PLP 4e cover

Preface to
Programming Language Pragmatics
Fourth Edition

A course in computer programming provides the typical student’s first exposure to the field of computer science.  Most students in such a course will have used computers all their lives, for social networking, email, games, web browsing, word processing, and a host of other tasks, but it is not until they write their first programs that they begin to appreciate how applications work.  After gaining a certain level of facility as programmers (presumably with the help of a good course in data structures and algorithms), the natural next step is to wonder how programming languages work.  This book provides an explanation.  It aims, quite simply, to be the most comprehensive and accurate languages text available, in a style that is engaging and accessible to the typical undergraduate.  This aim reflects my conviction that students will understand more, and enjoy the material more, if we explain what is really going on. 

In the conventional “systems” curriculum, the material beyond data structures (and possibly computer organization) tends to be compartmentalized into a host of separate subjects, including programming languages, compiler construction, computer architecture, operating systems, networks, parallel and distributed computing, database management systems, and possibly software engineering, object-oriented design, graphics, or user interface systems.  One problem with this compartmentalization is that the list of subjects keeps growing, but the number of semesters in a Bachelor’s program does not.  More important, perhaps, many of the most interesting discoveries in computer science occur at the boundaries between subjects.  Computer architecture and compiler construction, for example, have inspired each other for over 50 years, through generations of supercomputers, pipelined microprocessors, multicore chips, and modern GPUs.  Over the past decade, advances in virtualization have blurred boundaries among the hardware, operating system, compiler, and language run-time system, and have spurred the explosion in cloud computing.  Programming language technology is now routinely embedded in everything from dynamic web content, to gaming and entertainment, to security and finance. 

Increasingly, both educators and practitioners have come to emphasize these sorts of interactions.  Within higher education in particular, there is a growing trend toward integration in the core curriculum.  Rather than give the typical student an in-depth look at two or three narrow subjects, leaving holes in all the others, many schools have revised the programming languages and computer organization courses to cover a wider range of topics, with follow-on electives in various specializations.  This trend is very much in keeping with the ACM/IEEE-CS Computer Science Curricula 2013 guidelines, which emphasize the need to manage the size of the curriculum and to cultivate both a “system-level perspective” and an appreciation of the interplay between theory and practice.  In particular, the authors write,

Graduates of a computer science program need to think at multiple levels of detail and abstraction.  This understanding should transcend the implementation details of the various components to encompass an appreciation for the structure of computer systems and the processes involved in their construction and analysis [p. 24]. 
On the specific subject of this text, they write
Programming languages are the medium through which programmers precisely describe concepts, formulate algorithms, and reason about solutions.  In the course of a career, a computer scientist will work with many different languages, separately or together.  Software developers must understand the programming models underlying different languages and make informed design choices in languages supporting multiple complementary approaches.  Computer scientists will often need to learn new languages and programming constructs, and must understand the principles underlying how programming language features are defined, composed, and implemented.  The effective use of programming languages, and appreciation of their limitations, also requires a basic knowledge of programming language translation and static program analysis, as well as run-time components such as memory management [p. 155]. 

The first three editions of Programming Language Pragmatics (“PLP”) had the good fortune of riding the trend toward integrated understanding.  This fourth edition continues and strengthens the “systems perspective” while preserving the central focus on programming language design. 

At its core, PLP is a book about how programming languages work.  Rather than enumerate the details of many different languages, it focuses on concepts that underlie all the languages the student is likely to encounter, illustrating those concepts with a variety of concrete examples, and exploring the tradeoffs that explain why different languages were designed in different ways.  Similarly, rather than explain how to build a compiler or interpreter (a task few programmers will undertake in its entirety), PLP focuses on what a compiler does to an input program, and why.  Language design and implementation are thus explored together, with an emphasis on the ways in which they interact. 

Changes in the Fourth Edition

In comparison to the third edition, PLP-4e includes

  1. New chapters devoted to type systems and composite types, in place of the older single chapter on types
  2. Updated treatment of functional programming, with extensive coverage of OCaml
  3. Numerous other reflections of changes in the field
  4. Improvements inspired by instructor feedback or a fresh consideration of familiar topics

Item 1 in this list is perhaps the most visible change.  Chapter 7 was the longest in previous editions, and there is a natural split in the subject material.  Reorganization of this material for PLP-4e afforded an opportunity to devote more explicit attention to the subject of type inference, and of its role in ML-family languages in particular.  It also facilitated an update and reorganization of the material on parametric polymorphism, which was previously scattered across several different chapters and sections. 

