GitHubQL.swift

// Protocols

protocol Serializable {
    func serialized() -> String
}

protocol Field: Serializable {
    // ...
}

protocol Argument: Serializable {
    // ...
}

protocol Object: Serializable {
    associatedtype FieldType: Field
    
    var fields: [FieldType] { get }
    var arguments: [Argument]? { get }
}

extension Object {
    func serialized() -> String {
        return "{ \(fields.map({ $0.serialized() }).joined(separator: " ")) }"
    }
}

// Concrete Types

enum Schema: Serializable {
    case query(Query)
    case mutation(Mutation)
    
    func serialized() -> String {
        switch self {
        case .query(let query):
            return "query \(query.serialized())"
        case .mutation(let mutation):
            return "mutation \(mutation.serialized())"
        }
    }
}

enum Query: Serializable {
    case viewer(User)
    
    func serialized() -> String {
        switch self {
        case .viewer(let user):
            return "{ viewer \(user.serialized())}"
        }
    }
}

struct Mutation: Serializable {
    func serialized() -> String {
        return ""
    }
}

enum UserField: Field {
    case id
    case login
    case name
    
    func serialized() -> String {
        switch self {
        case .id:
            return "id"
        case .login:
            return "login"
        case .name:
            return "name"
        }
    }
}

struct User: Object {
    var fields: [UserField]
    var arguments: [Argument]?
}

let user = User(fields: [.id, .login, .name], arguments: nil)
let schema = Schema.query(.viewer(user))
schema.serialized() // output: "query { viewer { id login name }}"