Add parsing invariant structure even though I do not have a solid enough use-case to implement an invariant fully just yet.

.gitignore

@@ -2,3 +2,4 @@
 /.build
 /Packages
 /*.xcodeproj
+/*.xcworkspace

Examples.swift

@@ -1,78 +0,0 @@
-//
-//  Examples.swift
-//  JSONAPI
-//
-//  Created by Mathew Polzin on 11/12/18.
-//
-
-import Foundation
-import JSONAPI
-
-
-/*******
-
-Please enjoy these examples, but allow me the forced casting and the lack of error checking for the sake of brevity.
-
-********/
-
-
-typealias ExampleEntity<Description: EntityDescription> = Entity<Description, Id<String, Description>>
-
-// MARK: - A few resource objects (entities)
-enum PersonDescription: EntityDescription {
-
-	static var type: String { return "people" }
-	
-	struct Attributes: JSONAPI.Attributes {
-		let name: [String]
-		let favoriteColor: String
-	}
-	
-	struct Relationships: JSONAPI.Relationships {
-		let friends: ToManyRelationship<Person>
-		let dogs: ToManyRelationship<Dog>
-		let home: ToOneRelationship<House>
-	}
-}
-
-typealias Person = ExampleEntity<PersonDescription>
-
-enum DogDescription: EntityDescription {
-
-	static var type: String { return "dogs" }
-
-	struct Attributes: JSONAPI.Attributes {
-		let name: String
-	}
-
-	struct Relationships: JSONAPI.Relationships {
-		let owner: ToOneRelationship<Person?>
-	}
-}
-
-typealias Dog = ExampleEntity<DogDescription>
-
-enum HouseDescription: EntityDescription {
-
-	static var type: String { return "houses" }
-
-	typealias Attributes = NoAttributes
-	typealias Relationships = NoRelatives
-}
-
-typealias House = ExampleEntity<HouseDescription>
-
-// MARK: - Parse a response body with one Dog in it
-typealias SingleDogResponse = JSONAPIDocument<SingleResourceBody<Dog>, NoIncludes, BasicJSONAPIError>
-
-let dummyData = "".data(using: .utf8)!
-let dogResponse = try! JSONDecoder().decode(SingleDogResponse.self, from: dummyData)
-let dog = dogResponse.body.data?.primary.value
-
-// MARK: Parse a response body with multiple people in it and dogs and houses included
-typealias BatchPeopleResponse = JSONAPIDocument<ManyResourceBody<Person>, Include2<Dog, House>, BasicJSONAPIError>
-
-let peopleResponse = try! JSONDecoder().decode(BatchPeopleResponse.self, from: dummyData)
-let people = peopleResponse.body.data?.primary.values
-let dogs = peopleResponse.body.data?.included[Dog.self]
-let houses = peopleResponse.body.data?.included[House.self]

JSONAPI.playground/Pages/Test Library.xcplaygroundpage/Contents.swift

@@ -0,0 +1,24 @@
+//: [Previous](@previous)
+
+import Foundation
+import JSONAPI
+import JSONAPITestLib
+
+/*******
+
+Please enjoy these examples, but allow me the forced casting and the lack of error checking for the sake of brevity.
+
+********/
+
+// MARK: - Literal Expressibility
+// The JSONAPITestLib provides literal expressibility for key types to
+// make creating tests easier
+let dog = Dog(id: "1234", attributes: Dog.Attributes(name: "Buddy"), relationships: Dog.Relationships(owner: nil), meta: .none, links: .none)
+
+// MARK: - JSON API structure checking
+// The JSONAPITestLib provides a `check` function for each Entity type
+// that uses reflection to catch mistakes that are not forbidden by
+// Swift's type system but will result in unexpected results when
+// encoding/decoding. It is a good idea to add a `check` to each of
+// your unit tests that create Entities.
+try Dog.check(dog)

JSONAPI.playground/Pages/Usage.xcplaygroundpage/Contents.swift

@@ -0,0 +1,63 @@
+
+import Foundation
+import JSONAPI
+
+/*******
+
+Please enjoy these examples, but allow me the forced casting and the lack of error checking for the sake of brevity.
+
+********/
+
+// MARK: - Create a request or response body with one Dog in it
+let dogFromCode = try! Dog(name: "Buddy", owner: nil)
+
+typealias SingleDogDocument = JSONAPI.Document<SingleResourceBody<Dog>, NoMetadata, NoLinks, NoIncludes, NoAPIDescription, UnknownJSONAPIError>
+
+let singleDogDocument = SingleDogDocument(apiDescription: .none, body: .init(entity: dogFromCode), includes: .none, meta: .none, links: .none)
+
+let singleDogData = try! JSONEncoder().encode(singleDogDocument)
+
+// MARK: - Parse a request or response body with one Dog in it
+let dogResponse = try! JSONDecoder().decode(SingleDogDocument.self, from: singleDogData)
+let dogFromData = dogResponse.body.primaryData?.value
+
+// MARK: - Create a request or response with multiple people and dogs and houses included
+let personIds = [Person.Identifier(), Person.Identifier()]
+let dogs = try! [Dog(name: "Buddy", owner: personIds[0]), Dog(name: "Joy", owner: personIds[0]), Dog(name: "Travis", owner: personIds[1])]
+let houses = [House(attributes: .none, relationships: .none, meta: .none, links: .none), House(attributes: .none, relationships: .none, meta: .none, links: .none)]
+let people = try! [Person(id: personIds[0], name: ["Gary", "Doe"], favoriteColor: "Orange-Red", friends: [], dogs: [dogs[0], dogs[1]], home: houses[0]), Person(id: personIds[1], name: ["Elise", "Joy"], favoriteColor: "Red", friends: [], dogs: [dogs[2]], home: houses[1])]
+
+typealias BatchPeopleDocument = JSONAPI.Document<ManyResourceBody<Person>, NoMetadata, NoLinks, Include2<Dog, House>, NoAPIDescription, UnknownJSONAPIError>
+
+let includes = dogs.map { BatchPeopleDocument.Include($0) } + houses.map { BatchPeopleDocument.Include($0) }
+let batchPeopleDocument = BatchPeopleDocument(apiDescription: .none, body: .init(entities: people), includes: .init(values: includes), meta: .none, links: .none)
+let batchPeopleData = try! JSONEncoder().encode(batchPeopleDocument)
+
+// MARK: - Parse a request or response body with multiple people in it and dogs and houses included
+
+let peopleResponse = try! JSONDecoder().decode(BatchPeopleDocument.self, from: batchPeopleData)
+let peopleFromData = peopleResponse.body.primaryData?.values
+let dogsFromData = peopleResponse.body.includes?[Dog.self]
+let housesFromData = peopleResponse.body.includes?[House.self]
+
+print("-----")
+print(peopleResponse)
+print("-----")
+
+// MARK: - Pass successfully parsed body to other parts of the code
+
+if case let .data(bodyData) = peopleResponse.body {
+	print("first person's name: \(bodyData.primary.values[0][\.fullName])")
+} else {
+	print("no body data")
+}
+
+// MARK: - Work in the abstract
+
+func process<T: JSONAPIDocument>(document: T) {
+	guard case let .data(body) = document.body else {
+		return
+	}
+	let x: T.Body.Data = body
+}
+process(document: peopleResponse)

JSONAPI.playground/Sources/Entities.swift

@@ -0,0 +1,114 @@
+//
+//  Examples.swift
+//  JSONAPI
+//
+//  Created by Mathew Polzin on 11/12/18.
+//
+
+import Foundation
+import JSONAPI
+
+/*******
+
+Please enjoy these examples, but allow me the forced casting and the lack of error checking for the sake of brevity.
+
+********/
+
+// MARK: - String as CreatableRawIdType
+var GlobalStringId: Int = 0
+extension String: CreatableRawIdType {
+	public static func unique() -> String {
+		GlobalStringId += 1
+		return String(GlobalStringId)
+	}
+}
+
+// MARK: - typealiases for convenience
+public typealias ExampleEntity<Description: EntityDescription> = Entity<Description, NoMetadata, NoLinks, String>
+public typealias ToOne<E: Identifiable> = ToOneRelationship<E, NoMetadata, NoLinks>
+public typealias ToMany<E: Relatable> = ToManyRelationship<E, NoMetadata, NoLinks>
+
+// MARK: - A few resource objects (entities)
+public enum PersonDescription: EntityDescription {
+
+	public static var type: String { return "people" }
+	
+	public struct Attributes: JSONAPI.Attributes {
+		public let name: Attribute<[String]>
+		public let favoriteColor: Attribute<String>
+
+		public var fullName: Attribute<String> {
+			return name.map { $0.joined(separator: " ") }
+		}
+
+		public init(name: Attribute<[String]>, favoriteColor: Attribute<String>) {
+			self.name = name
+			self.favoriteColor = favoriteColor
+		}
+	}
+	
+	public struct Relationships: JSONAPI.Relationships {
+		public let friends: ToMany<Person>
+		public let dogs: ToMany<Dog>
+		public let home: ToOne<House>
+
+		public init(friends: ToMany<Person>, dogs: ToMany<Dog>, home: ToOne<House>) {
+			self.friends = friends
+			self.dogs = dogs
+			self.home = home
+		}
+	}
+}
+
+public typealias Person = ExampleEntity<PersonDescription>
+
+public extension Entity where Description == PersonDescription, MetaType == NoMetadata, LinksType == NoLinks, EntityRawIdType == String {
+	public init(id: Person.Id? = nil,name: [String], favoriteColor: String, friends: [Person], dogs: [Dog], home: House) throws {
+		self = try Person(id: id ?? Person.Id(), attributes: .init(name: .init(rawValue: name), favoriteColor: .init(rawValue: favoriteColor)), relationships: .init(friends: .init(entities: friends), dogs: .init(entities: dogs), home: .init(entity: home)), meta: .none, links: .none)
+	}
+}
+
+public enum DogDescription: EntityDescription {
+
+	public static var type: String { return "dogs" }
+
+	public struct Attributes: JSONAPI.Attributes {
+		public let name: Attribute<String>
+
+		public init(name: Attribute<String>) {
+			self.name = name
+		}
+	}
+
+	public struct Relationships: JSONAPI.Relationships {
+		public let owner: ToOne<Person?>
+
+		public init(owner: ToOne<Person?>) {
+			self.owner = owner
+		}
+	}
+}
+
+public typealias Dog = ExampleEntity<DogDescription>
+
+public extension Entity where Description == DogDescription, MetaType == NoMetadata, LinksType == NoLinks, EntityRawIdType == String {
+	public init(name: String, owner: Person?) throws {
+		self = try Dog(attributes: .init(name: .init(rawValue: name)), relationships: DogDescription.Relationships(owner: .init(entity: owner)), meta: .none, links: .none)
+	}
+
+	public init(name: String, owner: Person.Id) throws {
+		self = try Dog(attributes: .init(name: .init(rawValue: name)), relationships: .init(owner: .init(id: owner)), meta: .none, links: .none)
+	}
+}
+
+public enum HouseDescription: EntityDescription {
+
+	public static var type: String { return "houses" }
+
+	public typealias Attributes = NoAttributes
+	public typealias Relationships = NoRelationships
+}
+
+public typealias House = ExampleEntity<HouseDescription>
+
+

