Make Attribute a Functor to make computed attributes easier to write.

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))
+	}
+}

Tests/JSONAPITests/Attribute/Attribute+FunctorTests.swift

@@ -0,0 +1,70 @@
+//
+//  Attribute+FunctorTests.swift
+//  JSONAPITests
+//
+//  Created by Mathew Polzin on 11/28/18.
+//
+
+import XCTest
+import JSONAPI
+import JSONAPITestLib
+
+class Attribute_FunctorTests: XCTestCase {
+	func test_mapGuaranteed() {
+		let entity = try? TestType(attributes: .init(name: "Frankie", number: .init(rawValue: 22.0)))
+
+		XCTAssertNotNil(entity)
+
+		XCTAssertEqual(entity?[\.computedString], "Frankie2")
+	}
+
+	func test_mapOptionalSuccess() {
+		let entity = try? TestType(attributes: .init(name: "Frankie", number: .init(rawValue: 22.0)))
+
+		XCTAssertNotNil(entity)
+
+		XCTAssertEqual(entity?[\.computedNumber], 22)
+	}
+
+	func test_mapOptionalFailure() {
+		let entity = try? TestType(attributes: .init(name: "Frankie", number: .init(rawValue: 22.5)))
+
+		XCTAssertNotNil(entity)
+
+		XCTAssertNil(entity?[\.computedNumber])
+	}
+}
+
+// MARK: Test types
+extension Attribute_FunctorTests {
+	enum TestTypeDescription: EntityDescription {
+		public static var type: String { return "test" }
+
+		public struct Attributes: JSONAPI.Attributes {
+			let name: Attribute<String>
+			let number: TransformedAttribute<Double, DoubleToString>
+			var computedString: Attribute<String> {
+				return name.map { $0 + "2" }
+			}
+			var computedNumber: Attribute<Int>? {
+				return try? number.map { string in
+					let num = Double(string).flatMap { Int(exactly: $0) }
+					guard let ret = num else {
+						throw DecodingError.typeMismatch(Int.self, .init(codingPath: [], debugDescription: "String was not an Int."))
+					}
+					return ret
+				}
+			}
+		}
+
+		public typealias Relationships = NoRelationships
+	}
+
+	typealias TestType = Entity<TestTypeDescription>
+
+	enum DoubleToString: Transformer {
+		public static func transform(_ from: Double) -> String {
+			return String(from)
+		}
+	}
+}

Tests/JSONAPITests/Computed Properties/ComputedPropertiesTests.swift

@@ -26,7 +26,7 @@ class ComputedPropertiesTests: XCTestCase {
 	func test_ComputedAttributeAccess() {
 		let entity = decoded(type: TestType.self, data: computed_property_attribute)
 
-		XCTAssertEqual(entity[\.computed], "Sarah")
+		XCTAssertEqual(entity[\.computed], "Sarah2")
 	}
 
 	func test_ComputedRelationshipAccess() {
@@ -44,7 +44,7 @@ extension ComputedPropertiesTests {
 		public struct Attributes: JSONAPI.Attributes {
 			public let name: Attribute<String>
 			public var computed: Attribute<String> {
-				return name
+				return name.map { $0 + "2" }
 			}
 		}