Item 2 reflects the increasing adoption of functional techniques into mainstream imperative languages, as well as the increasing prominence of SML, OCaml, and Haskell in both education and industry.  Throughout the text, OCaml is now co-equal with Scheme as a source of functional programming examples.  As noted in the previous paragraph, there is an expanded section (7.2.4) on the ML type system, and Section 11.4 includes an OCaml overview, with coverage of equality and ordering, bindings and lambda expressions, type constructors, pattern matching, and control flow and side effects.  The choice of OCaml, rather than Haskell, as the ML-family exemplar reflects its prominence in industry, together with classroom experience suggesting that—at least for many students—the initial exposure to functional thinking is easier in the context of eager evaluation.  To colleagues who wish I’d chosen Haskell, my apologies! 

Other new material (Item 3) appears throughout the text.  Wherever appropriate, reference has been made to features of the latest languages and standards, including C & C++11, Java 8, C# 5, Scala, Go, Swift, Python 3, and HTML 5.  Section 3.6.4 pulls together previously scattered coverage of lambda expressions, and shows how these have been added to various imperative languages.  Complementary coverage of object closures, including C++11’s std::function and std::bind, appears in Section 10.4.4.  Section C-5.4.5 introduces the x86-64 and ARM architectures in place of the x86-32 and MIPS used in previous editions.  Examples using these same two architectures subsequently appear in the sections on calling sequences (9.2) and linking (15.6).  Coverage of the x86 calling sequence continues to rely on gcc; the ARM case study uses LLVM.  Section 8.5.3 introduces smart pointers.  R-value references appear in Section 9.3.1.  JavaFX replaces Swing in the graphics examples of Section 9.6.2.  Appendix A has new entries for Go, Lua, Rust, Scala, and Swift. 

Finally, Item 4 encompasses improvements to almost every section of the text.  Among the more heavily updated topics are FOLLOW and PREDICT sets (Section 2.3.3); Wirth’s error recovery algorithm for recursive descent (Section C-2.3.5); overloading (Section 3.5.2); modules (Section 3.3.4); duck typing (Section 7.3); records and variants (Section 8.1); intrusive lists (removed from the running example of Chapter 10); static fields and methods (Section 10.2.2); mix-in inheritance (moved from the companion site back into the main text, and updated to cover Scala traits and Java 8 default methods); multicore processors (pervasive changes to Chapter 13); phasers (Section 13.3.1); memory models (Section 13.3.3); semaphores (Section13.3.5); futures (Section 13.4.5); GIMPLE and RTL (Section C-15.2.1); QEMU (Section 16.2.2); DWARF (Section 16.3.2); and language genealogy (Figure A.1). 

To accommodate new material, coverage of some topics has been condensed or even removed.  Examples include modules (Chapters 3 and 10), variant records and with statements (Chapter 8), and metacircular interpretation (Chapter 11).  Additional material&emdash;the Common Language Infrastructure (CLI) in particular&emdash;has moved to the companion site.  Throughout the text, examples drawn from languages no longer in widespread use have been replaced with more recent equivalents wherever appropriate.  Almost all remaining references to Pascal and Modula are merely historical.  Most coverage of Occam and Tcl has also been dropped. 

Overall, the printed text has grown by roughly 40 pages.  There are 5 more “Design & Implementation” sidebars, 35 more numbered examples, and about 25 new end-of-chapter exercises and explorations.  Considerable effort has been invested in creating a consistent and comprehensive index.  As in earlier editions, Morgan Kaufmann has maintained its commitment to providing definitive texts at reasonable cost: PLP-4e is far less expensive than competing alternatives, but larger and more comprehensive. 

The Companion Site

To minimize the physical size of the text, make way for new material, and allow students to focus on the fundamentals when browsing, over 350 pages of more advanced or peripheral material can be found on a companion web site:  Each companion-site (CS) section is represented in the main text by a brief introduction to the subject and an “In More Depth” paragraph that summarizes the elided material. 