JSONAPI.playground/contents.xcplayground

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='6.0' target-platform='macos' executeOnSourceChanges='false'>
+    <pages>
+        <page name='Test Library'/>
+        <page name='Usage'/>
+    </pages>
+</playground>

Package.resolved

@@ -1,15 +1,7 @@
 {
   "object": {
     "pins": [
-      {
-        "package": "Result",
-        "repositoryURL": "https://github.com/antitypical/Result.git",
-        "state": {
-          "branch": null,
-          "revision": "8fc088dcf72802801efeecba76ea8fb041fb773d",
-          "version": "4.0.0"
-        }
-      }
+
     ]
   },
   "version": 1

Package.swift

@@ -6,23 +6,25 @@ import PackageDescription
 let package = Package(
     name: "JSONAPI",
     products: [
-        // Products define the executables and libraries produced by a package, and make them visible to other packages.
         .library(
             name: "JSONAPI",
             targets: ["JSONAPI"]),
+		.library(
+			name: "JSONAPITestLib",
+			targets: ["JSONAPITestLib"])
     ],
     dependencies: [
-        .package(url: "https://github.com/antitypical/Result.git", from: "4.0.0")
     ],
     targets: [
-        // Targets are the basic building blocks of a package. A target can define a module or a test suite.
-        // Targets can depend on other targets in this package, and on products in packages which this package depends on.
         .target(
             name: "JSONAPI",
-            dependencies: ["Result"]),
+            dependencies: []),
+		.target(
+			name: "JSONAPITestLib",
+			dependencies: ["JSONAPI"]),
         .testTarget(
             name: "JSONAPITests",
-            dependencies: ["JSONAPI"]),
+            dependencies: ["JSONAPITestLib"])
     ],
   	swiftLanguageVersions: [.v4_2]
 )

README.md

@@ -1,91 +1,88 @@
 # JSONAPI
 [![MIT license](http://img.shields.io/badge/license-MIT-lightgrey.svg)](http://opensource.org/licenses/MIT) [![Swift 4.2](http://img.shields.io/badge/Swift-4.2-blue.svg)](https://swift.org) [![Build Status](https://app.bitrise.io/app/c8295b9589aa401e/status.svg?token=vzcyqWD5bQ4xqQfZsaVzNw&branch=master)](https://app.bitrise.io/app/c8295b9589aa401e)
 
-A Swift package for encoding and decoding to *JSON API* compliant requests and responses.
+A Swift package for encoding to- and decoding from **JSON API** compliant requests and responses.
 
 See the JSON API Spec here: https://jsonapi.org/format/
 
 ## Primary Goals
 
 The primary goals of this framework are:
-1. Allow creation of Swift types that are easy to use in code but also can be encoded to- or decoded from *JSON API* compliant payloads without lots of boilerplate code.
+1. Allow creation of Swift types that are easy to use in code but also can be encoded to- or decoded from **JSON API v1.0 Spec** compliant payloads without lots of boilerplate code.
 2. Leverage `Codable` to avoid additional outside dependencies and get operability with non-JSON encoders/decoders for free.
 3. Do not sacrifice type safety.
 4. Be platform agnostic so that Swift code can be written once and used by both the client and the server.
 
-## Project Status
+### Caveat
+The big caveat is that, although the aim is to support the JSON API spec, this framework ends up being _naturally_ opinionated about certain things that the API Spec does not specify. These caveats are largely a side effect of attempting to write the library in a "Swifty" way.
 
-### Decoding
-#### Document
-- [x] `data`
-- [x] `included`
-- [x] `errors` (untested)
-- [ ] `meta`
-- [ ] `jsonapi`
-- [ ] `links`
+If you find something wrong with this library and it isn't already mentioned under **Project Status**, let me know! I want to keep working towards a library implementation that is useful in any application.
 
-#### Resource Object
-- [x] `id`
-- [x] `type`
-- [x] `attributes`
-- [x] `relationships`
-- [ ] `links`
-- [ ] `meta`
-
-#### Relationship Object
-- [x] `data`
-- [ ] `links`
-- [ ] `meta`
-
-### Encoding
-#### Document
-- [x] `data`
-- [x] `included`
-- [x] `errors` (untested)
-- [ ] `meta`
-- [ ] `jsonapi`
-- [ ] `links`
-
-#### Resource Object
-- [x] `id`
-- [x] `type`
-- [x] `attributes`
-- [x] `relationships`
-- [ ] `links`
-- [ ] `meta`
-
-#### Relationship Object
-- [x] `data`
-- [ ] `links`
-- [ ] `meta`
-
-### EntityDescription Validator (using reflection)
-- [ ] Disallow optional array in `Attribute` and `Relationship` (should be empty array, not `null`).
-- [ ] Only allow `Attribute` and `TransformAttribute` within `Attributes` struct.
-- [ ] Only allow `ToManyRelationship` and `ToOneRelationship` within `Relationships` struct.
-
-### Strict Decoding/Encoding Settings
-- [ ] Error (potentially while still encoding/decoding successfully) if an included entity is not related to a primary entity (Turned off by default).
-
-### Misc
-- [x] Support transforms on `Attributes` values (e.g. to support different representations of `Date`)
-- [x] Support ability to distinguish between `Attributes` fields that are optional (i.e. the key might not be there) and `Attributes` values that are optional (i.e. the key is guaranteed to be there but it might be `null`).
-- [x] Fix `ToOneRelationship` so that it is possible to specify an optional relationship where the value is `null` rather than the key being omitted.
-- [ ] Conform to `CustomStringConvertible`
-- [ ] More tests around failing to decode improperly structured JSON (not bad JSON, but JSON that is not to spec)
-- [ ] Use `KeyPath` to specify `Includes` thus creating type safety around the relationship between a primary resource type and the types of included resources????
-- [x] For `NoIncludes`, do not even loop over the "included" JSON API section if it exists.
-- [ ] Property-based testing (using `SwiftCheck`)
-- [ ] Roll my own `Result` or find an alternative that doesn't use `Foundation`.
-- [ ] Create more descriptive errors that are easier to use for troubleshooting.
-
-## Usage
+## Dev Environment
 ### Prerequisites
 1. Swift 4.2+ and Swift Package Manager
 
-### `EntityDescription`
+### Xcode project
+To create an Xcode project for JSONAPI, run
+`swift package generate-xcodeproj`
 
-An `EntityDescription` is the `JSONAPI` framework's specification for what the JSON API spec calls a *Resource Object*. You might create the following `EntityDescription` to represent a person in a network of friends:
+### Running the Playground
+To run the included Playground files, create an Xcode project using Swift Package Manager, then create an Xcode Workspace in the root of the repository and add both the generated Xcode project and the playground to the Workspace.
+
+Note that Playground support for importing non-system Frameworks is still a bit touchy as of Swift 4.2. Sometimes building, cleaning and building, or commenting out and then uncommenting import statements (especially in the Entities.swift Playground Source file) can get things working for me when I am getting an error about `JSONAPI` not being found.
+
+## Project Status
+
+### Encoding/Decoding
+#### Document
+- [x] `data`
+- [x] `included`
+- [x] `errors`
+- [x] `meta`
+- [x] `jsonapi`
+- [x] `links`
+
+#### Resource Object
+- [x] `id`
+- [x] `type`
+- [x] `attributes`
+- [x] `relationships`
+- [x] `links`
+- [x] `meta`
+
+#### Relationship Object
+- [x] `data`
+- [x] `links`
+- [x] `meta`
+
+#### Links Object
+- [x] `href`
+- [x] `meta`
+
+### Misc
+- [x] Support transforms on `Attributes` values (e.g. to support different representations of `Date`)
+- [x] Support validation on `Attributes`.
+- [ ] Create more descriptive errors that are easier to use for troubleshooting.
+
+### JSONAPITestLib
+#### Entity Validator
+- [x] Disallow optional array in `Attribute` (should be empty array, not `null`).
+- [x] Only allow `TransformedAttribute` and its derivatives as stored properties within `Attributes` struct. Computed properties can still be any type because they do not get encoded or decoded.
+- [x] Only allow `ToManyRelationship` and `ToOneRelationship` within `Relationships` struct.
+
+### Potential Improvements
+- [ ] (Maybe) Use `KeyPath` to specify `Includes` thus creating type safety around the relationship between a primary resource type and the types of included resources.
+- [ ] (Maybe) Replace `SingleResourceBody` and `ManyResourceBody` with support at the `Document` level to just interpret `PrimaryResource`, `PrimaryResource?`, or `[PrimaryResource]` as the same decoding/encoding strategies.
+- [ ] Property-based testing (using `SwiftCheck`).
+- [ ] Error or warning if an included entity is not related to a primary entity or another included entity (Turned off or at least not throwing by default).
+
+## Usage
+
+In this documentation, in order to draw attention to the difference between the `JSONAPI` framework (this Swift library) and the **JSON API Spec** (the specification this library helps you follow), the specification will consistently be referred to below as simply the **SPEC**.
+
+### `JSONAPI.EntityDescription`
+
+An `EntityDescription` is the `JSONAPI` framework's representation of what the **SPEC** calls a *Resource Object*. You might create the following `EntityDescription` to represent a person in a network of friends:
 
 ```
 enum PersonDescription: IdentifiedEntityDescription {
@@ -102,14 +99,14 @@ enum PersonDescription: IdentifiedEntityDescription {
 }
 ```
 
-To enumerate them, the requirements of an `EntityDescription` are
-1. A static `var` "type" that matches the JSON type; The JSON spec requires every *Resource Object* to have a "type".
+The requirements of an `EntityDescription` are:
+1. A static `var` "type" that matches the JSON type; The **SPEC** requires every *Resource Object* to have a "type".
 2. A `struct` of `Attributes` **- OR -** `typealias Attributes = NoAttributes`
-3. A `struct` of `Relationships` **- OR -** `typealias Relationships = NoRelatives`
+3. A `struct` of `Relationships` **- OR -** `typealias Relationships = NoRelationships`
 
 Note that an `enum` type is used here for the `EntityDescription`; it could have been a `struct`, but `EntityDescription`s do not ever need to be created so an `enum` with no `case`s is a nice fit for the job.
 
