicMember keyPath: KeyPath<Collection.Element, Value>) -> [Value] {
        collection.compactMap { $0[keyPath: keyPath] }
    }

    subscript<Value: OptionalType>(dynamicMember keyPath: KeyPath<Collection.Element, Value>) -> [Value.Wrapped] {
        collection.compactMap { $0[keyPath: keyPath].value }
    }
}

protocol DynamicKePathLiteralFunctionExpressions: Collection {
    var map: Map<Self> { get }
    var filter: Filter<Self> { get }
    var compactMap: CompactMap<Self> { get }
}

extension DynamicKePathLiteralFunctionExpressions {
    var map: Map<Self> { .init(collection: self) }
    var filter: Filter<Self> { .init(collection: self) }
    var compactMap: CompactMap<Self> { .init(collection: self) }
}

extension Array: DynamicKePathLiteralFunctionExpressions {}

struct User {
    let name: String
    let age: Int
    let bestFriend: String?

    var canVote: Bool {
        age >= 18
    }
}

let eric = User(name: "Eric Effiong", age: 18, bestFriend: "Otis Milburn")
let maeve = User(name: "Maeve Wiley", age: 19, bestFriend: nil)
let otis = User(name: "Otis Milburn", age: 17, bestFriend: "Eric Effiong")
let users = [eric, maeve, otis]

let userNames: [String] = users.map.name
print(userNames)

let voters: [User] = users.filter.canVote
print(voters)

let bestFriends: [String] = users.compactMap.bestFriend
print(bestFriends)