Formal Syntax and Semantics of Programming Languages

654 Pages·1999·1.62 MB·English

Formal Syntax
and
Semantics of
Programming Languages
A Laboratory Based Approach
Kenneth Slonneger
University of Iowa
Barry L. Kurtz
Louisiana Tech University
Addison-Wesley Publishing Company
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The procedures and applications presented in this book have been included
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Library of Congr ess Cataloging-in-Publication Data
Slonneger, Kenneth.
Formal syntax and semantics of programming languages: a laboratory
based approach / Kenneth Slonneger, Barry L. Kurtz.
p.cm.
Includes bibliographical references and index.
ISBN 0-201-65697-3
1.Pr ogramming languages (Electronic computers)--Syntax.
2.Pr ogramming languages (Electronic computers)--Semantics.
I. Kurtz, Barry L. II. Title.
QA76.7.S59 1995
005.13'1--dc20 94-4203
CIP
Reproduced by Addison-Wesley from camera-ready copy supplied by the
authors.
Copyright © 1995 by Addison-Wesley Publishing Company, Inc.
All rights reserved. No part of this publication may be reproduced, stored in
a retrieval system, or transmitted, in any form or by any means, electronic,
mechanical, photocopying, recording, or otherwise, without the prior written
permission of the publisher. Printed in the United States of America.
ISBN 0-201-65697-3
1234 5 6 7 8 9 10-MA-979695
Dedications
To my father, Robert
Barry L. Kurtz
To Marybeth and my family
Ken Slonneger
Preface
This text developed out of our experiences teaching courses covering the
formal semantics of programming languages. Independently we both devel-
oped laboratory exercises implementing small programming languages in
Prolog following denotational definitions. Prolog proved to be an excellent
tool for illustrating the formal semantics of programming languages. We
found that these laboratory exercises were highly successful in motivating
students since the hands-on experience helped demystify the study of for-
mal semantics. At a professional meeting we became aware of each other’s
experiences with a laboratory approach to semantics, and this book evolved
from that conference.
Although this text has been carefully written so that the laboratory activities
can be omitted without loss of continuity, we hope that most readers will try
the laboratory approach and experience the same success that we have ob-
served in our classes.
Overall Goals
We have pursued a broad spectrum of definitional techniques, illustrated
with numerous examples. Although the specification methods are formal,
the presentation is “gentle”, providing just enough in the way of mathemati-
cal underpinnings to produce an understanding of the metalanguages. We
hope to supply enough coverage of mathematics and formal methods to jus-
tify the definitional techniques, but the text is accessible to students with a
basic grounding in discrete mathematics as presented to undergraduate
computer science students.
There has been a tendency in the area of formal semantics to create cryptic,
overly concise semantic definitions that intimidate students new to the study
of programming languages. The emphasis in this text is on clear notational
conventions with the goals of readability and understandability foremost in
our minds.
As with other textbooks in this field, we introduce the basic concepts using
mini-languages that are rich enough to illustrate the fundamental concepts,
yet sparse enough to avoid being overwhelming. We have named our mini-
languages after birds.
v
vi PREFACE
Wren is a simple imperative language with two types, integer and Bool-
ean, thus allowing for context-sensitive type and declaration checking. It
has assignment, if, while, and input/output commands.
Pelican, a block-structured, imperative language, is an extension of Wren
containing the declaration of constants, anonymous blocks, procedures,
and recursive definitions.
The description of continuations in denotational semantics requires a modi-
fied version of Wren with goto statements, which we call Gull. This mini-
language can be skipped without loss of continuity if continuations are not
covered.
Organization of the Text
The primary target readership of our text is first-year graduate students,
although by careful selection of materials it is also accessible to advanced
undergraduate students. The text contains more material than can be cov-
ered in a one semester course. We have provided a wide variety of tech-
niques so that instructors may choose materials to suit the particular needs
of their students.
Dependencies between chapters are indicated in the graph below. We have
purposely attempted to minimize mutual interdependencies and to make
our presentation as broad as possible.
