R is a free software environment for statistical computing and graphics. It started as a free implementation of the S language, which was back then commercially available as S-Plus, and has since around ten years become the lingua franca of statistics, the main language people use to communicate statistical computation. R’s popularity stems partly from the fact that it is free and open source, partly from the fact that it is easily extendible: through add-on packages that follow a clearly defined structure, new statistical ideas can be implemented, shared, and used by others. Using R, the computational aspects of research can be communicated in a reproducible way, understood by a large audience.
Written between 1984 and 1998, John Chambers is (co-)author of
the four leading – “brown”, “blue”, “white”, “green” – books
that describe the S language as it evolved and as it is now. He
has designed it, implemented it, and improved it in all its phases.
Being part of the R core team, he is author of the
part of every R installation, providing the S4 approach to object
This book, Extending R, appeared as a volume in “The R Series”. The book is organized in four parts:
- Understanding R,
- Programming with R,
- Object-oriented programming, and
The first part starts with explaining three principles underlying R:
- Everything that exists in R is an object
- Everything that happens in R is a function call
- Interfaces to other software are part of R.
These principle form the basis for parts II, III and IV. The first chapter introduces them. Chapter two, “Evolution”, describes the history of the S language, from its earliest days to Today: the coming and going of S-Plus, the arrival of R and its dominance Today. It also describes the evolution of functional S, and the evolution of object-oriented programming in S. Chapter 3, “R in action”, explains a number of basics of R, such as how function calls work, how objects are implemented, and how the R evaluator works.
Part II, “Programming with R”, discusses functions in depth, explains what objects are and how they are managed, and explains what extension packages do to the R environment. It discusses small, medium and large programming exercises, and what they demand.
Part III, “Object-oriented programming”, largely focuses on the difference between functional object oriented programming (as implemented in S4) and encapsulated object oriented programming as implemented in reference classes (similar to C++ and java), and shows examples for which purpose each paradigm is most useful.
Part IV, “Interfaces”, explains the potential and challenges of interfacing R with other programming languages. It discusses several of such interfaces, and describes a general framework for creating such interfaces. As instances of this framework it provides interfaces to the Python and Julia languages, and discusses the existing Rcpp framework.
For who was this book written? It is clearly not an introductory text, nor a how-to or hands-on book for learning how to program R or write R packages, and it refers to the two volumes Advanced R and R packages, both written by Hadley Wickham. For those with a bit of experience with R programming and a general interest in the language, this book may give a number of new insights and a deeper, often evolutionary motivated understanding.
Not surprisingly, the book also gives clear advice on how software development should take place: object-oriented with formally defined classes (S4 or reference classes), and it argues why this is a good idea. One of these arguments is the ability to do method dispatch based on more than one argument. This needs all arguments to be evaluated, and does not work well with non-standard evaluation. Many R packages currently promoted by Hadley Wickham and many others (“tidyverse”) often favor non-standard evaluation, and constrain to S3. I think that both arguments have some merit, and would look forward to a good user study that compares the usability of the two approaches.