Move Poly into its own library.

Package.resolved

@@ -1,7 +1,15 @@
 {
   "object": {
     "pins": [
-
+      {
+        "package": "Poly",
+        "repositoryURL": "https://github.com/mattpolzin/Poly.git",
+        "state": {
+          "branch": "master",
+          "revision": "92d889c2cdc027bb94814187e84d34783ae159ae",
+          "version": null
+        }
+      }
     ]
   },
   "version": 1

Package.swift

@@ -14,11 +14,12 @@ let package = Package(
 			targets: ["JSONAPITestLib"])
     ],
     dependencies: [
+		.package(url: "https://github.com/mattpolzin/Poly.git", .branch("master"))
     ],
     targets: [
         .target(
             name: "JSONAPI",
-            dependencies: []),
+            dependencies: ["Poly"]),
 		.target(
 			name: "JSONAPITestLib",
 			dependencies: ["JSONAPI"]),

Sources/JSONAPI/Document/Document.swift

@@ -5,6 +5,8 @@
 //  Created by Mathew Polzin on 11/5/18.
 //
 
+import Poly
+
 public protocol JSONAPIDocument: Codable, Equatable {
 	associatedtype PrimaryResourceBody: JSONAPI.ResourceBody
 	associatedtype MetaType: JSONAPI.Meta

Sources/JSONAPI/Document/Includes.swift

@@ -5,6 +5,8 @@
 //  Created by Mathew Polzin on 11/10/18.
 //
 
+import Poly
+
 public typealias Include = JSONPoly
 
 public struct Includes<I: Include>: Codable, Equatable {

Sources/JSONAPI/Resource/Poly+PrimaryResource.swift

@@ -0,0 +1,55 @@
+//
+//  Poly+PrimaryResource.swift
+//  JSONAPI
+//
+//  Created by Mathew Polzin on 11/22/18.
+//
+
+import Poly
+
+/// Poly is a protocol to which types that
+/// are polymorphic belong to. Specifically,
+/// Poly1, Poly2, Poly3, etc. types conform
+/// to the Poly protocol. These types allow
+/// typesafe grouping of a number of
+/// disparate types under one roof for
+/// the purposes of JSON API compliant
+/// encoding or decoding.
+public typealias JSONPoly = Poly & PrimaryResource
+
+extension Poly0: PrimaryResource {
+	public init(from decoder: Decoder) throws {
+		throw JSONAPIEncodingError.illegalDecoding("Attempted to decode Poly0, which should represent a thing that is not expected to be found in a document.")
+	}
+
+	public func encode(to encoder: Encoder) throws {
+		throw JSONAPIEncodingError.illegalEncoding("Attempted to encode Poly0, which should represent a thing that is not expected to be found in a document.")
+	}
+}
+
+// MARK: - 1 type
+extension Poly1: PrimaryResource, MaybePrimaryResource where A: Codable & Equatable {}
+
+// MARK: - 2 types
+extension Poly2: PrimaryResource, MaybePrimaryResource where A: PolyWrapped, B: PolyWrapped {}
+
+// MARK: - 3 types
+extension Poly3: PrimaryResource, MaybePrimaryResource where A: PolyWrapped, B: PolyWrapped, C: PolyWrapped {}
+
+// MARK: - 4 types
+extension Poly4: PrimaryResource, MaybePrimaryResource where A: PolyWrapped, B: PolyWrapped, C: PolyWrapped, D: PolyWrapped {}
+
+// MARK: - 5 types
+extension Poly5: PrimaryResource, MaybePrimaryResource where A: PolyWrapped, B: PolyWrapped, C: PolyWrapped, D: PolyWrapped, E: PolyWrapped {}
+
+// MARK: - 6 types
+extension Poly6: PrimaryResource, MaybePrimaryResource where A: PolyWrapped, B: PolyWrapped, C: PolyWrapped, D: PolyWrapped, E: PolyWrapped, F: PolyWrapped {}
+
+// MARK: - 7 types
+extension Poly7: PrimaryResource, MaybePrimaryResource where A: PolyWrapped, B: PolyWrapped, C: PolyWrapped, D: PolyWrapped, E: PolyWrapped, F: PolyWrapped, G: PolyWrapped {}
+
+// MARK: - 8 types
+extension Poly8: PrimaryResource, MaybePrimaryResource where A: PolyWrapped, B: PolyWrapped, C: PolyWrapped, D: PolyWrapped, E: PolyWrapped, F: PolyWrapped, G: PolyWrapped, H: PolyWrapped {}
+
+// MARK: - 9 types
+extension Poly9: PrimaryResource, MaybePrimaryResource where A: PolyWrapped, B: PolyWrapped, C: PolyWrapped, D: PolyWrapped, E: PolyWrapped, F: PolyWrapped, G: PolyWrapped, H: PolyWrapped, I: PolyWrapped {}

Tests/JSONAPITests/Document/DocumentTests.swift

@@ -7,6 +7,7 @@
 
 import XCTest
 import JSONAPI
+import Poly
 
 class DocumentTests: XCTestCase {
 

Tests/JSONAPITests/Poly/PolyProxyTests.swift

@@ -7,6 +7,7 @@
 
 import XCTest
 import JSONAPI
+import Poly
 
 public class PolyProxyTests: XCTestCase {
 	func test_generalReasonableness() {

Tests/JSONAPITests/Poly/PolyTests.swift

@@ -7,6 +7,7 @@
 
 import XCTest
 import JSONAPI
+import Poly
 
 // MARK: - init
 class PolyTests: XCTestCase {