Preface


Welcome

This electronic book is a course on Software Foundations, the mathematical underpinnings of reliable software. Topics include basic concepts of logic, computer-assisted theorem proving, the Coq proof assistant, functional programming, operational semantics, Hoare logic, and static type systems. The exposition is intended for a broad range of readers, from advanced undergraduates to PhD students and researchers. No specific background in logic or programming languages is assumed, though a degree of mathematical maturity will be helpful.
The principal novelty of the course is that it is one hundred percent formalized and machine-checked: the entire text is literally a script for Coq. It is intended to be read alongside an interactive session with Coq. All the details in the text are fully formalized in Coq, and the exercises are designed to be worked using Coq.
The files are organized into a sequence of core chapters, covering about one semester's worth of material and organized into a coherent linear narrative, plus a number of "appendices" covering additional topics. All the core chapters are suitable for both upper-level undergraduate and graduate students.

Overview

Building reliable software is hard. The scale and complexity of modern systems, the number of people involved in building them, and the range of demands placed on them render it extremely difficult to build software that is even more-or-less correct, much less 100% correct. At the same time, the increasing degree to which information processing is woven into every aspect of society continually amplifies the cost of bugs and insecurities.
Computer scientists and software engineers have responded to these challenges by developing a whole host of techniques for improving software reliability, ranging from recommendations about managing software projects and organizing programming teams (e.g., extreme programming) to design philosophies for libraries (e.g., model-view-controller, publish-subscribe, etc.) and programming languages (e.g., object-oriented programming, aspect-oriented programming, functional programming, ...) to mathematical techniques for specifying and reasoning about properties of software and tools for helping validate these properties.
The present course is focused on this last set of techniques. The text weaves together five conceptual threads:
(1) basic tools from logic for making and justifying precise claims about programs;
(2) the use of proof assistants to construct rigorous logical arguments;
(3) the idea of functional programming, both as a method of programming that simplifies reasoning about programs and as a bridge between programming and logic;
(4) formal techniques for reasoning about the properties of specific programs (e.g., the fact that a sorting function or a compiler obeys some formal specification); and
(5) the use of type systems for establishing well-behavedness guarantees for all programs in a given programming language (e.g., the fact that well-typed Java programs cannot be subverted at runtime).
Each of these topics is easily rich enough to fill a whole course in its own right, so tackling all of them together naturally means that much will be left unsaid. Nevertheless, we hope readers will find that the themes illuminate and amplify each other and that bringing them together creates a foundation from which it will be easy to dig into any of them more deeply. Some suggestions for further reading can be found in the Postscript chapter.

Logic

Logic is the field of study whose subject matter is proofs — unassailable arguments for the truth of particular propositions. Volumes have been written about the central role of logic in computer science. Manna and Waldinger called it "the calculus of computer science," while Halpern et al.'s paper On the Unusual Effectiveness of Logic in Computer Science catalogs scores of ways in which logic offers critical tools and insights. Indeed, they observe that "As a matter of fact, logic has turned out to be significiantly more effective in computer science than it has been in mathematics. This is quite remarkable, especially since much of the impetus for the development of logic during the past one hundred years came from mathematics."
In particular, the fundamental notion of inductive proofs is ubiquitous in all of computer science. You have surely seen them before, in contexts from discrete math to analysis of algorithms, but in this course we will examine them much more deeply than you have probably done so far.