-This readme doesn't go into detail on the JSON API Spec, but the following JSON API *Resource Object* would be described by the above `PersonDescription`:
+This readme doesn't go into detail on the **SPEC**, but the following *Resource Object* would be described by the above `PersonDescription`:
 
 ```
 {
@@ -139,58 +136,76 @@ This readme doesn't go into detail on the JSON API Spec, but the following JSON
 }
 ```
 
-### `Entity`
+### `JSONAPI.Entity`
 
-Once you have an `EntityDescription`, you _create_, _encode_, and _decode_ `Entity`s that "fit the description". If you have a `CreatableRawIdType` (see the section on `RawIdType`s below) then you can create new `Entity`s that will automatically be given unique Ids, but even without a `CreatableRawIdType` you can encode, decode and work with entities.
+Once you have an `EntityDescription`, you _create_, _encode_, and _decode_ `Entities` that "fit the description". If you have a `CreatableRawIdType` (see the section on `RawIdType`s below) then you can create new `Entities` that will automatically be given unique Ids, but even without a `CreatableRawIdType` you can encode, decode and work with entities.
 
-The `Entity` and `EntityDescription` together embody the rules and properties of a JSON API *Resource Object*.
+The `Entity` and `EntityDescription` together with a `JSONAPI.Meta` type and a `JSONAPI.Links` type embody the rules and properties of a JSON API *Resource Object*.
 
-An `Entity` needs to be specialized on two generic types. The first is the `EntityDescription` described above. The second is the type of Id to use for the `Entity`.
+An `Entity` needs to be specialized on four generic types. The first is the `EntityDescription` described above. The others are a `Meta`, `Links`, and `MaybeRawId`.
+
+#### `Meta`
+
+The second generic specialization on `Entity` is `Meta`. This is described in its own section [below](#jsonapimeta). All `Meta` at any level of a JSON API Document follow the same rules.
+
+#### `Links`
+
+The third generic specialization on `Entity` is `Links`. This is described in its own section [below](#jsonnapilinks). All `Links` at any level of a JSON API Document follow the same rules, although the **SPEC** makes different suggestions as to what types of links might live on which parts of the Document.
 
 #### `IdType`
 
-An `IdType` packages up two pieces of information: A unique identifier of a given `RawIdType` and the `EntityDescription` of the type of entity the Id identifies. Having the `EntityDescription` type associated with the Id makes it easy to store all of your entities in a local hash broken out by `EntityDescription`; You can pass Ids around and always know where to look for the `Entity` to which the Id refers. `RawIdType`s are documented below.
+ The second is the raw type of `Id` to use for the `Entity`. The actual `Id` of the `Entity` will not be a `RawIdType`, though. The `Id` will package a value of `RawIdType` with a specialized reference back to the `Entity` type it identifies. This just looks like `Id<RawIdType, Entity<EntityDescription, RawIdType>>`.
+
+Having the `Entity` type associated with the `Id` makes it easy to store all of your entities in a hash broken out by `Entity` type; You can pass `Ids` around and always know where to look for the `Entity` to which the `Id` refers.
+
+A `RawIdType` is the underlying type that uniquely identifies an `Entity`. This is often a `String` or a `UUID`.
+
+#### `MaybeRawId`
+
+`MaybeRawId` is either a `RawIdType` that can be used to uniquely identify `Entities` or it is `Unidentified` which is used to indicate an `Entity` does not have an `Id` (which is useful when a client is requesting that the server create an `Entity` and assign it a new `Id`).
 
 #### Convenient `typealiases`
 
 Often you can use one `RawIdType` for many if not all of your `Entities`. That means you can save yourself some boilerplate by using `typealias`es like the following:
 ```
-public typealias Entity<Description: JSONAPI.EntityDescription> = JSONAPI.Entity<Description, Id<String, Description>>
+public typealias Entity<Description: JSONAPI.EntityDescription, Meta: JSONAPI.Meta, Links: JSONAPI.Links> = JSONAPI.Entity<Description, Meta, Links, String>
 
-public typealias NewEntity<Description: JSONAPI.EntityDescription> = JSONAPI.Entity<Description, Unidentified>
+public typealias NewEntity<Description: JSONAPI.EntityDescription, Meta: JSONAPI.Meta, Links: JSONAPI.Links> = JSONAPI.Entity<Description, Meta, Links, Unidentified>
 ```
 
 It can also be nice to create a `typealias` for each type of entity you want to work with:
 ```
-typealias Person = Entity<PersonDescription>
+typealias Person = Entity<PersonDescription, NoMetadata, NoLinks>
 
-typealias NewPerson = NewEntity<PersonDescription>
+typealias NewPerson = NewEntity<PersonDescription, NoMetadata, NoLinks>
 ```
 
-Note that I am assuming an unidentified person is a "new" person. I suspect that is generally an acceptable conflation because the only time JSON API spec allows a *Resource Object* to be encoded without an Id is when a client is requesting the given *Resource Object* be created by the server and the client wants the server to create the Id for that object.
+Note that I am assuming an unidentified person is a "new" person. I suspect that is generally an acceptable conflation because the only time the **SPEC** allows a *Resource Object* to be encoded without an `Id` is when a client is requesting the given *Resource Object* be created by the server and the client wants the server to create the `Id` for that object.
 
-### `Relationships`
+### `JSONAPI.Relationships`
 
-There are two types of `Relationship`s: `ToOneRelationship` and `ToManyRelationship`. An `EntityDescription`'s `Relationships` type can contain any number of `Relationship`s of either of these types. Do not store anything other than `Relationship`s in the `Relationships` struct of an `EntityDescription`.
+There are two types of `Relationships`: `ToOneRelationship` and `ToManyRelationship`. An `EntityDescription`'s `Relationships` type can contain any number of `Relationship` properties of either of these types. Do not store anything other than `Relationship` properties in the `Relationships` struct of an `EntityDescription`.
 
-To describe a relationship that may be omitted (i.e. the key is not even present in the JSON object), you make the entire `ToOneRelationship` or `ToManyRelationship` optional. However, this is not recommended because you can also represent optional relationships as nullable which means the key is always present. A `ToManyRelationship` can naturally represent no related objects exist with an empty array, so `ToManyRelationship` does not support nullability at all. A `ToOneRelationship` can be marked as nullable (i.e. the value might be `null` or it might be a resource identifier) like this:
+In addition to identifying entities by Id and type, `Relationships` can contain `Meta` or `Links` that follow the same rules as [`Meta`](#jsonapimeta) and [`Links`](#jsonapilinks) elsewhere in the JSON API Document.
+
+To describe a relationship that may be omitted (i.e. the key is not even present in the JSON object), you make the entire `ToOneRelationship` or `ToManyRelationship` optional. However, this is not recommended because you can also represent optional relationships as nullable which means the key is always present. A `ToManyRelationship` can naturally represent the absence of related values with an empty array, so `ToManyRelationship` does not support nullability at all. A `ToOneRelationship` can be marked as nullable (i.e. the value could be either `null` or a resource identifier) like this:
 ```
-let nullableRelative: ToOneRelationship<Person?>
+let nullableRelative: ToOneRelationship<Person?, NoMetadata, NoLinks>
 ```
 
 An entity that does not have relationships can be described by adding the following to an `EntityDescription`:
 ```
-typealias Relationships = NoRelatives
+typealias Relationships = NoRelationships
 ```
 
-`Relationship`s boil down to Ids of other entities. To access the Id of a related entity, you can use the shorthand `~>` operator with the `KeyPath` of the `Relationship` from which you want the Id. The friends of the above `Person` entity could be accessed as follows (type annotations for clarity):
+`Relationship` values boil down to `Ids` of other entities. To access the `Id` of a related `Entity`, you can use the custom `~>` operator with the `KeyPath` of the `Relationship` from which you want the `Id`. The friends of the above `Person` `Entity` can be accessed as follows (type annotations for clarity):
 ```
 let friendIds: [Person.Identifier] = person ~> \.friends
 ```
 
-### `Attributes`
+### `JSONAPI.Attributes`
 
-The `Attributes` of an `EntityDescription` can contain any JSON encodable/decodable types as long as they are wrapped in an `Attribute` or `TransformAttribute` `struct`. This is the place to store all attributes of an entity.
+The `Attributes` of an `EntityDescription` can contain any JSON encodable/decodable types as long as they are wrapped in an `Attribute`, `ValidatedAttribute`, or `TransformedAttribute` `struct`.
 