1
2 3 4 5 6 8 9 11 12
7 8 10 13
Only sections 2 and 3 of Chapter 8 depend on Chapter 5. The text contains
a laboratory component that we describe in more detail in a moment. How-
ever, materials have been carefully organized so that no components of the
non-laboratory sections of the text are dependent on any laboratory activi-
PREFACE vii
ties. All of the laboratory activities except those in Chapter 6 depend on
Chapter 2.
Overview
The first four chapters deal primarily with the syntax of programming lan-
guages. Chapter 1 treats context-free syntax in the guise of BNF grammars
and their variants. Since most methods of semantic specification use ab-
stract syntax trees, the abstract syntax of languages is presented and con-
trasted with concrete syntax.
Language processing with Prolog is introduced in Chapter 2 by describing a
scanner for Wren and a parser defined in terms of Prolog logic grammars.
These utilities act as the front end for the prototype context checkers, inter-
preters, and translators developed later in the text. Extensions of BNF gram-
mars that provide methods of verifying the context-sensitive aspects of pro-
gramming languages—namely, attribute grammars and two-level grammars—
are described in Chapters 3 and 4.
Chapters 5 through 8 are devoted to semantic formalisms that can be clas-
sified as operational semantics. Chapter 5 introduces the lambda calculus
by describing its syntax and the evaluation of lambda expressions by reduc-
tion rules. Metacircular interpreters are consider in Chapter 6, which intro-
duces the self-definition of programming languages.
Chapter 7 describes the translation of Wren into assembly language using
an attribute grammar that constructs the target code as a program is parsed.
Two well-known operational formalisms are treated in Chapter 8: the SECD
machine—an abstract machine for evaluating the lambda calculus—and
structural operational semantics—an operational methodology for describ-
ing the semantics of programming languages in terms of logical rules of
inference. We use this technique to specify the semantics of Wren formally.
The last five chapters present three traditional methods of defining the se-
mantics of programming languages formally and one recently proposed tech-
nique. Denotational semantics, one of the most complete and successful
methods of specifying a programming language, is covered in Chapter 9.
Specifications of several languages are provided, including a calculator lan-
guage, Wren, Pelican, and Gull, a language whose semantics requires con-
tinuation semantics. Denotational semantics is also used to check the con-
text constraints for Wren. Chapter 10 deals with the mathematical founda-
tions of denotational semantics in domain theory by describing the data
structures employed by denotational definitions. Chapter 10 also includes a
justification for recursive definitions via fixed-point semantics, which is then
applied in lambda calculus evaluation.
viii PREFACE
Axiomatic semantics, dealt with in Chapter 11, has become an important
component of software development by means of proofs of correctness for
algorithms. The approach here presents axiomatic specifications of Wren
and Pelican, but the primary examples involve proofs of partial correctness
and termination. The chapter concludes with a brief introduction to using
assertions as program specifications and deriving program code based on
these assertions. Chapter 12 investigates the algebraic specification of ab-
stract data types and uses these formalisms to specify the context constraints
and the semantics of Wren. Algebraic semantics also provides an explana-
tion of abstract syntax.
Chapter 13 introduces a specification method, action semantics, that has
been proposed recently in response to criticisms arising from the difficulty
of using formal methods. Action semantics resembles denotational seman-
tics but can be viewed in terms of operational behavior without sacrificing
mathematical rigor. We use it to specify the semantics of the calculator lan-
guage, Wren, and Pelican. The text concludes with two short appendices
introducing the basics of programming in Prolog and Scheme, which is used
in Chapter 6.
The Laboratory Component
A unique feature of this text is the laboratory component. Running through-
out the text is a series of exercises and examples that involve implementing
syntactic and semantic specifications on real systems. We have chosen Prolog
as the primary vehicle for these implementations for several reasons:
1. Prolog provides high-level programming enabling the construction of deri-
vation trees and abstract syntax trees as structures without using pointer
programming as needed in most imperative languages.
2. Most Prolog systems provide a programming environment that is easy to
use, especially in the context of rapid prototyping; large systems can be
developed one predicate at a time and can be tested during their con-
struction.
3. Logic programming creates a framework for drawing out the logical prop-
erties of abstract specifications that encourages students to approach
problems in a disciplined and logical manner. Furthermore, the specifi-
cations described in logic become executable specifications with Prolog.
