ProtocolsPlayground.swift

// Given that a protocol unifies two types: BlogPost and PhotoPost...
protocol PostType {
    var title: String { get }
}

struct BlogPost: PostType {
    let title = "My Hot New Blog Post"
}

struct PhotoPost: PostType {
    let title = "Photos from Vacation in Cleveland with Lebron"
}

// And that we have an array of items of such protocol...
let posts: [PostType] = [BlogPost(), PhotoPost()]

// And thus we construct a protocol to communicate view models of these types...
protocol PostTypeViewModel {
    typealias Post = PostType
    var post: Post { get }
}

struct BlogPostViewModel: PostTypeViewModel {
    let post: BlogPost
}

struct PhotoPostViewModel: PostTypeViewModel {
    let post: PhotoPost
}

// ...how do we generically go from PostType -> PostTypeViewModel while getting out the correct concrete types of BlogPostViewModel and PhotoPostViewModel?
let postViewModels: [PostTypeViewModel] = posts.map( *** What do I do here? *** )

// ...so that we could theoretically get an array of post titles out?
postViewModels.map { $0.post.title }