 To describe an attribute that may be omitted (i.e. the key might not even be in the JSON object), you make the entire `Attribute` optional:
 ```
@@ -214,52 +229,155 @@ let favoriteColor: String = person[\.favoriteColor]
 
 #### `Transformer`
 
-Sometimes you need to use a type that does not encode or decode itself in the way you need to represent it as a serialized JSON object. For example, the Swift `Foundation` type `Date` can encode/decode itself to `Double` out of the box, but you might want to represent dates as ISO 8601 compliant `String`s instead. To do this, you create a `Transformer`.
+Sometimes you need to use a type that does not encode or decode itself in the way you need to represent it as a serialized JSON object. For example, the Swift `Foundation` type `Date` can encode/decode itself to `Double` out of the box, but you might want to represent dates as ISO 8601 compliant `String`s instead. The Foundation library `JSONDecoder` has a setting to make this adjustment, but for the sake of an example, you could create a `Transformer`.
 
 A `Transformer` just provides one static function that transforms one type to another. You might define one for an ISO 8601 compliant `Date` like this:
 ```
 enum ISODateTransformer: Transformer {
-	public static func transform(_ from: String) throws -> Date {
+	public static func transform(_ value: String) throws -> Date {
 		// parse Date out of input and return
 	}
 }
 ```
 
-Then you define the attribute as a `TransformAttribute` instead of an `Attribute`:
+Then you define the attribute as a `TransformedAttribute` instead of an `Attribute`:
 ```
-let date: TransformAttribute<String, ISODateTransformer>
+let date: TransformedAttribute<String, ISODateTransformer>
 ```
 
 Note that the first generic parameter of `TransformAttribute` is the type you expect to decode from JSON, not the type you want to end up with after transformation.
 
-### `JSONAPIDocument`
+If you make your `Transformer` a `ReversibleTransformer` then your life will be a bit easier when you construct `TransformedAttributes` because you have access to initializers for both the pre- and post-transformed value types. Continuing with the above example of a `ISODateTransformer`:
+```
+extension ISODateTransformer: ReversibleTransformer {
+	public static func reverse(_ value: Date) throws -> String {
+		// serialize Date to a String
+	}
+}
 
-The entirety of a JSON API request or response is encoded or decoded from- or to a `JSONAPIDocument`. As an example, a JSON API response containing one `Person` and no included entities could be decoded as follows:
+let exampleAttribute = try? TransformedAttribute<String, ISODateTransformer>(transformedValue: Date())
+let otherAttribute = try? TransformedAttribute<String, ISODateTransformer>(rawValue: "2018-12-01 09:06:41 +0000")
+```
+
+#### `Validator`
+
+You can also creator `Validators` and `ValidatedAttribute`s. A `Validator` is just a `Transformer` that by convention does not perform a transformation. It simply `throws` if an attribute value is invalid.
+
+#### Computed `Attribute`
+
+You can add computed properties to your `EntityDescription.Attributes` struct if you would like to expose attributes that are not explicitly represented by the JSON. These computed properties should still have the type `Attribute` because that way you can take advantage of the quick access provided by `Entity`'s subscript operator. Here's an example of how you might take the `Person[\.name]` attribute from the example above and create a `fullName` computed property.
+
+```
+public var fullName: Attribute<String> {
+	return name.map { $0.joined(separator: " ") }
+}
+```
+
+### Copying `Entities`
+`Entity` is a value type, so copying is its default behavior. There are two common mutations you might want to make when copying an `Entity`:
+1. Assigning a new `Identifier` to the copy of an identified `Entity`.
+2. Assigning a new `Identifier` to the copy of an unidentified `Entity`.
+
+The above can be accomplished with code like the following:
+
+```
+// use case 1
+let person1 = person.withNewIdentifier()
+
+// use case 2
+let newlyIdentifiedPerson1 = unidentifiedPerson.identified(byType: String.self)
+
+let newlyIdentifiedPerson2 = unidentifiedPerson.identified(by: "2232")
+```
+
+### `JSONAPI.Document`
+
+The entirety of a JSON API request or response is encoded or decoded from- or to a `Document`. As an example, a JSON API response containing one `Person` and no included entities could be decoded as follows:
 ```
 let decoder = JSONDecoder()
 
-let responseStructure = JSONAPIDocument<SingleResourceBody<Person>, NoIncludes, BasicJSONAPIError>.self
+let responseStructure = JSONAPI.Document<SingleResourceBody<Person>, NoMetadata, NoLinks, NoIncludes, UnknownJSONAPIError>.self
 