Note that placement of material on the companion site does not constitute a judgment about its technical importance.  It simply reflects the fact that there is more material worth covering than will fit in a single volume or a single-semester course.  Since preferences and syllabi vary, most instructors will probably want to assign reading from the CS, and most will refrain from assigning certain sections of the printed text.  My intent has been to retain in print the material that is likely to be covered in the largest number of courses.  Also included on the CS are pointers to on-line resources and compilable copies of all significant code fragments found in the text (in more than two dozen languages). 

Design & Implementation Sidebars

Like its predecessors, PLP-4e places heavy emphasis on the ways in which language design constrains implementation options, and the ways in which anticipated implementations have influenced language design.  Many of these connections and interactions are highlighted in some 140 “Design & Implementation” sidebars.  A more detailed introduction appears in Sidebar 1.1.  A numbered list appears in Appendix B. 

Numbered and Titled Examples

Examples in PLP-4e are intimately woven into the flow of the presentation.  To make it easier to find specific examples, to remember their content, and to refer to them in other contexts, a number and a title for each is displayed in a marginal note.  There are over 1000 such examples across the main text and the CS.  A detailed list appears in Appendix C. 

Exercise Plan

Review questions appear throughout the text at roughly 10-page intervals, at the ends of major sections.  These are based directly on the preceding material, and have short, straightforward answers. 

More detailed questions appear at the end of each chapter.  These are divided into Exercises and Explorations.  The former are generally more challenging than the per-section review questions, and should be suitable for homework or brief projects.  The latter are more open-ended, requiring web or library research, substantial time commitment, or the development of subjective opinion.  Solutions to many of the exercises (but not the explorations) are available to registered instructors from a password-protected web site: visit

How to Use the Book

Programming Language Pragmatics covers almost all of the material in the PL “knowledge units” of the Computer Science Curricula 2013 report.  The languages course at the University of Rochester, for which this book was designed, is in fact one of the featured “course exemplars” in the report (pp. 369–371).  Figure 1 illustrates several possible paths through the text. 

Chart of paths through the text

Figure 1  Paths through the text.  Darker shaded regions indicate supplemental “In More Depth” sections on the companion site.  Section numbers are shown for breaks that do not correspond to supplemental material. 

For self-study, or for a full-year course (track F in Figure 1), I recommend working through the book from start to finish, turning to the companion site as each “In More Depth” section is encountered.  The one-semester course at Rochester (track R) also covers most of the book, but leaves out most of the CS sections, as well as bottom-up parsing (2.3.4), logic languages (Chapter 12), and the second halves of Chapters 15 (Building a Runnable Program) and 16 (Run-time Program Management).  Note that the material on functional programming (Chapter 11 in particular) can be taught in either OCaml or Scheme. 

Some chapters (2, 4, 5, 15, 16, 17) have a heavier emphasis than others on implementation issues.  These can be reordered to a certain extent with respect to the more design-oriented chapters.  Many students will already be familiar with much of the material in Chapter 5, most likely from a course on computer organization; hence the placement of the chapter on the companion site.  Some students may also be familiar with some of the material in Chapter 2, perhaps from a course on automata theory.  Much of this chapter can then be read quickly as well, pausing perhaps to dwell on such practical issues as recovery from syntax errors, or the ways in which a scanner differs from a classical finite automaton. 

A traditional programming languages course (track P in Figure 1) might leave out all of scanning and parsing, plus all of Chapter 4.  It would also de-emphasize the more implementation-oriented material throughout.  In place of these, it could add such design-oriented CS sections as multiple inheritance (10.6), Smalltalk (10.7.1), lambda calculus (11.7), and predicate calculus (12.3). 

PLP has also been used at some schools for an introductory compiler course (track C in Figure 1).  The typical syllabus leaves out most of Part III (Chapters 11 through 14), and de-emphasizes the more design-oriented material throughout.  In place of these, it includes all of scanning and parsing, Chapters 15 through 17, and a slightly different mix of other CS sections. 