4. Prolog’s logic grammars provide a simple-to-use parser that can serve as
a front end to language processors. It also serves as a direct implemen-
tation of attribute grammars and provides an immediate application of
BNF specifications of the context-free part of a language’s grammar.
PREFACE ix
An appendix covering the basics of Prolog is provided for students unfamil-
iar with logic programming.
Our experience has shown that the laboratory practice greatly enhances the
learning experience. The only way to master formal methods of language
definition is to practice writing and reading language specifications. We in-
volve students in the implementation of general tools that can be applied to
a variety of examples and that provide increased motivation and feedback to
the students. Submitting specifications to a prototyping system can un-
cover oversights and subtleties that are not apparent to a casual reader. As
authors, we have frequently used these laboratory approaches to help “de-
bug” our formal specifications!
Laboratory materials found in this textbook are available on the Internet via
anonymous ftp from ftp.cs.uiowa.edu in the subdirectory pub/slonnegr.
Laboratory Activities
Chapter 2: Scanning and parsing Wren
Chapter 3: Context checking Wren using an attribute grammar
Chapter 4: Context checking Hollerith literals using a two-level grammar
Chapter 5: Evaluating the lambda calculus using its reduction rules
Chapter 6: Self-definition of Scheme (Lisp)
Self-definition of Prolog
Chapter 7: Translating (compiling) Wren programs following an attribute
grammar
Chapter 8: Interpreting the lambda calculus using the SECD machine
Interpreting Wren according to a definition using structural
operational semantics
Chapter 9: Interpreting Wren following a denotational specification
Chapter 10: Evaluating a lambda calculus that includes recursive defini-
tions
Chapter 12: Interpreting Wren according to an algebraic specification of
the language
Chapter 13: Translating Pelican programs into action notation following a
specification in action semantics.
x PREFACE
Acknowledgments
We would like to thank Addison-Wesley for their support in developing this
text—in particular, Tom Stone, senior editor for Computer Science, Kathleen
Billus, assistant editor, Marybeth Mooney, production coordinator, and the
many other people who helped put this text together.
We would like to acknowledge the following reviewers for their valuable feed-
back that helped us improve the text: Doris Carver (Louisiana State Univer-
sity), Art Fleck (University of Iowa), Ray Ford (University of Montana), Phokion
Kolaitis (Santa Cruz), William Purdy (Syracuse University), and Roy Rubinstein
(Worcester Polytech). The comments and suggestions of a number of stu-
dents contributed substantially to the text; those students include Matt Clay,
David Frank, Sun Kim, Kent Lee, Terry Letsche, Sandeep Pal, Ruth Ruei,
Matt Tucker, and Satish Viswanantham.
We used Microsoft Word and Aldus PageMaker for the Macintosh to develop
this text. We owe a particular debt to the Internet, which allowed us to ex-
change and develop materials smoothly. Finally, we each would like to thank
our respective family members whose encouragement and patience made
this text possible.
Ken Slonneger
Barry L. Kurtz
Contents
Chapter 1
SPECIFYING SYNTAX 1
1.1 GRAMMARS AND BNF 2
Context-Free Grammars 4
Context-Sensitive Grammars 8
Exercises 8
1.2 THE PROGRAMMING LANGUAGE WREN 10
Ambiguity 12
Context Constraints in Wren 13
Semantic Errors in Wren 15
Exercises 16
1.3 VARIANTS OF BNF 18
Exercises 20
1.4 ABSTRACT SYNTAX 21
Abstract Syntax Trees 21
Abstract Syntax of a Programming Language 23
Exercises 29
1.5 FURTHER READING 30
Chapter 2
INTRODUCTION TO LABORATORY ACTIVITIES 31
2.1 SCANNING 33
Exercises 39
2.2 LOGIC GRAMMARS 40
Motivating Logic Grammars 41
Improving the Parser 44
Prolog Grammar Rules 46
Parameters in Grammars 47
Executing Goals in a Logic Grammar 49
Exercises 49
2.3 PARSING WREN 50
Handling Left Recursion 52
Left Factoring 55
Exercises 56
2.4 FURTHER READING 57
Chapter 3
ATTRIBUTE GRAMMARS 59
3.1 CONCEPTS AND EXAMPLES 59
Examples of Attribute Grammars 60
Formal Definitions 66
xi