 let document = try decoder.decode(responseStructure, from: data)
 ```
 
+A JSON API Document is guaranteed by the **SPEC** to be "data", "metadata", or "errors." If it is "data", it may also contain "metadata" and/or other "included" resources. If it is "errors," it may also contain "metadata."
+
 #### `ResourceBody`
 
-The first generic type of a `JSONAPIDocument` is a `ResourceBody`. This can either be a `SingleResourceBody` or a `ManyResourceBody`. You will find zero or one `Entity` values in a JSON API document that has a `SingleResourceBody` and you will find zero or more `Entity` values in a JSON API document that has a `ManyResourceBody`.
+The first generic type of a `JSONAPIDocument` is a `ResourceBody`. This can either be a `SingleResourceBody<PrimaryResource>` or a `ManyResourceBody<PrimaryResource>`. You will find zero or one `PrimaryResource` values in a JSON API document that has a `SingleResourceBody` and you will find zero or more `PrimaryResource` values in a JSON API document that has a `ManyResourceBody`. You can use the `Poly` types (`Poly1` through `Poly6`) to specify that a `ResourceBody` will be one of a few different types of `Entity`. These `Poly` types work in the same way as the `Include` types described below.
 
-#### `IncludeDecoder`
+If you expect a response to not have a "data" top-level key at all, then use `NoResourceBody` instead.
 
-The second generic type of a `JSONAPIDocument` is an `IncludeDecoder`. This type controls which types of `Entity` are looked for when decoding the "included" part of the JSON API document. If you do not expect any included entities to be in the document, `NoIncludes` is the way to go. The `JSONAPI` framework provides `IncludeDecoder`s for up to six types of included entities. These are named `Include1`, `Include2`, `Include3`, and so on.
+##### nullable `PrimaryResource`
+
+If you expect a `SingleResourceBody` to sometimes come back `null`, you should make your `PrimaryResource` optional. If you do not make your `PrimaryResource` optional then a `null` primary resource will be considered an error when parsing the JSON.
+
+You cannot, however, use an optional `PrimaryResource` with a `ManyResourceBody` because the **SPEC** requires that an empty document in that case be represented by an empty array rather than `null`.
+
+#### `MetaType`
+
+The second generic type of a `JSONAPIDocument` is a `Meta`. This `Meta` follows the same rules as `Meta` at any other part of a JSON API Document. It is described below in its own section, but as an example, the JSON API document could contain the following pagination info in its meta entry:
+```
+{
+	"meta": {
+		"total": 100,
+		"limit": 50,
+		"offset": 50
+	}
+}
+```
+
+You would then create the following `Meta` type:
+```
+struct PageMetadata: JSONAPI.Meta {
+	let total: Int
+	let limit: Int
+	let offset: Int
+}
+```
+
+You can always use `NoMetadata` if this JSON API feature is not needed.
+
+#### `LinksType`
+
+The third generic type of a `JSONAPIDocument` is a `Links` struct. `Links` are described in their own section [below](#jsonapilinks).
+
+#### `IncludeType`
+
+The fourth generic type of a `JSONAPIDocument` is an `Include`. This type controls which types of `Entity` are looked for when decoding the "included" part of the JSON API document. If you do not expect any included entities to be in the document, `NoIncludes` is the way to go. The `JSONAPI` framework provides `Include`s for up to six types of included entities. These are named `Include1`, `Include2`, `Include3`, and so on.
 
 **IMPORTANT**: The number trailing "Include" in these type names does not indicate a number of included entities, it indicates a number of _types_ of included entities. `Include1` can be used to decode any number of included entities as long as all the entities are of the same _type_.
 
 To specify that we expect friends of a person to be included in the above example `JSONAPIDocument`, we would use `Include1<Person>` instead of `NoIncludes`.
 
+#### `APIDescriptionType`
+
+The fifth generic type of a `JSONAPIDocument` is an `APIDescription`. The type represents the "JSON:API Object" described by the **SPEC**. This type describes the highest version of the **SPEC** supported and can carry additional metadata to describe the API.
+
+You can specify this is not part of the document by using the `NoAPIDescription` type.
+
+You can describe the API by a version with no metadata by using `APIDescription<NoMetadata>`.
+
+You can supply any `JSONAPI.Meta` type as the metadata type of the API description.
+
 #### `Error`
 
-The final generic type of a `JSONAPIDocument` is the `Error`. You should create an error type that can decode all the errors you expect your `JSONAPIDocument` to be able to decode. As prescribed by the JSON API Spec, these errors will be found in the root document member `errors`.
+The final generic type of a `JSONAPIDocument` is the `Error`. You should create an error type that can decode all the errors you expect your `JSONAPIDocument` to be able to decode. As prescribed by the **SPEC**, these errors will be found in the root document member `errors`.
 
-### `RawIdType`
+### `JSONAPI.Meta`
+
+A `Meta` struct is totally open-ended. It is described by the **SPEC** as a place to put any information that does not fit into the standard JSON API Document structure anywhere else.
+
+You can specify `NoMetadata` if the part of the document being described should not contain any `Meta`.
+
+### `JSONAPI.Links`
+
+A `Links` struct must contain only `Link` properties. Each `Link` property can either be a `URL` or a `URL` and some `Meta`. Each part of the document has some suggested common `Links` to include but generally any link can be included.
+
+You can specify `NoLinks` if the part of the document being described should not contain any `Links`.
+
+### `JSONAPI.RawIdType`
 
 If you want to create new `JSONAPI.Entity` values and assign them Ids then you will need to conform at least one type to `CreatableRawIdType`. Doing so is easy; here are two example conformances for `UUID` and `String` (via `UUID`):
 ```
@@ -275,3 +393,6 @@ extension String: CreatableRawIdType {
 	}
 }
 ```
+
+# JSONAPITestLib
+The `JSONAPI` framework is packaged with a test library to help you test your `JSONAPI` integration. The test library is called `JSONAPITestLib`. It provides literal expressibility for `Attribute`, `ToOneRelationship`, and `Id` in many situations so that you can easily write test `Entity` values into your unit tests. It also provides a `check()` function for each `Entity` type that can be used to catch problems with your `JSONAPI` structures that are not caught by Swift's type system. You can see the `JSONAPITestLib` in action in the Playground included with the `JSONAPI` repository.

Sources/JSONAPI/Document/APIDescription.swift

@@ -0,0 +1,57 @@
+//
+//  APIDescription.swift
+//  JSONAPI
+//
+//  Created by Mathew Polzin on 12/1/18.
+//
+
+/// This is what the JSON API Spec calls the "JSON:API Object"
+public protocol APIDescriptionType: Codable, Equatable {
+	associatedtype Meta
+}
+
+/// This is what the JSON API Spec calls the "JSON:API Object"
+public struct APIDescription<Meta: JSONAPI.Meta>: APIDescriptionType {
+	public let version: String
+	public let meta: Meta
+}
+
+public struct NoAPIDescription: APIDescriptionType, CustomStringConvertible {
+	public typealias Meta = NoMetadata
+
+	public init() {}
+
+	public static var none: NoAPIDescription { return .init() }
+
+	public var description: String { return "No JSON:API Object" }
+}
+
+extension APIDescription {
+	private enum CodingKeys: String, CodingKey {
+		case version
+		case meta
+	}
+
+	public init(from decoder: Decoder) throws {
+		let container = try decoder.container(keyedBy: CodingKeys.self)
+
+		// The spec says that if a version is not specified, it should be assumed to be at least 1.0
+		version = (try? container.decode(String.self, forKey: .version)) ?? "1.0"
+
+		if let metaVal = NoMetadata() as? Meta {
+			meta = metaVal
+		} else {
+			meta = try container.decode(Meta.self, forKey: .meta)
+		}
+	}
+
+	public func encode(to encoder: Encoder) throws {
+		var container = encoder.container(keyedBy: CodingKeys.self)
+
+		try container.encode(version, forKey: .version)
+
+		if Meta.self != NoMetadata.self {
+			try container.encode(meta, forKey: .meta)
+		}
+	}
+}

Sources/JSONAPI/Document/Document.swift

@@ -5,6 +5,19 @@
 //  Created by Mathew Polzin on 11/5/18.
 //
 
+public protocol JSONAPIDocument: Codable, Equatable {
+	associatedtype PrimaryResourceBody: JSONAPI.ResourceBody
+	associatedtype MetaType: JSONAPI.Meta
+	associatedtype LinksType: JSONAPI.Links
+	associatedtype IncludeType: JSONAPI.Include
+	associatedtype APIDescription: APIDescriptionType
+	associatedtype Error: JSONAPIError
+
+	typealias Body = Document<PrimaryResourceBody, MetaType, LinksType, IncludeType, APIDescription, Error>.Body
+
+	var body: Body { get }
+}
+
 /// A JSON API Document represents the entire body
 /// of a JSON API request or the entire body of
 /// a JSON API response.
@@ -12,29 +25,171 @@
 /// API uses snake case, you will want to use
 /// a conversion such as the one offerred by the
 /// Foundation JSONEncoder/Decoder: `KeyDecodingStrategy`
-public struct JSONAPIDocument<ResourceBody: JSONAPI.ResourceBody, Include: IncludeDecoder, Error: JSONAPIError>: Equatable {
+public struct Document<PrimaryResourceBody: JSONAPI.ResourceBody, MetaType: JSONAPI.Meta, LinksType: JSONAPI.Links, IncludeType: JSONAPI.Include, APIDescription: APIDescriptionType, Error: JSONAPIError>: JSONAPIDocument {
+	public typealias Include = IncludeType
+
+	/// The JSON API Spec calls this the JSON:API Object. It contains version
+	/// and metadata information about the API itself.
+	public let apiDescription: APIDescription
+
+	/// The Body of the Document. This body is either one or more errors
+	/// with links and metadata attempted to parse but not guaranteed or
+	/// it is a successful data struct containing all the primary and
+	/// included resources, the metadata, and the links that this
+	/// document type specifies.
 	public let body: Body
-//	public let meta: Meta?
-//	public let jsonApi: APIDescription?
-//	public let links: Links?
 	
 	public enum Body: Equatable {
-		case errors([Error])
-		case data(primary: ResourceBody, included: Includes<Include>)
-		
+		case errors([Error], meta: MetaType?, links: LinksType?)
+		case data(Data)
+
+		public struct Data: Equatable {
+			public let primary: PrimaryResourceBody
+			public let includes: Includes<Include>
+			public let meta: MetaType
+			public let links: LinksType
+
+			public init(primary: PrimaryResourceBody, includes: Includes<Include>, meta: MetaType, links: LinksType) {
+				self.primary = primary
+				self.includes = includes
+				self.meta = meta
+				self.links = links
+			}
+		}
+
 		public var isError: Bool {
 			guard case .errors = self else { return false }
 			return true
 		}
-		
-		public var data: (primary: ResourceBody, included: Includes<Include>)? {
-			guard case let .data(primary: body, included: includes) = self else { return nil }
-			return (primary: body, included: includes)
+
+		public var errors: [Error]? {
+			guard case let .errors(errors, meta: _, links: _) = self else { return nil }
+			return errors
 		}
+
+		public var data: Data? {
+			guard case let .data(data) = self else { return nil }
+			return data
+		}
+		
+		public var primaryResource: PrimaryResourceBody? {
