Undergraduate Topics in Computer Science
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Gilles Dowek

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Undergraduate Topics in Computer Science

Undergraduate Topics in Computer Science (UTiCS) delivers high-quality instructional content for undergraduates studying in all areas of computing and information science. From core foundational and theoretical material to ﬁnal-year topics and applications, UTiCS books take a fresh, concise, and modern approach and are ideal for self-study or for a one- or two-semester course. The texts are all authored by established experts in their ﬁelds, reviewed by an international advisory board, and contain numerous examples and problems. Many include fully worked solutions.

Also in this series Iain D. Craig Object-Oriented Programming Languages: Interpretation 978-1-84628-773-2 Max Bramer Principles of Data Mining 978-1-84628-765-7 Hanne Riis Nielson and Flemming Nielson Semantics with Applications: An Appetizer 978-1-84628-691-9 Michael Kifer and Scott A. Smolka Introduction to Operating System Design and Implementation: The OSP 2 Approcah 978-1-84628-842-5 Phil Brooke and Richard Paige Practical Distributed Processing 978-1-84628-840-1 Frank Klawonn Computer Graphics with Java 978-1-84628-847-0 David Salomon A Concise Introduction to Data Compression 978-1-84800-071-1 David Makinson Sets, Logic and Maths for Computing 978-1-84628-844-9 Orit Hazzan Agile Software Engineering 978-1-84800-198-5 Pankaj Jalote A Concise Introduction to Software Engineering 978-1-84800-301-9 Alan P. Parkes A Concise Introduction to Languages and Machines 978-1-84800-120-6

Gilles Dowek

Principles of Programming Languages

123

Gilles Dowek École Polytechnique France Series editor Ian Mackie, École Polytechnique, France Advisory board Samson Abramsky, University of Oxford, UK Chris Hankin, Imperial College London, UK Dexter Kozen, Cornell University, USA Andrew Pitts, University of Cambridge, UK Hanne Riis Nielson, Technical University of Denmark, Denmark Steven Skiena, Stony Brook University, USA Iain Stewart, University of Durham, UK David Zhang, The Hong Kong Polytechnic University, Hong Kong

Undergraduate Topics in Computer Science ISSN 1863-7310 ISBN: 978-1-84882-031-9 e-ISBN: 978-1-84882-032-6 DOI: 10.1007/978-1-84882-032-6 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Control Number: 2008943965 Based on course notes by Gilles Dowek published in 2006 by L’Ecole Polytechnique with the following title: “Les principes des langages de programmation.” c Springer-Verlag London Limited 2009 Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licenses issued by the Copyright Licensing Agency. Enquiries concerning reproduction outside those terms should be sent to the publishers. The use of registered names, trademarks, etc., in this publication does not imply, even in the absence of a speciﬁc statement, that such names are exempt from the relevant laws and regulations and therefore free for general use. The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made. Printed on acid-free paper Springer Science+Business Media springer.com

The author wants to thank François Pottier, Philippe Baptiste, Julien Cervelle, Albert Cohen, Olivier Delande, Olivier Hermant, Ian Mackie, François Morain, Jean-Marc Steyaert and Paul Zimmermann for their remarks on a ﬁrst version of this book.

Preface

We’ve known about algorithms for millennia, but we’ve only been writing computer programs for a few decades. A big diﬀerence between the Euclidean or Eratosthenes age and ours is that since the middle of the twentieth century, we express the algorithms we conceive using formal languages: programming languages. Computer scientists are not the only ones who use formal languages. Optometrists, for example, prescribe eyeglasses using very technical expressions, such as “OD: -1.25 (-0.50) 180◦ OS: -1.00 (-0.25) 180◦ ”, in which the parentheses are essential. Many such formal languages have been created throughout history: musical notation, algebraic notation, etc. In particular, such languages have long been used to control machines, such as looms and cathedral chimes. However, until the appearance of programming languages, those languages were only of limited importance: they were restricted to specialised ﬁelds with only a few specialists and written texts of those languages remained relatively scarce. This situation has changed with the appearance of programming languages, which have a wider range of applications than the prescription of eyeglasses or the control of a loom, are used by large communities, and have allowed the creation of programs of many hundreds of thousands of lines. The appearance of programming languages has allowed the creation of artiﬁcial objects, programs, of a complexity incomparable to anything that has come before, such as steam engines or radios. These programs have, in return, allowed the creation of other complex objects, such as integrated circuits made of millions of transistors, or mathematical proofs that are hundreds of thousands of pages long. It is very surprising that we have succeeded in writing such complex programs in languages comprising such a small number of constructs — assignment, loops, etc. — that is to say in languages barely more sophisticated than the language of prescription eyeglasses. vii

