The latest draft of my book is available. This will be my last pre-publication update, as I’m in the process of editing the remainder of the book. That said, the first part is packed with examples and provides a solid foundation on its own. I’m including the preface below to whet people’s appetite for the complete book when it is published.

### Preface

This book is about programming. Not just any programming, but programming for data science and numerical systems. This type of programming usually starts as a mathematical modeling problem that needs to be translated into computer code. With functional programming, the same reasoning used for the mathematical model can be used for the software model. This not only reduces the impedance mismatch between model and code, it also makes code easier to understand, maintain, change, and reuse. Functional programming is the conceptual force that binds the these two models together. Most books that cover numerical and/or quantitative methods focus primarily on the mathematics of the model. Once this model is established the computational algorithms are presented without fanfare as imperative, step-by-step, algorithms. These detailed steps are designed for machines. It is often difficult to reason about the algorithm in a way that can meaningfully leverage the properties of the mathematical model. This is a shame because mathematical models are often quite beautiful and elegant yet are transformed into ugly and cumbersome software. This unfortunate outcome is exacerbated by the real world, which is messy: data does not always behave as desired; data sources change; computational power is not always as great as we wish; reporting and validation workflows complicate model implementations. Most theoretical books ignore the practical aspects of working in the field. The need for a theoretical and structured bridge from the quantitative methods to programming has grown as data science and the computational sciences become more pervasive.

The goal is to re-establish the intimate relationship between mathematics and computer science. By the end of the book, readers will be able to write computer programs that clearly convey the underlying mathematical model, while being easy to modify and maintain. This is possible in R by leveraging the functional programming features built into the language. Functional programming has a long history within academia and its popularity in industry has recently been rising. Two prominent reasons for this upswing are the growing computational needs commensurate with large data sets and the data-centric computational model that is consistent with the analytical model. The superior modularity of functional programs isolates data management from model development, which keeps the model clean and reduces development overhead. Derived from a formal mathematical language, functional programs can be reasoned about with a level of clarity and certainty not possible in other programming paradigms. Hence the same level of rigor applied to the quantitative model can be applied to the software model.

Divided into three parts, foundations are first established that uses the world of mathematics as an introduction to functional programming concepts (Chapter 2). Topics discussed include basic concepts in set theory, statistics, linear algebra, and calculus. As core tools for data scientists, this material should be accessible to most practitioners, graduate students, and even upper class undergraduates. The idea is to show you that you already know most of the concepts used in functional programming. Writing code using functional programming extends this knowledge to operations beyond the mathematical model. This chapter also shows counter examples using other programming styles, some in other languages. The point isn’t to necessarily criticize other implementation approaches, but rather make the differences in styles tangible to the reader. After establishing some initial familiarity, Chapter 3 dives into functions, detailing the various properties and behaviors they have. Some important features include higher-order functions, first-class functions, and closures. This chapter gives you a formal vocabulary for talking about functional programs in R. This distinction is important as other functional languages have plenty more terms and theory ignored in this book. Again, the goal is how to leverage functional programming ideas to be a better data scientist. Finally, Chapter 4 reviews the various packages in R that provide functionality related to functional programming. These packages include some built-in implementations, paradigms for parallel computing, a subset of the so-called tidyverse, and my own lambda.r package, which offers a more comprehensive approach to writing functional programs.

While the first part provides a working overview of functional programming in R, Part II takes it a step further. This part is for readers that want to exploit functional programming principles to their fullest. Many topics in Part I reference deeper discussions in Part II. This canon begins by exploring the nature of vectorization in Chapter 5. Often taken for granted, we look at how colloquial vectorization conflates a few concepts. This chapter unravels the concepts, showing you what can be done and what to expect with each type of vectorization. Three primary higher-order functions map, fold, and filter follow. Chapter 6 shows how the concept of map appears throughout R, particularly in the apply family of functions. I show how to reason about data structures with respect to the ordinals (or index) of the data structure. This approach can simplify code by enabling the separation of data structures, so long as an explicit ordinal mapping exists. While fold is more fundamental than map, its use is less frequent in R. Discussed in Chapter 7, fold provides a common structure for repeated function application. Optimization and many iterative methods, as well as stochastic systems make heavy use of repeated function application, but this is usually implemented as a loop. With fold, the same function used to implement a single step can be used for multiple steps, reducing code complexity and making it easier to test. The final function of the canon is filter, which creates subsets using a predicate. This concept is so integral to R that the notation is deeply integrated into the language. Chapter 8 shows how the native R syntax is tied to this higher-order function, which can be useful when data structures are more complex. Understanding this connection also simplifies porting code to or from R when the other language doesn’t have native syntax for these operations.

Programming languages can’t do much without data structures. Chapter 9 shows how to use native R data structures to implement numerous algorithms as well as emulate other data structures. For example, lists can be used to emulate trees, while environments can emulate hash tables.

The last part focuses on applications and advanced topics. Part III can act as a reference for implementing various algorithms in a functional style. This provides readers with tangible examples of using functional programming concepts for algorithm development. This part begins by proposing a simple model development pipeline in Chapter 11. The intention is to provide some reusable structure that can be tailored to each reader’s needs. For example, the process of backtesting is often implemented in reverse. Loops become integral to the implementation, which makes it difficult to test individual update steps in an algorithm. This chapter also shows some basic organizational tricks to simplify model development. A handful of machine learning models, such as random forest, are also presented in Chapter 11. Remarkably, many of these algorithms can be implemented in less than 30 lines fo code when using functional programming techniques. Optimization methods, such as Newton’s method, linear programming, and dynamic follow in Chapter 13. These methods are usually iterative and therefore benefit from functional programming. State-based systems are explored in Chapter 12. Ranging from iterative function systems to context-free grammars, state is central to many models and simulations. This chapter shows how functional programming can simplify these models. This part also discusses two case studies (Chapter 14 and Chapter 15) for implementing more complete systems. In essence, Part III shows the reader how to apply the concepts presented in the book to real-world problems.

Each chapter presents a number of exercises to test what you’ve learned. By the end of the book, you will know how to leverage functional program- ming to improve your life as a data scientist. Not only will you be able to quickly and easily implement mathematical ideas, you’ll be able to incrementally change your exploratory model into a production model, possibly scaling to multiple cores for big data. Doing so also facilitates repeatable research since others can review and modify your work more easily.