+			guard case let .data(data) = self else { return nil }
+			return data.primary
+		}
+
+		public var includes: Includes<Include>? {
+			guard case let .data(data) = self else { return nil }
+			return data.includes
+		}
+
+		public var meta: MetaType? {
+			switch self {
+			case .data(let data):
+				return data.meta
+			case .errors(_, meta: let metadata?, links: _):
+				return metadata
+			default:
+				return nil
+			}
+		}
+
+		public var links: LinksType? {
+			switch self {
+			case .data(let data):
+				return data.links
+			case .errors(_, meta: _, links: let links?):
+				return links
+			default:
+				return nil
+			}
+		}
+	}
+
+	public init(apiDescription: APIDescription, errors: [Error], meta: MetaType? = nil, links: LinksType? = nil) {
+		body = .errors(errors, meta: meta, links: links)
+		self.apiDescription = apiDescription
+	}
+
+	public init(apiDescription: APIDescription, body: PrimaryResourceBody, includes: Includes<Include>, meta: MetaType, links: LinksType) {
+		self.body = .data(.init(primary: body, includes: includes, meta: meta, links: links))
+		self.apiDescription = apiDescription
+	}
+}
+/*
+extension Document where IncludeType == NoIncludes {
+	public init(apiDescription: APIDescription, body: PrimaryResourceBody, meta: MetaType, links: LinksType) {
+		self.init(apiDescription: apiDescription, body: body, includes: .none, meta: meta, links: links)
 	}
 }
 
-extension JSONAPIDocument: Codable {
+extension Document where MetaType == NoMetadata {
+	public init(apiDescription: APIDescription, body: PrimaryResourceBody, includes: Includes<Include>, links: LinksType) {
+		self.init(apiDescription: apiDescription, body: body, includes: includes, meta: .none, links: links)
+	}
+}
+
+extension Document where LinksType == NoLinks {
+	public init(apiDescription: APIDescription, body: PrimaryResourceBody, includes: Includes<Include>, meta: MetaType) {
+		self.init(apiDescription: apiDescription, body: body, includes: includes, meta: meta, links: .none)
+	}
+}
+
+extension Document where APIDescription == NoAPIDescription {
+	public init(body: PrimaryResourceBody, includes: Includes<Include>, meta: MetaType, links: LinksType) {
+		self.init(apiDescription: .none, body: body, includes: includes, meta: meta, links: links)
+	}
+}
+
+extension Document where IncludeType == NoIncludes, LinksType == NoLinks {
+	public init(apiDescription: APIDescription, body: PrimaryResourceBody, meta: MetaType) {
+		self.init(apiDescription: apiDescription, body: body, meta: meta, links: .none)
+	}
+}
+
+extension Document where IncludeType == NoIncludes, MetaType == NoMetadata {
+	public init(apiDescription: APIDescription, body: PrimaryResourceBody, links: LinksType) {
+		self.init(apiDescription: apiDescription, body: body, meta: .none, links: links)
+	}
+}
+
+extension Document where IncludeType == NoIncludes, APIDescription == NoAPIDescription {
+	public init(body: PrimaryResourceBody, meta: MetaType, links: LinksType) {
+		self.init(apiDescription: .none, body: body, meta: meta, links: links)
+	}
+}
+
+extension Document where MetaType == NoMetadata, LinksType == NoLinks {
+	public init(apiDescription: APIDescription, body: PrimaryResourceBody, includes: Includes<Include>) {
+		self.init(apiDescription: apiDescription, body: body, includes: includes, links: .none)
+	}
+}
+
+extension Document where MetaType == NoMetadata, APIDescription == NoAPIDescription {
+	public init(body: PrimaryResourceBody, includes: Includes<Include>, links: LinksType) {
+		self.init(apiDescription: .none, body: body, includes: includes, links: links)
+	}
+}
+
+extension Document where IncludeType == NoIncludes, MetaType == NoMetadata, LinksType == NoLinks {
+	public init(apiDescription: APIDescription, body: PrimaryResourceBody) {
+		self.init(apiDescription: apiDescription, body: body, includes: .none)
+	}
+}
+
+extension Document where MetaType == NoMetadata, LinksType == NoLinks, APIDescription == NoAPIDescription {
+	public init(body: PrimaryResourceBody, includes: Includes<Include>) {
+		self.init(apiDescription: .none, body: body, includes: includes)
+	}
+}
+
+extension Document where IncludeType == NoIncludes, MetaType == NoMetadata, LinksType == NoLinks, APIDescription == NoAPIDescription {
+	public init(body: PrimaryResourceBody) {
+		self.init(apiDescription: .none, body: body)
+	}
+}
+*/
+
+extension Document {
 	private enum RootCodingKeys: String, CodingKey {
 		case data
 		case errors
@@ -47,38 +202,127 @@ extension JSONAPIDocument: Codable {
 	public init(from decoder: Decoder) throws {
 		let container = try decoder.container(keyedBy: RootCodingKeys.self)
 
+		if let noData = NoAPIDescription() as? APIDescription {
+			apiDescription = noData
+		} else {
+			apiDescription = try container.decode(APIDescription.self, forKey: .jsonapi)
+		}
+
 		let errors = try container.decodeIfPresent([Error].self, forKey: .errors)
 
+		let meta: MetaType?
+		if let noMeta = NoMetadata() as? MetaType {
+			meta = noMeta
+		} else {
+			do {
+				meta = try container.decode(MetaType.self, forKey: .meta)
+			} catch {
+				meta = nil
+			}
+		}
+
+		let links: LinksType?
+		if let noLinks = NoLinks() as? LinksType {
+			links = noLinks
+		} else {
+			do {
+				links = try container.decode(LinksType.self, forKey: .links)
+			} catch {
+				links = nil
+			}
+		}
+
+		// If there are errors, there cannot be a body. Return errors and any metadata found.
 		if let errors = errors {
-			body = .errors(errors)
+			body = .errors(errors, meta: meta, links: links)
 			return
 		}
 
-		let data = try container.decode(ResourceBody.self, forKey: .data)
+		let data: PrimaryResourceBody
+		if let noData = NoResourceBody() as? PrimaryResourceBody {
+			data = noData
+		} else {
+			data = try container.decode(PrimaryResourceBody.self, forKey: .data)
+		}
+
 		let maybeIncludes = try container.decodeIfPresent(Includes<Include>.self, forKey: .included)
 		
 		// TODO come back to this and make robust
+
+		guard let metaVal = meta else {
+			throw JSONAPIEncodingError.missingOrMalformedMetadata
+		}
+		guard let linksVal = links else {
+			throw JSONAPIEncodingError.missingOrMalformedLinks
+		}
 		
-		body = .data(primary: data, included: maybeIncludes ?? Includes<Include>.none)
+		body = .data(.init(primary: data, includes: maybeIncludes ?? Includes<Include>.none, meta: metaVal, links: linksVal))
 	}
 
 	public func encode(to encoder: Encoder) throws {
 		var container = encoder.container(keyedBy: RootCodingKeys.self)
 
 		switch body {
-		case .errors(let errors):
+		case .errors(let errors, meta: let meta, links: let links):
 			var errContainer = container.nestedUnkeyedContainer(forKey: .errors)
 
 			for error in errors {
 				try errContainer.encode(error)
 			}
 
-		case .data(primary: let resourceBody, included: let includes):
-			try container.encode(resourceBody, forKey: .data)
+			if MetaType.self != NoMetadata.self,
+				let metaVal = meta {
+				try container.encode(metaVal, forKey: .meta)
+			}
+
+			if LinksType.self != NoLinks.self,
+				let linksVal = links {
+				try container.encode(linksVal, forKey: .links)
+			}
+
+		case .data(let data):
+			try container.encode(data.primary, forKey: .data)
 
 			if Include.self != NoIncludes.self {
-				try container.encode(includes, forKey: .included)
+				try container.encode(data.includes, forKey: .included)
 			}
+
+			if MetaType.self != NoMetadata.self {
+				try container.encode(data.meta, forKey: .meta)
+			}
+
+			if LinksType.self != NoLinks.self {
+				try container.encode(data.links, forKey: .links)
+			}
+		}
+
+		if APIDescription.self != NoAPIDescription.self {
+			try container.encode(apiDescription, forKey: .jsonapi)
 		}
 	}
 }
+
+// MARK: - CustomStringConvertible
+
+extension Document: CustomStringConvertible {
+	public var description: String {
+		return "Document(\(String(describing: body)))"
+	}
+}
+
+extension Document.Body: CustomStringConvertible {
+	public var description: String {
+		switch self {
+		case .errors(let errors, meta: let meta, links: let links):
+			return "errors: \(String(describing: errors)), meta: \(String(describing: meta)), links: \(String(describing: links))"
+		case .data(let data):
+			return String(describing: data)
+		}
+	}
+}
+
+extension Document.Body.Data: CustomStringConvertible {
+	public var description: String {
+		return "primary: \(String(describing: primary)), includes: \(String(describing: includes)), meta: \(String(describing: meta)), links: \(String(describing: links))"
+	}
+}

Sources/JSONAPI/Document/Error.swift

@@ -9,7 +9,7 @@ public protocol JSONAPIError: Swift.Error, Equatable, Codable {
 	static var unknown: Self { get }
 }
 
-public enum BasicJSONAPIError: JSONAPIError {
+public enum UnknownJSONAPIError: JSONAPIError {
 	case unknownError
 	
 	public init(from decoder: Decoder) throws {
@@ -21,7 +21,7 @@ public enum BasicJSONAPIError: JSONAPIError {
 		try container.encode("unknown")
 	}
 	
-	public static var unknown: BasicJSONAPIError {
+	public static var unknown: UnknownJSONAPIError {
 		return .unknownError
 	}
 }

Sources/JSONAPI/Document/Includes.swift

@@ -5,16 +5,14 @@
 //  Created by Mathew Polzin on 11/10/18.
 //
 
-import Result
+public typealias Include = Poly
 
-public protocol IncludeDecoder: Codable, Equatable {}
-
-public struct Includes<I: IncludeDecoder>: Codable, Equatable {
+public struct Includes<I: Include>: Codable, Equatable {
 	public static var none: Includes { return .init(values: []) }
 	
 	let values: [I]
 	
-	private init(values: [I]) {
+	public init(values: [I]) {
 		self.values = values
 	}
 	
@@ -52,433 +50,76 @@ public struct Includes<I: IncludeDecoder>: Codable, Equatable {
 	}
 }
 
-// MARK: - Decoding
+extension Includes: CustomStringConvertible {
+	public var description: String {
+		return "Includes(\(String(describing: values))"
+	}
+}
 
-func decode<Entity: JSONAPI.EntityType>(_ type: Entity.Type, from container: SingleValueDecodingContainer) throws -> Result<Entity, EncodingError> {
-	let ret: Result<Entity, EncodingError>
-	do {
-		ret = try .success(container.decode(Entity.self))
-	} catch (let err as EncodingError) {
-		ret = .failure(err)
-	} catch (let err) {
-		ret = .failure(EncodingError.invalidValue(Entity.Description.self,
-												  .init(codingPath: container.codingPath,
-														debugDescription: err.localizedDescription,
-														underlyingError: err)))
+extension Includes where I == NoIncludes {
+	public init() {
+		values = []
 	}
-	return ret
 }
 
 // MARK: - 0 includes
-
-public protocol _Include0: IncludeDecoder { }
-public struct Include0: _Include0 {
-	
-	public init(from decoder: Decoder) throws {
-	}
-
-	public func encode(to encoder: Encoder) throws {
-		throw JSONAPIEncodingError.illegalEncoding("Attempted to encode Include0, which should be represented by the absence of an 'included' entry altogether.")
-	}
-}
+public typealias Include0 = Poly0
 public typealias NoIncludes = Include0
 
 // MARK: - 1 include
-public protocol _Include1: _Include0 {
-	associatedtype A: EntityType
-	var a: A? { get }
-}
-public enum Include1<A: EntityType>: _Include1 {
-	case a(A)
-	
-	public var a: A? {
-		guard case let .a(ret) = self else { return nil }
-		return ret
-	}
-	
-	public init(from decoder: Decoder) throws {
-		let container = try decoder.singleValueContainer()
-		
-		self = .a(try container.decode(A.self))
-	}
-
-	public func encode(to encoder: Encoder) throws {
-		var container = encoder.singleValueContainer()
-
-		switch self {