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Programs written in these programming languages have the novelty of not only being understandable by humans, which brings them closer to the scores used by organists, but also readable by machines, which brings them closer to the punch cards used in Barbarie organs. The appearance of programming languages has therefore profoundly impacted our relationship with language, complexity, and machines. This book is an introduction to the principles of programming languages. It uses the Java language for support. It is intended for students who already have some experience with computer programming. It is assumed that they have learned some programming empirically, in a single programming language, other than Java. The ﬁrst objective of this book will then be to learn the fundamentals of the Java programming language. However, knowing a single programming language is not suﬃcient to be a good programmer. For this, you must not only know several languages, but be able to easily learn new ones. This requires that you understand universal concepts like functions or cells, which exist in one form or another in all programming languages. This can only be done by comparing two or more languages. In this book, two comparison languages have been chosen: Caml and C. Therefore, the goal is not for the students to learn three programming languages simultaneously, but that with the comparison with Caml and C, they can learn the principles around which programming languages are created. This understanding will allow them to develop, if they wish, a real competence in Caml or in C, or in any other programming language. Another objective of this book is for the students to begin acquiring the tools which permit them to precisely deﬁne the meaning of the program. This precision is, indeed, the only means to clearly understand what happens when a program is executed, and to reason in situations where complexity deﬁes intuition. The idea is to describe the meaning of a statement by a function operating on a set of states. However, our expectations of this objective remain modest: students wishing to pursue this goal will have to do so elsewhere. The ﬁnal objective of this course is to learn basic algorithms for lists and trees. Here too, our expectations remain modest: students wishing to pursue this will also have to look elsewhere.

Contents

1.

Imperative Core . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Five Constructs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.1 Assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.2 Variable Declaration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.3 Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1.4 Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1.5 Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2 Input and Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.2.1 Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.2.2 Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.3 The Semantics of the Imperative Core . . . . . . . . . . . . . . . . . . . . . . . 8 1.3.1 The Concept of a State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.3.2 Decomposition of the State . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.3.3 A Visual Representation of a State . . . . . . . . . . . . . . . . . . . 10 1.3.4 The Value of Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 1.3.5 Execution of Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.

Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 The Concept of Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Avoiding Repetition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3 Return Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4 The return Construct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.5 Functions and Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.6 Global Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.7 The Main Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

19 19 19 21 22 23 24 25 25 ix

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2.1.8 Global Variables Hidden by Local Variables . . . . . . . . . . . . 2.1.9 Overloading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 The Semantics of Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 The Value of Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Execution of Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Order of Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Caml . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5 C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Expressions as Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Passing Arguments by Value and Reference . . . . . . . . . . . . . . . . . . 2.4.1 Pascal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Caml . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3 C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.4 Java . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

27 28 29 30 31 34 34 36 37 37 39 40 41 45

3.

Recursion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Calling a Function from Inside the Body of that Function . . . . . 3.2 Recursive Deﬁnitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Recursive Deﬁnitions and Circular Deﬁnitions . . . . . . . . . . 3.2.2 Recursive Deﬁnitions and Deﬁnitions by Induction . . . . . . 3.2.3 Recursive Deﬁnitions and Inﬁnite Programs . . . . . . . . . . . . 3.2.4 Recursive Deﬁnitions and Fixed Point Equations . . . . . . . 3.3 Caml . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Programming Without Assignment . . . . . . . . . . . . . . . . . . . . . . . . .

47 47 48 48 49 49 51 53 54 55

4.

Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Tuples with Named Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 The Deﬁnition of a Record Type . . . . . . . . . . . . . . . . . . . . . 4.1.2 Allocation of a Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3 Accessing Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.4 Assignment of Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.5 Constructors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.6 The Semantics of Records . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Sharing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Sharing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Equality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 Wrapper Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Caml . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Deﬁnition of a Record Type . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Creating a Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3 Accessing Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

59 59 60 60 62 62 64 65 66 66 68 68 73 73 73 74

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4.3.4 Assigning to Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Deﬁnition of a Record Type . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Creating a Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.3 Accessing Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.4 Assigning to Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1 Array Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2 Allocation of an Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.3 Accessing and Assigning to Fields . . . . . . . . . . . . . . . . . . . . 4.5.4 Arrays of Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.5 Arrays in Caml . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.6 Arrays in C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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74 76 76 76 77 77 79 79 80 80 82 83 84

5.

