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))
+	}
+}