ABSTRACT
In chapter 8 we will discuss R’s condition system, which provides a paired set of tools that allow the author of a function to indicate that something unusual is happening, and the user of that function to deal with it. The function author signals conditions with functions like stop(), warning(), and message(), then the function user can handle them with functions like tryCatch() and withCallingHandlers(). Understanding the condition system is important because you’ll often need to play both roles: signalling conditions from the functions you create, and handle conditions signalled by the functions you call.
R’s condition system is based on ideas from Common Lisp. Like R’s approach to object-oriented programming, it is rather different to currently popular programming languages so it is easy to misunderstand, and there has been relatively little written about how to use it effectively. Historically, this has meant that few people have taken full advantage of its power. The goal of this chapter is to remedy that situation. Here you will learn about the big ideas of R’s condition system, as well as learning a bunch of practical tools that will make your code stronger.
“Signalling conditions” introduces the basic tools for signalling conditions, and discusses when it is appropriate to use each type. “Ignoring conditions” teaches you about the simplest tools for handling conditions: functions like try() and supressMessages() that swallow conditions and prevent them from getting to the top level. “Handling conditions” introduces the condition object, and the two fundamental tools of condition handling: tryCatch() for error conditions, and withCallingHandlers() for everything else. “Custom conditions” shows you how to extend the built-in condition objects to store useful data that condition handlers can use to make more informed decisions. “Applications” closes out the chapter with a grab bag of practical applications based on the low-level tools found in earlier sections.