Dynamic Data Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 5.1 Recursive Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 5.1.1 Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 5.1.2 The null Value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 5.1.3 An Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 5.1.4 Recursive Deﬁnitions and Fixed Point Equations . . . . . . . 88 5.1.5 Inﬁnite Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 5.2 Disjunctive Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 5.3 Dynamic Data Types and Computability . . . . . . . . . . . . . . . . . . . . 92 5.4 Caml . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 5.5 C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 5.6 Garbage Collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 5.6.1 Inaccessible Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 5.6.2 Programming without Garbage Collection . . . . . . . . . . . . . 98 5.6.3 Global Methods of Memory Management . . . . . . . . . . . . . . 100 5.6.4 Garbage Collection and Functions . . . . . . . . . . . . . . . . . . . . 102

6.

Programming with Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 6.1 Finite Sets and Functions of a Finite Domain . . . . . . . . . . . . . . . . 103 6.1.1 Membership . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 6.1.2 Association Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 6.2 Concatenation: Modify or Copy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 6.2.1 Modify . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 6.2.2 Copy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 6.2.3 Using Recursion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 6.2.4 Chemical Reactions and Mathematical Functions . . . . . . . 111 6.3 List Inversion: an Extra Argument . . . . . . . . . . . . . . . . . . . . . . . . . . 112 6.4 Lists and Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

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6.5 Stacks and Queues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 6.5.1 Stacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 6.5.2 Queues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 6.5.3 Priority Queues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 7.

Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 7.1 Exceptional Circumstances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 7.2 Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 7.3 Catching Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 7.4 The Propagation of Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 7.5 Error Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 7.6 The Semantics of Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 7.7 Caml . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

8.

Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 8.1 Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 8.1.1 Functions as Part of a Type . . . . . . . . . . . . . . . . . . . . . . . . . 127 8.1.2 The Semantics of Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 8.2 Dynamic Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 8.3 Methods and Functional Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 8.4 Static Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 8.5 Static Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 8.6 Inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 8.7 Caml . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

9.

Programming with Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 9.1 Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 9.2 Traversing a Tree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 9.2.1 Depth First Traversal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 9.2.2 Breadth First Traversal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 9.3 Search Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 9.3.1 Membership . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 9.3.2 Balanced Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 9.3.3 Dictionaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 9.4 Priority Queues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 9.4.1 Partially Ordered Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 9.4.2 Partially Ordered Balanced Trees . . . . . . . . . . . . . . . . . . . . . 153

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

1 Imperative Core

1.1 Five Constructs Most programming languages have, among others, ﬁve constructs: assignment, variable declaration, sequence, test, and loop. These constructs form the imperative core of the language.

1.1.1 Assignment The assignment construct allows the creation of a statement with a variable x and an expression t. In Java, this statement is written as x = t;. Variables are identiﬁers which are written as one of more letters. Expressions are composed of variables and constants with operators, such as +, -, *, / — division — and % — modulo. Therefore, the following statements x = y % 3; x = y; y = 3; x = x + 1; G. Dowek, Principles of Programming Languages, Undergraduate Topics in Computer Science, DOI 10.1007/978-1-84882-032-6_1, c Springer-Verlag London Limited 2009

1

2

1. Imperative Core

are all proper Java statements, while y + 3 = x; x + 2 = y + 5; are not. To understand what happens when you execute the statement x = t; suppose that within the recesses of your computer’s memory, there is a compartment labelled x. Executing the statement x = t; consists of ﬁlling this compartment with the value of the expression t. The value previously contained in compartment x is erased. If the expression t is a constant, for example 3, its value is the same constant. If it is an expression with no variables, such as 3 + 4, its value is obtained by carrying out mathematical operations, in this case, addition. If expression t contains variables, the values of these variables must be looked up in the computer’s memory. The whole of the contents of the computer’s memory is called a state. Let us consider, initially, that expressions, such as x + 3, and statements, such as y = x + 3;, form two disjoint categories. Later, however, we shall be brought to revise this premise. In these examples, the values of expressions are integers. Computers can only store integers within a ﬁnite interval. In Java, integers must be between -231 and 231 - 1, so there are 232 possible values. When a mathematical operation produces a value outside of this interval, the result is kept within the interval by taking its modulo 232 remainder. Thus, by adding 1 to 231 - 1, that is to say 2147483647, we leave the interval and then return to it by removing 232 , which gives -231 or -2147483648.