Proof Assistants

The flow of ideas between logic and computer science has not been in just one direction: CS has also made important contributions to logic. One of these has been the development of software tools for helping construct proofs of logical propositions. These tools fall into two broad categories:
  • Automated theorem provers provide "push-button" operation: you give them a proposition and they return either true, false, or ran out of time. Although their capabilities are limited to fairly specific sorts of reasoning, they have matured tremendously in recent years and are used now in a huge variety of settings. Examples of such tools include SAT solvers, SMT solvers, and model checkers.
  • Proof assistants are hybrid tools that automate the more routine aspects of building proofs while depending on human guidance for more difficult aspects. Widely used proof assistants include Isabelle, Agda, Twelf, ACL2, PVS, and Coq, among many others.
This course is based around Coq, a proof assistant that has been under development, mostly in France, since 1983 and that in recent years has attracted a large community of users in both research and industry. Coq provides a rich environment for interactive development of machine-checked formal reasoning. The kernel of the Coq system is a simple proof-checker, which guarantees that only correct deduction steps are performed. On top of this kernel, the Coq environment provides high-level facilities for proof development, including powerful tactics for constructing complex proofs semi-automatically, and a large library of common definitions and lemmas.
Coq has been a critical enabler for a huge variety of work across computer science and mathematics:
  • As a platform for modeling programming languages, it has become a standard tool for researchers who need to describe and reason about complex language definitions. It has been used, for example, to check the security of the JavaCard platform, obtaining the highest level of common criteria certification, and for formal specifications of the x86 and LLVM instruction sets and programming languages such as C.
  • As an environment for developing formally certified software, Coq has been used, for example, to build CompCert, a fully-verified optimizing compiler for C, for proving the correctness of subtle algorithms involving floating point numbers, and as the basis for CertiCrypt, an environment for reasoning about the security of cryptographic algorithms.
  • As a realistic environment for functional programming with dependent types, it has inspired numerous innovations. For example, the Ynot project at Harvard embedded "relational Hoare reasoning" (an extension of the Hoare Logic we will see later in this course) in Coq.
  • As a proof assistant for higher-order logic, it has been used to validate a number of important results in mathematics. For example, its ability to include complex computations inside proofs made it possible to develop the first formally verified proof of the 4-color theorem. This proof had previously been controversial among mathematicians because part of it included checking a large number of configurations using a program. In the Coq formalization, everything is checked, including the correctness of the computational part. More recently, an even more massive effort led to a Coq formalization of the Feit-Thompson Theorem — the first major step in the classification of finite simple groups.
By the way, in case you're wondering about the name, here's what the official Coq web site says: "Some French computer scientists have a tradition of naming their software as animal species: Caml, Elan, Foc or Phox are examples of this tacit convention. In French, 'coq' means rooster, and it sounds like the initials of the Calculus of Constructions (CoC) on which it is based." The rooster is also the national symbol of France, and C-o-q are the first three letters of the name of Thierry Coquand, one of Coq's early developers.

Functional Programming

The term functional programming refers both to a collection of programming idioms that can be used in almost any programming language and to a family of programming languages designed to emphasize these idioms, including Haskell, OCaml, Standard ML, F#, Scala, Scheme, Racket, Common Lisp, Clojure, Erlang, and Coq.
Functional programming has been developed over many decades — indeed, its roots go back to Church's lambda-calculus, which was invented in the 1930s, before there were even any computers! But since the early '90s it has enjoyed a surge of interest among industrial engineers and language designers, playing a key role in high-value systems at companies like Jane St. Capital, Microsoft, Facebook, and Ericsson.
The most basic tenet of functional programming is that, as much as possible, computation should be pure, in the sense that the only effect of execution should be to produce a result: the computation should be free from side effects such as I/O, assignments to mutable variables, redirecting pointers, etc. For example, whereas an imperative sorting function might take a list of numbers and rearrange its pointers to put the list in order, a pure sorting function would take the original list and return a new list containing the same numbers in sorted order.
One significant benefit of this style of programming is that it makes programs easier to understand and reason about. If every operation on a data structure yields a new data structure, leaving the old one intact, then there is no need to worry about how that structure is being shared and whether a change by one part of the program might break an invariant that another part of the program relies on. These considerations are particularly critical in concurrent programs, where every piece of mutable state that is shared between threads is a potential source of pernicious bugs. Indeed, a large part of the recent interest in functional programming in industry is due to its simpler behavior in the presence of concurrency.
Another reason for the current excitement about functional programming is related to the first: functional programs are often much easier to parallelize than their imperative counterparts. If running a computation has no effect other than producing a result, then it does not matter where it is run. Similarly, if a data structure is never modified destructively, then it can be copied freely, across cores or across the network. Indeed, the "Map-Reduce" idiom, which lies at the heart of massively distributed query processors like Hadoop and is used by Google to index the entire web is a classic example of functional programming.
For this course, functional programming has yet another significant attraction: it serves as a bridge between logic and computer science. Indeed, Coq itself can be viewed as a combination of a small but extremely expressive functional programming language plus with a set of tools for stating and proving logical assertions. Moreover, when we come to look more closely, we find that these two sides of Coq are actually aspects of the very same underlying machinery — i.e., proofs are programs.

