Welcome
Here's a good way to build formally verified correct software:
- Write your program in an expressive language with a good proof theory
(the Gallina language embedded in Coq's logic).
- Prove it correct in Coq.
- Compile it with an optimizing ML compiler.
Since you want your programs to be
efficient, you'll want to
implement sophisticated data structures and algorithms. Since
Gallina is a
purely functional language, it helps to have
purely functional algorithms.
In this volume you will learn how to specify and verify (prove the
correctness of) sorting algorithms, binary search trees, balanced
binary search trees, and priority queues. Before using this book,
you should have some understanding of these algorithms and data
structures, available in any standard undergraduate algorithms
textbook.
This electronic book is Volume 3 of the
Software Foundations
series, which presents the mathematical underpinnings of reliable
software. It builds on
Software Foundations Volume 1
(Logical Foundations), but does not depend on Volume 2.
The exposition here is intended for a broad range of readers, from
advanced undergraduates to PhD students and researchers.
The principal novelty of
Software Foundations 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.
Practicalities
Chapter Dependencies
Before using
Verified Functional Algorithms, read
(and do the exercises in) these chapters of
Software Foundations Volume I:
Preface, Basics, Induction, Lists, Poly, Tactics, Logic,
IndProp, Maps, and perhaps (ProofObjects), (IndPrinciples).
In this volume, the core path is:
Preface -> Perm -> Sort -> SearchTree -> Extract -> Redblack
with many optional chapters whose dependencies are,
The
Color chapter is advanced material that should not be
attempted until the student has had experience with most
of the earlier chapters, or other experience using Coq.
System Requirements
Coq runs on Windows, Linux, and OS X. The Preface of Volume 1
describes the Coq installation you will need. This edition was
built with Coq 8.17.
In addition, two of the chapters ask you to compile and run an
OCaml program; having OCaml installed on your computer is helpful,
but not essential.
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 this book (as a collection of Coq scripts and HTML files) is
available at
https://softwarefoundations.cis.upenn.edu.
(If you are using the book as part of a class, your professor may
give you access to a locally modified version of the files, which
you should use instead of the release version.)
Lecture Videos
Lectures on for an intensive summer course based on some chapters
of this book at the DeepSpec summer school in 2017 can be
found at
https://deepspec.org/event/dsss17/lecture_appel.html.
For Instructors and Contributors
If you plan 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 see the
Preface
to
Logical Foundations for instructions.
Recommended Citation Format
If you want to refer to this volume in your own writing, please
do so as follows:
@book {Appel:SF3,
author = {Andrew W. Appel},
editor = {Benjamin C. Pierce},
title = "Verified Functional Algorithms",
series = "Software Foundations",
volume = "3",
year = "2024",
publisher = "Electronic textbook",
note = {Version 1.5.4, \URL{http://softwarefoundations.cis.upenn.edu} },
}