For a school on the quarter system, an appealing option is to offer an introductory one-quarter course and two optional follow-on courses (track Q in Figure 1).  The introductory quarter might cover the main (non-CS) sections of Chapters 1, 3, 6, 7, and 8, plus the first halves of Chapters 2 and 9.  A language-oriented follow-on quarter might cover the rest of Chapter 9, all of Part III, CS sections from Chapters 6 through 9, and possibly supplemental material on formal semantics, type theory, or other related topics.  A compiler-oriented follow-on quarter might cover the rest of Chapter 2; Chapters 4–5 and 15–17 CS sections from Chapters 3 and 9–10, and possibly supplemental material on automatic code generation, aggressive code improvement, programming tools, and so on. 

Whatever the path through the text, I assume that the typical reader has already acquired significant experience with at least one imperative language.  Exactly which language it is shouldn’t matter.  Examples are drawn from a wide variety of languages, but always with enough comments and other discussion that readers without prior experience should be able to understand easily.  Single-paragraph introductions to more than 60 different languages appear in Appendix A.  Algorithms, when needed, are presented in an informal pseudocode that should be self-explanatory.  Real programming language code is set in "typewriter" font.  Pseudocode is set in a sans-serif font

Supplemental Materials

In addition to supplemental sections, the companion site contains complete source code for all nontrivial examples, and a list of all known errors in the book.  Additional resources are available on-line at  For instructors who have adopted the text, a password-protected page provides access to

Acknowledgments for the Fourth Edition

In preparing the fourth edition, I have been blessed with the generous assistance of a very large number of people.  Many provided errata or other feedback on the third edition, among them Yacine Belkadi, Björn Brandenburg, Bob Cochran, Daniel Crisman, Chen Ding, Peter Drake, Michael Edgar, Michael Glass, Sérgio Gomes, Allan Gottlieb, Hossein Hadavi, Chris Hart, Thomas Helmuth, Wayne Heym, Scott Hoge, Kelly Jones, Ahmed Khademzadeh, Eleazar Enrique Leal, Kyle Liddell, Annie Liu, Hao Luo, Dirk Müller, Holger Peine, Mikhail Prokharau, Harsh Raju, and Jingguo Yao.  I also remain indebted to the many individuals acknowledged in previous editions, and to the reviewers, adopters, and readers who made those editions a success. 

Anonymous reviewers for the fourth edition provided a wealth of useful suggestions; my thanks to all of you! Special thanks to Adam Chlipala of MIT for his detailed and insightful suggestions on the coverage of functional programming.  My thanks as well to Nelson Beebe (University of Utah) for pointing out that compilers cannot safely use integer comparisons for floating-point numbers that may be NaNs; to Dan Scarafoni for prompting me to distinguish between FIRST/EPS of symbols and FIRST/EPS of strings in the algorithm to generate PREDICT sets; to Dave Musicant for suggested improvements to the description of deep binding; to Allan Gottlieb (NYU) for several key clarifications regarding Ada semantics; and to Benjamin Kowarsch for similar clarifications regarding Objective-C.  Problems that remain in all these areas are entirely my own. 

In preparing the fourth edition, I have drawn on 25 years of experience teaching this material to upper-level undergraduates at the University of Rochester.  I am grateful to all my students for their enthusiasm and feedback.  My thanks as well to my colleagues and graduate students, and to the department’s administrative, secretarial, and technical staff for providing such a supportive and productive work environment.  Finally, my thanks to David Padua, whose work I have admired since I was in graduate school; I am deeply honored to have him as the author of the Foreword. 

As they were on previous editions, the staff at Morgan Kaufmann has been a genuine pleasure to work with, on both a professional and a personal level.  My thanks in particular to Nate McFadden, Senior Development Editor, who shepherded both this and the previous two editions with unfailing patience, good humor, and a fine eye for detail; to Mohana Natarajan, who managed the book’s production; and to Todd Green, Publisher, who upholds the personal touch of the Morgan Kauffman imprint within the larger Elsevier universe. 

Most important, I am indebted to my wife, Kelly, for her patience and support through endless months of writing and revising.  Computing is a fine profession, but family is what really matters. 

Michael L. Scott      
Rochester, NY      
August 2015      

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