Program Verification

Approximately the first third of the book is devoted to developing the conceptual framework of logic and functional programming and gaining enough fluency with Coq to use it for modeling and reasoning about nontrivial artifacts. From this point on, we increasingly turn our attention to two broad topics of critical importance to the enterprise of building reliable software (and hardware): techniques for proving specific properties of particular programs and for proving general properties of whole programming languages.
For both of these, the first thing we need is a way of representing programs as mathematical objects, so we can talk about them precisely, together with ways of describing their behavior in terms of mathematical functions or relations. Our tools for these tasks are abstract syntax and operational semantics, a method of specifying programming languages by writing abstract interpreters. At the beginning, we work with operational semantics in the so-called "big-step" style, which leads to somewhat simpler and more readable definitions when it is applicable. Later on, we switch to a more detailed "small-step" style, which helps make some useful distinctions between different sorts of "nonterminating" program behaviors and is applicable to a broader range of language features, including concurrency.
The first programming language we consider in detail is Imp, a tiny toy language capturing the core features of conventional imperative programming: variables, assignment, conditionals, and loops. We study two different ways of reasoning about the properties of Imp programs.
First, we consider what it means to say that two Imp programs are equivalent in the intuitive sense that they yield the same behavior when started in any initial memory state. This notion of equivalence then becomes a criterion for judging the correctness of metaprograms — programs that manipulate other programs, such as compilers and optimizers. We build a simple optimizer for Imp and prove that it is correct.
Second, we develop a methodology for proving that particular Imp programs satisfy formal specifications of their behavior. We introduce the notion of Hoare triples — Imp programs annotated with pre- and post-conditions describing what should be true about the memory in which they are started and what they promise to make true about the memory in which they terminate — and the reasoning principles of Hoare Logic, a "domain-specific logic" specialized for convenient compositional reasoning about imperative programs, with concepts like "loop invariant" built in.
This part of the course is intended to give readers a taste of the key ideas and mathematical tools used in a wide variety of real-world software and hardware verification tasks.

Type Systems

Our final major topic, covering approximately the last third of the course, is type systems, a powerful set of tools for establishing properties of all programs in a given language.
Type systems are the best established and most popular example of a highly successful class of formal verification techniques known as lightweight formal methods. These are reasoning techniques of modest power — modest enough that automatic checkers can be built into compilers, linkers, or program analyzers and thus be applied even by programmers unfamiliar with the underlying theories. Other examples of lightweight formal methods include hardware and software model checkers, contract checkers, and run-time property monitoring techniques for detecting when some component of a system is not behaving according to specification.
This topic brings us full circle: the language whose properties we study in this part, the simply typed lambda-calculus, is essentially a simplified model of the core of Coq itself!

Practicalities


Chapter Dependencies

A diagram of the dependencies between chapters and some suggested paths through the material can be found in the file deps.html.

System Requirements

Coq runs on Windows, Linux, and OS X. You will need:
  • A current installation of Coq, available from the Coq home page. Everything should work with version 8.4. (Version 8.5 will not work, due to a few incompatible changes in Coq between 8.4 and 8.5.)
  • An IDE for interacting with Coq. Currently, there are two choices:
    • Proof General is an Emacs-based IDE. It tends to be preferred by users who are already comfortable with Emacs. It requires a separate installation (google "Proof General").
    • CoqIDE is a simpler stand-alone IDE. It is distributed with Coq, so it should "just work" once you have Coq installed. It can also be compiled from scratch, but on some platforms this may involve installing additional packages for GUI libraries and such.

Exercises

Each chapter includes numerous exercises. Each is marked with a "star rating," which can be interpreted as follows:
  • One star: easy exercises that underscore points in the text and that, for most readers, should take only a minute or two. Get in the habit of working these as you reach them.
  • Two stars: straightforward exercises (five or ten minutes).
  • Three stars: exercises requiring a bit of thought (ten minutes to half an hour).
  • Four and five stars: more difficult exercises (half an hour and up).
Also, some exercises are marked "advanced", and some are marked "optional." Doing just the non-optional, non-advanced exercises should provide good coverage of the core material. Optional exercises provide a bit of extra practice with key concepts and introduce secondary themes that may be of interest to some readers. Advanced exercises are for readers who want an extra challenge (and, in return, a deeper contact with the material).
Please do not post solutions to the exercises in any public place: Software Foundations is widely used both for self-study and for university courses. Having solutions easily available makes it much less useful for courses, which typically have graded homework assignments. The authors especially request that readers not post solutions to the exercises anyplace where they can be found by search engines.

Downloading the Coq Files

A tar file containing the full sources for the "release version" of these notes (as a collection of Coq scripts and HTML files) is available here:
        http://www.cis.upenn.edu/~bcpierce/sf
If you are using the notes as part of a class, you may be given access to a locally extended version of the files, which you should use instead of the release version.

Note for Instructors

If you intend to use these materials in your own course, you will undoubtedly find things you'd like to change, improve, or add. Your contributions are welcome!
Please send an email to Benjamin Pierce describing yourself and how you would like to use the materials, and including the result of doing "htpasswd -s -n NAME", where NAME is your preferred user name. We'll set you up with read/write access to our subversion repository and developers' mailing list; in the repository you'll find a README with further instructions.

Translations

Thanks to the efforts of a team of volunteer translators, Software Foundations can now be enjoyed in Japanese at http://proofcafe.org/sf. A Chinese translation is underway.