-		case .a(let a):
-			try container.encode(a)
-		}
-	}
-}
-
-extension Includes where I: _Include1 {
+public typealias Include1 = Poly1
+extension Includes where I: _Poly1 {
 	public subscript(_ lookup: I.A.Type) -> [I.A] {
 		return values.compactMap { $0.a }
 	}
 }
 
 // MARK: - 2 includes
-public protocol _Include2: _Include1 {
-	associatedtype B: EntityType
-	var b: B? { get }
-}
-public enum Include2<A: EntityType, B: EntityType>: _Include2 {
-	case a(A)
-	case b(B)
-	
-	public var a: A? {
-		guard case let .a(ret) = self else { return nil }
-		return ret
-	}
-	
-	public var b: B? {
-		guard case let .b(ret) = self else { return nil }
-		return ret
-	}
-	
-	public init(from decoder: Decoder) throws {
-		let container = try decoder.singleValueContainer()
-		
-		let attempts = [
-			try decode(A.self, from: container).map { Include2.a($0) },
-			try decode(B.self, from: container).map { Include2.b($0) } ]
-		
-		let maybeVal: Include2<A, B>? = attempts
-			.compactMap { $0.value }
-			.first
-		
-		guard let val = maybeVal else {
-			throw EncodingError.invalidValue(Include2<A, B>.self, .init(codingPath: decoder.codingPath, debugDescription: "Failed to find an include of the expected type. Attempts: \(attempts.map { $0.error }.compactMap { $0 })"))
-		}
-		
-		self = val
-	}
-
-	public func encode(to encoder: Encoder) throws {
-		var container = encoder.singleValueContainer()
-
-		switch self {
-		case .a(let a):
-			try container.encode(a)
-		case .b(let b):
-			try container.encode(b)
-		}
-	}
-}
-
-extension Includes where I: _Include2 {
+public typealias Include2 = Poly2
+extension Includes where I: _Poly2 {
 	public subscript(_ lookup: I.B.Type) -> [I.B] {
 		return values.compactMap { $0.b }
 	}
 }
 
 // MARK: - 3 includes
-public protocol _Include3: _Include2 {
-	associatedtype C: EntityType
-	var c: C? { get }
-}
-public enum Include3<A: EntityType, B: EntityType, C: EntityType>: _Include3 {
-	case a(A)
-	case b(B)
-	case c(C)
-	
-	public var a: A? {
-		guard case let .a(ret) = self else { return nil }
-		return ret
-	}
-	
-	public var b: B? {
-		guard case let .b(ret) = self else { return nil }
-		return ret
-	}
-	
-	public var c: C? {
-		guard case let .c(ret) = self else { return nil }
-		return ret
-	}
-	
-	public init(from decoder: Decoder) throws {
-		let container = try decoder.singleValueContainer()
-		
-		let attempts = [
-			try decode(A.self, from: container).map { Include3.a($0) },
-			try decode(B.self, from: container).map { Include3.b($0) },
-			try decode(C.self, from: container).map { Include3.c($0) }]
-		
-		let maybeVal: Include3<A, B, C>? = attempts
-			.compactMap { $0.value }
-			.first
-		
-		guard let val = maybeVal else {
-			throw EncodingError.invalidValue(Include3<A, B, C>.self, .init(codingPath: decoder.codingPath, debugDescription: "Failed to find an include of the expected type. Attempts: \(attempts.map { $0.error }.compactMap { $0 })"))
-		}
-		
-		self = val
-	}
-
-	public func encode(to encoder: Encoder) throws {
-		var container = encoder.singleValueContainer()
-
-		switch self {
-		case .a(let a):
-			try container.encode(a)
-		case .b(let b):
-			try container.encode(b)
-		case .c(let c):
-			try container.encode(c)
-		}
-	}
-}
-
-extension Includes where I: _Include3 {
+public typealias Include3 = Poly3
+extension Includes where I: _Poly3 {
 	public subscript(_ lookup: I.C.Type) -> [I.C] {
 		return values.compactMap { $0.c }
 	}
 }
 
 // MARK: - 4 includes
-public protocol _Include4: _Include3 {
-	associatedtype D: EntityType
-	var d: D? { get }
-}
-public enum Include4<A: EntityType, B: EntityType, C: EntityType, D: EntityType>: _Include4 {
-	case a(A)
-	case b(B)
-	case c(C)
-	case d(D)
-	
-	public var a: A? {
-		guard case let .a(ret) = self else { return nil }
-		return ret
-	}
-	
-	public var b: B? {
-		guard case let .b(ret) = self else { return nil }
-		return ret
-	}
-	
-	public var c: C? {
-		guard case let .c(ret) = self else { return nil }
-		return ret
-	}
-	
-	public var d: D? {
-		guard case let .d(ret) = self else { return nil }
-		return ret
-	}
-	
-	public init(from decoder: Decoder) throws {
-		let container = try decoder.singleValueContainer()
-		
-		let attempts = [
-			try decode(A.self, from: container).map { Include4.a($0) },
-			try decode(B.self, from: container).map { Include4.b($0) },
-			try decode(C.self, from: container).map { Include4.c($0) },
-			try decode(D.self, from: container).map { Include4.d($0) }]
-		
-		let maybeVal: Include4<A, B, C, D>? = attempts
-			.compactMap { $0.value }
-			.first
-		
-		guard let val = maybeVal else {
-			throw EncodingError.invalidValue(Include4<A, B, C, D>.self, .init(codingPath: decoder.codingPath, debugDescription: "Failed to find an include of the expected type. Attempts: \(attempts.map { $0.error }.compactMap { $0 })"))
-		}
-		
-		self = val
-	}
-
-	public func encode(to encoder: Encoder) throws {
-		var container = encoder.singleValueContainer()
-
-		switch self {
-		case .a(let a):
-			try container.encode(a)
-		case .b(let b):
-			try container.encode(b)
-		case .c(let c):
-			try container.encode(c)
-		case .d(let d):
-			try container.encode(d)
-		}
-	}
-}
-
-extension Includes where I: _Include4 {
+public typealias Include4 = Poly4
+extension Includes where I: _Poly4 {
 	public subscript(_ lookup: I.D.Type) -> [I.D] {
 		return values.compactMap { $0.d }
 	}
 }
 
 // MARK: - 5 includes
-public protocol _Include5: _Include4 {
-	associatedtype E: EntityType
-	var e: E? { get }
-}
-public enum Include5<A: EntityType, B: EntityType, C: EntityType, D: EntityType, E: EntityType>: _Include5 {
-	case a(A)
-	case b(B)
-	case c(C)
-	case d(D)
-	case e(E)
-	
-	public var a: A? {
-		guard case let .a(ret) = self else { return nil }
-		return ret
-	}
-	
-	public var b: B? {
-		guard case let .b(ret) = self else { return nil }
-		return ret
-	}
-	
-	public var c: C? {
-		guard case let .c(ret) = self else { return nil }
-		return ret
-	}
-	
-	public var d: D? {
-		guard case let .d(ret) = self else { return nil }
-		return ret
-	}
-	
-	public var e: E? {
-		guard case let .e(ret) = self else { return nil }
-		return ret
-	}
-	
-	public init(from decoder: Decoder) throws {
-		let container = try decoder.singleValueContainer()
-		
-		let attempts = [
-			try decode(A.self, from: container).map { Include5.a($0) },
-			try decode(B.self, from: container).map { Include5.b($0) },
-			try decode(C.self, from: container).map { Include5.c($0) },
-			try decode(D.self, from: container).map { Include5.d($0) },
-			try decode(E.self, from: container).map { Include5.e($0) }]
-		
-		let maybeVal: Include5<A, B, C, D, E>? = attempts
-			.compactMap { $0.value }
-			.first
-		
-		guard let val = maybeVal else {
-			throw EncodingError.invalidValue(Include5<A, B, C, D, E>.self, .init(codingPath: decoder.codingPath, debugDescription: "Failed to find an include of the expected type. Attempts: \(attempts.map { $0.error }.compactMap { $0 })"))
-		}
-		
-		self = val
-	}
-
-	public func encode(to encoder: Encoder) throws {
-		var container = encoder.singleValueContainer()
-
-		switch self {
-		case .a(let a):
-			try container.encode(a)
-		case .b(let b):
-			try container.encode(b)
-		case .c(let c):
-			try container.encode(c)
-		case .d(let d):
-			try container.encode(d)
-		case .e(let e):
-			try container.encode(e)
-		}
-	}
-}
-
-extension Includes where I: _Include5 {
+public typealias Include5 = Poly5
+extension Includes where I: _Poly5 {
 	public subscript(_ lookup: I.E.Type) -> [I.E] {
 		return values.compactMap { $0.e }
 	}
 }
 
 // MARK: - 6 includes
-public protocol _Include6: _Include5 {
-	associatedtype F: EntityType
-	var f: F? { get }
-}
-public enum Include6<A: EntityType, B: EntityType, C: EntityType, D: EntityType, E: EntityType, F: EntityType>: _Include6 {
-	case a(A)
-	case b(B)
-	case c(C)
-	case d(D)
-	case e(E)
-	case f(F)
-	
-	public var a: A? {
-		guard case let .a(ret) = self else { return nil }
-		return ret
-	}
-	
-	public var b: B? {
-		guard case let .b(ret) = self else { return nil }
-		return ret
-	}
-	
-	public var c: C? {
-		guard case let .c(ret) = self else { return nil }
-		return ret
-	}
-	
-	public var d: D? {
-		guard case let .d(ret) = self else { return nil }
-		return ret
-	}
-	
-	public var e: E? {
-		guard case let .e(ret) = self else { return nil }
-		return ret
-	}
-	
-	public var f: F? {
-		guard case let .f(ret) = self else { return nil }
-		return ret
-	}
-	
-	public init(from decoder: Decoder) throws {
-		let container = try decoder.singleValueContainer()
-		
-		let attempts = [
-			try decode(A.self, from: container).map { Include6.a($0) },
-			try decode(B.self, from: container).map { Include6.b($0) },
-			try decode(C.self, from: container).map { Include6.c($0) },
-			try decode(D.self, from: container).map { Include6.d($0) },
-			try decode(E.self, from: container).map { Include6.e($0) },
-			try decode(F.self, from: container).map { Include6.f($0) }]
-		
-		let maybeVal: Include6<A, B, C, D, E, F>? = attempts
-			.compactMap { $0.value }
-			.first
-		
-		guard let val = maybeVal else {
-			throw EncodingError.invalidValue(Include6<A, B, C, D, E, F>.self, .init(codingPath: decoder.codingPath, debugDescription: "Failed to find an include of the expected type. Attempts: \(attempts.map { $0.error }.compactMap { $0 })"))
-		}
-		
-		self = val
-	}
-
-	public func encode(to encoder: Encoder) throws {
-		var container = encoder.singleValueContainer()
-
-		switch self {
-		case .a(let a):
-			try container.encode(a)
-		case .b(let b):
-			try container.encode(b)
-		case .c(let c):
-			try container.encode(c)
-		case .d(let d):
-			try container.encode(d)
-		case .e(let e):
-			try container.encode(e)
-		case .f(let f):
-			try container.encode(f)
-		}
-	}
-}
-
-extension Includes where I: _Include6 {
+public typealias Include6 = Poly6
+extension Includes where I: _Poly6 {
 	public subscript(_ lookup: I.F.Type) -> [I.F] {
 		return values.compactMap { $0.f }
 	}
 }
+
+// MARK: - 7 includes
+public typealias Include7 = Poly7
+extension Includes where I: _Poly7 {
+	public subscript(_ lookup: I.G.Type) -> [I.G] {
+		return values.compactMap { $0.g }
+	}
+}
+
+// MARK: - 8 includes

Sources/JSONAPI/Document/ResourceBody.swift

@@ -1,28 +1,55 @@
 //
-//  ResourceBody.swift
+//  PrimaryResourceBody.swift
 //  JSONAPI
 //
 //  Created by Mathew Polzin on 11/10/18.
 //
 
+public protocol MaybePrimaryResource: Equatable, Codable {}
+
+/// A PrimaryResource is a type that can be used in the body of a JSON API
+/// document as the primary resource.
+public protocol PrimaryResource: MaybePrimaryResource {}
+
+extension Optional: MaybePrimaryResource where Wrapped: PrimaryResource {}
+
+/// A ResourceBody is a representation of the body of the JSON API Document.
+/// It can either be one resource (which can be specified as optional or not)
+/// or it can contain many resources (and array with zero or more entries).
 public protocol ResourceBody: Codable, Equatable {
 }
 
-public struct SingleResourceBody<Entity: JSONAPI.EntityType>: ResourceBody {
-	public let value: Entity?
+public struct SingleResourceBody<Entity: JSONAPI.MaybePrimaryResource>: ResourceBody {
+	public let value: Entity
+
+	public init(entity: Entity) {
+		self.value = entity
+	}
 }
 
-public struct ManyResourceBody<Entity: JSONAPI.EntityType>: ResourceBody {
+public struct ManyResourceBody<Entity: JSONAPI.PrimaryResource>: ResourceBody {
 	public let values: [Entity]
+
+	public init(entities: [Entity]) {
