comparable-keypaths.swift

/// Returns a binary predicate using the given key path to create an ascending order
/// for elements of type `Root`.
func ascending<Root, Value: Comparable>(_ path: KeyPath<Root, Value>) -> (Root, Root) -> Bool {
  return { $0[keyPath: path] < $1[keyPath: path] }
}

/// Returns a binary predicate using the given key path to create a descending order
/// for elements of type `Root`.
func descending<Root, Value: Comparable>(_ path: KeyPath<Root, Value>) -> (Root, Root) -> Bool {
  return { $0[keyPath: path] > $1[keyPath: path] }
}

let chrises = ["Pratt", "Hemsworth", "Rock", "Lattner"]
chrises.sorted(by: ascending(\.count))    // ["Rock", "Pratt", "Lattner", "Hemsworth"]
chrises.sorted(by: descending(\.count))   // ["Hemsworth", "Lattner", "Pratt", "Rock"]