What Are Hack and HHVM?Who This Book Is ForPhilosophyProgram TypesGradual MigrationHow the Book Is OrganizedVersionsConventions Used in This BookSafari® Books OnlineHow to Contact UsContent UpdatesJanuary 15, 2016Acknowledgments Why Use the Typechecker?Setting Up the TypecheckerAutoload EverythingReading Error MessagesType Annotation SyntaxFunction Return TypesFunction ParametersPropertiesHack’s T