+		values = entities
+	}
 }
 
-// MARK: Decodable
+/// Use NoResourceBody to indicate you expect a JSON API document to not
+/// contain a "data" top-level key.
+public struct NoResourceBody: ResourceBody {
+	public static var none: NoResourceBody { return NoResourceBody() }
+}
+
+// MARK: Codable
 extension SingleResourceBody {
 	public init(from decoder: Decoder) throws {
 		let container = try decoder.singleValueContainer()
 
-		if container.decodeNil() {
-			value = nil
+		let anyNil: Any? = nil
+		if container.decodeNil(),
+			let val = anyNil as? Entity {
+			value = val
 			return
 		}
 
@@ -32,7 +59,7 @@ extension SingleResourceBody {
 	public func encode(to encoder: Encoder) throws {
 		var container = encoder.singleValueContainer()
 
-		if value == nil {
+		if (value as Any?) == nil {
 			try container.encodeNil()
 			return
 		}
@@ -59,3 +86,17 @@ extension ManyResourceBody {
 		}
 	}
 }
+
+// MARK: CustomStringConvertible
+
+extension SingleResourceBody: CustomStringConvertible {
+	public var description: String {
+		return "PrimaryResourceBody(\(String(describing: value)))"
+	}
+}
+
+extension ManyResourceBody: CustomStringConvertible {
+	public var description: String {
+		return "PrimaryResourceBody(\(String(describing: values)))"
+	}
+}

Sources/JSONAPI/EncodingError.swift

@@ -0,0 +1,14 @@
+//
+//  EncodingError.swift
+//  JSONAPI
+//
+//  Created by Mathew Polzin on 12/7/18.
+//
+
+public enum JSONAPIEncodingError: Swift.Error {
+	case typeMismatch(expected: String, found: String)
+	case illegalEncoding(String)
+	case illegalDecoding(String)
+	case missingOrMalformedMetadata
+	case missingOrMalformedLinks
+}

Sources/JSONAPI/Invariants/ParsingInvariant.swift

@@ -0,0 +1,76 @@
+//
+//  ParsingInvariant.swift
+//  JSONAPI
+//
+//  Created by Mathew Polzin on 12/24/18.
+//
+
+/// An Error produced when an invariant fails.
+public enum InvariantError: Swift.Error {
+	case failure(description: String)
+}
+
+/// Set a reaction to a particular type of invariant failure.
+/// `Warning` and `Quiet` will take a default strategy for handling
+/// the invariant and parsing will not fail. `Error` will cause parsing
+/// to fail due to the given invariant failing.
+public protocol IReaction {
+	/// Perform the given Invariant strategy.
+	static func trigger(invariant: InvariantType.Type) throws
+}
+
+public enum Invariant {
+	/// Used to indicate an invariant failure shoud be considered an error.
+	/// Parsing will fail if an Error invariant fails.
+	public enum Error: IReaction {
+		public static func trigger(invariant: InvariantType.Type) throws {
+			throw InvariantError.failure(description: String(describing: invariant))
+		}
+	}
+
+	/// Used to indicate an invariant failure should be considered a warning.
+	/// The invariant description will be printed to the console but parsing
+	/// will not fail. A default strategy for addressing the invariant will
+	/// be taken. See the given invariant's documentation for details.
+	public enum Warning: IReaction {
+		public static func trigger(invariant: InvariantType.Type) throws {
+			print("**WARN** \(String(describing: invariant))")
+		}
+	}
+
+	/// Used to indicate an invariant failures should be considered inconsequential.
+	/// Parsing will continue and a default strategy for addressing the invariant
+	/// will be taken. See the given invariant's documentation for details.
+	public enum Quiet: IReaction {
+		public static func trigger(invariant: InvariantType.Type) throws {}
+	}
+
+	/// A strategy pairs an invariant type with the desired reaction.
+	public enum Strategy<IType: InvariantType, IReaction: JSONAPI.IReaction>: IStrategy {}
+}
+
+public protocol InvariantType {}
+
+public protocol IStrategy {
+	associatedtype IType: InvariantType
+	associatedtype IReaction: JSONAPI.IReaction
+}
+
+// TODO: Does this invariant make sense? I can't actually know whether arbitrary
+// Id types are "empty" so I think I started down this path without having fully
+// thought through it. Perhaps there are still other obvious invariants to add
+// though.
+public protocol RelationshipNonEmptyIdInvariant: IStrategy where IType == Invariant.Relationships.NonEmptyId {}
+
+extension Invariant {
+	public enum Relationships {
+
+		/// The Empty Id Invariant holds as long as a relationships's Id
+		/// is not "empty"
+		public enum NonEmptyId: InvariantType {}
+
+		public enum Strategies<NonEmptyId: RelationshipNonEmptyIdInvariant> {}
+	}
+}
+
+// Would be used like `ToOneRelationship<OtherEntity, Invariant.Relationships.Strategies<Invariant.Strategy<Invariant.Relationships.NonEmptyId, Invariant.Error>>>`

Sources/JSONAPI/Meta/Links.swift

@@ -0,0 +1,67 @@
+//
+//  Links.swift
+//  JSONAPI
+//
+//  Created by Mathew Polzin on 11/24/18.
+//
+
+/// A Links structure should contain nothing but JSONAPI.Link properties.
+public protocol Links: Codable, Equatable {}
+
+/// Use NoLinks where no links should belong to a JSON API component
+public struct NoLinks: Links, CustomStringConvertible {
+	public static var none: NoLinks { return NoLinks() }
+	public init() {}
+
+	public var description: String { return "No Links" }
+}
+
+public protocol JSONAPIURL: Codable, Equatable {}
+
+public struct Link<URL: JSONAPI.JSONAPIURL, Meta: JSONAPI.Meta>: Equatable, Codable {
+	public let url: URL
+	public let meta: Meta
+
+	public init(url: URL, meta: Meta) {
+		self.url = url
+		self.meta = meta
+	}
+}
+
+extension Link where Meta == NoMetadata {
+	public init(url: URL) {
+		self.init(url: url, meta: .none)
+	}
+}
+
+public extension Link {
+	private enum CodingKeys: String, CodingKey {
+		case href
+		case meta
+	}
+
+	init(from decoder: Decoder) throws {
+		guard Meta.self == NoMetadata.self,
+			let noMeta = NoMetadata() as? Meta else {
+				let container = try decoder.container(keyedBy: CodingKeys.self)
+				meta = try container.decode(Meta.self, forKey: .meta)
+				url = try container.decode(URL.self, forKey: .href)
+				return
+		}
+		let container = try decoder.singleValueContainer()
+		url = try container.decode(URL.self)
+		meta = noMeta
+	}
+
+	func encode(to encoder: Encoder) throws {
+		guard Meta.self == NoMetadata.self else {
+			var container = encoder.container(keyedBy: CodingKeys.self)
+			try container.encode(url, forKey: .href)
+			try container.encode(meta, forKey: .meta)
+			return
+		}
+		var container = encoder.singleValueContainer()
+
+		try container.encode(url)
+	}
+}

Sources/JSONAPI/Meta/Meta.swift

@@ -0,0 +1,28 @@
+//
+//  Meta.swift
+//  JSONAPI
+//
+//  Created by Mathew Polzin on 11/21/18.
+//
+
+/// Conform a type to this protocol to indicate it can be encoded to or decoded from
+/// the meta data attached to a component of a JSON API document. Different meta data
+/// can be stored all over the place: On the root document, on a resource object, on
+/// link objects, etc.
+///
+/// JSON API Metadata is totally open ended. It can take whatever JSON-compliant structure
+/// the server and client agree upon.
+public protocol Meta: Codable, Equatable {
+}
+
+// We make Optional a Meta if it wraps a Meta so that Metadata can be specified as
+// nullable.
+extension Optional: Meta where Wrapped: Meta {}
+
+public struct NoMetadata: Meta, CustomStringConvertible {
+	public static var none: NoMetadata { return NoMetadata() }
+
+	public init() { }
+
+	public var description: String { return "No Metadata" }
+}

Sources/JSONAPI/Resource/Attribute+Functor.swift

@@ -0,0 +1,21 @@
+//
+//  Attribute+Functor.swift
+//  JSONAPI
+//
+//  Created by Mathew Polzin on 11/28/18.
+//
+
+public extension TransformedAttribute {
+	/// Map an Attribute to a new wrapped type.
+	/// Note that the resulting Attribute will have no transformer, even if the
+	/// source Attribute has a transformer.
+	/// You are mapping the output of the source transform into
+	/// the RawValue of a new transformerless Attribute.
+	///
+	/// Generally, this is the most useful operation. The transformer gives you
+	/// control over the decoding of the Attribute, but once the Attribute exists,
+	/// mapping on it is most useful for creating computed Attribute properties.
+	public func map<T: Codable>(_ transform: (Transformer.To) throws -> T) rethrows -> Attribute<T> {
+		return Attribute<T>(value: try transform(value))
+	}
+}

Sources/JSONAPI/Resource/Attribute.swift

@@ -5,8 +5,13 @@
 //  Created by Mathew Polzin on 11/13/18.
 //
 
-public struct TransformAttribute<RawValue: Codable, Transformer: JSONAPI.Transformer>: Codable where Transformer.From == RawValue {
-	private let rawValue: RawValue
+public protocol AttributeType: Codable {
+}
+
+// MARK: TransformedAttribute
+/// A TransformedAttribute takes a Codable type and attempts to turn it into another type.
+public struct TransformedAttribute<RawValue: Codable, Transformer: JSONAPI.Transformer>: AttributeType where Transformer.From == RawValue {
+	let rawValue: RawValue
 	
 	public let value: Transformer.To
 	
@@ -16,12 +21,41 @@ public struct TransformAttribute<RawValue: Codable, Transformer: JSONAPI.Transfo
 	}
 }
 
-public typealias Attribute<T: Codable> = TransformAttribute<T, IdentityTransformer<T>>
+// MARK: ValidatedAttribute
+/// A ValidatedAttribute does not transform its raw value, but it throws
+/// an error if the raw value does not match expectations.
+public typealias ValidatedAttribute<RawValue: Codable, Validator: JSONAPI.Validator> = TransformedAttribute<RawValue, Validator> where RawValue == Validator.From
 
-extension TransformAttribute: Equatable where Transformer.From: Equatable, Transformer.To: Equatable {}
+// MARK: Attribute
+/// An Attribute simply represents a type that can be encoded and decoded.
+public typealias Attribute<T: Codable> = TransformedAttribute<T, IdentityTransformer<T>>
+
+extension TransformedAttribute where Transformer: ReversibleTransformer {
+	public init(transformedValue: Transformer.To) throws {
+		self.value = transformedValue
+		rawValue = try Transformer.reverse(value)
+	}
+}
+
+extension TransformedAttribute: CustomStringConvertible {
+	public var description: String {
+		return "Attribute<\(String(describing: Transformer.From.self)) -> \(String(describing: Transformer.To.self))>(\(String(describing: value)))"
+	}
+}
+
+extension TransformedAttribute: Equatable where Transformer.From: Equatable, Transformer.To: Equatable {}
+
+extension TransformedAttribute where Transformer == IdentityTransformer<RawValue> {
+	// If we are using the identity transform, we can skip the transform and guarantee no
+	// error is thrown.
+	public init(value: RawValue) {
+		rawValue = value
+		self.value = value
+	}
+}
 
 // MARK: - Codable
-extension TransformAttribute {
+extension TransformedAttribute {
 	public init(from decoder: Decoder) throws {
 		let container = try decoder.singleValueContainer()
 		
@@ -46,28 +80,7 @@ extension TransformAttribute {
 	
 	public func encode(to encoder: Encoder) throws {
 		var container = encoder.singleValueContainer()
-		
-		// See note in decode above about the weirdness
-		// going on here.
-		let anyNil: Any? = nil
-		if let _ = anyNil as? Transformer.From,
-			(rawValue as Any?) == nil {
-			try container.encodeNil()
-		}
-		
+
 		try container.encode(rawValue)
 	}
 }
-
-// MARK: - Transformers
-
-public protocol Transformer {
-	associatedtype From
-	associatedtype To
-	
-	static func transform(_ from: From) throws -> To
-}
-
-public enum IdentityTransformer<T>: Transformer {
-	public static func transform(_ from: T) throws -> T { return from }
-}