Exercise 1.1 What is the value of the variable x after executing the following statement? x = 2 * 1500000000; In Caml, assignment is written x := t. In the expression t, we designate the value of x, not with the expression x itself, but with the expression !x. Thus, in Caml we write y := !x + 1 while in Java we write y = x + 1;. In C, assignment is written as it is in Java.

1.1 Five Constructs

3

1.1.2 Variable Declaration Before being able to assign values to a variable x, it must be declared, which associates the name x to a location in the computer’s memory. Variable declaration is a construct that allows the creation of a statement composed of a variable, an expression, and a statement. In Java, this statement is written {int x = t; p} where p is a statement, for example {int x = 4; x = x + 1;}. The variable x can then be used in the statement p, which is called the scope of variable x. It is also possible to declare a variable without giving it an initial value, for example, {int x; x = y + 4;}. We must of course be careful not to use a variable which has been declared without an initial value and that has not been assigned a value. This produces an error. Apart from the int type, Java has three other integer types that have diﬀerent intervals. These types are deﬁned in Table 1.1. When a mathematical operation produces a value outside of these intervals, the result is returned to the interval by taking its remainder, modulo the size of the interval. In Java, there are also other scalar types for decimal numbers, booleans, and characters. These types are deﬁned in Table 1.1. Operations allowed in the construction of expressions for each of these types are described in Table 1.2. Variables can also contain objects that are of composite types, like arrays and character strings, which we will address later. Because we will need them shortly, character strings are described brieﬂy in Table 1.3. The integers are of type byte, short, int or long corresponding to the intervals [-27 , 27 - 1], [-215 , 215 - 1], [-231 , 231 - 1] and [-263 , 263 - 1], Respectively. Constants are written in base 10, for example, -666. Decimal numbers are of type float or double. Constants are written in scientiﬁc notation, for example 3.14159, 666 or 6.02E23. Booleans are of type boolean. Constants are written as false and true. Characters are of type char. Constants are written between apostrophes, for example ‘b’. Table 1.1 Scalars types in Java

To declare a variable of type T, replace the type int with T. The general form of a declaration is thus {T x = t; p}.

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The basic operations that allow for arithmetical expressions are +, -, *, / — division — and % — modulo. When one of the numbers a or b is negative, the number a / b is the quotient rounded towards 0. So the result of a / b is the quotient of the absolute values of a and b, and is positive when a and b have the same sign, and negative if they have diﬀerent signs. The number a % b is a - b * (a / b). So (-29) / 4 equals -7 and (-29) % 4 equals -1. The operations for decimal numbers are +, -, *, /, along with some transcendental functions: Math.sin, Math.cos, ... The operations allowed in boolean expressions are ==, != — diﬀerent —, , =, & — and —, &&, | — or —, || and ! — not. For all data types, the expression (b) ? t : u evaluates to the value of t if the boolean expression b has the value true, and evaluates to the value of u if the boolean expression b has the value false. Table 1.2 Expressions in Java Character strings are of type String. Constants are written inside quotation marks, for example "Principles of Programming Languages". Table 1.3 Character strings in Java In Caml, variable declaration is written as let x = ref t in p and it isn’t necessary to explicitly declare the variable’s type. It is not possible in Caml to declare a variable without giving it an initial value. In C, like in Java, declaration is written {T x = t; p}. It is possible to declare a variable without giving it an initial value, and in this case, it could have any value. In Java and in C, it is impossible to declare the same variable twice, and the following program is not valid. int int int y =

y = 4; x = 5; x = 6; x;

In contrast, nothing in Caml stops you from writing

1.1 Five Constructs

5

let y = ref 4 in let x = ref 5 in let x = ref 6 in y := !x and this program assigns the value 6 to the variable y, so it is the most recent declaration of x that is used. We say that the ﬁrst declaration of x is hidden by the second. Java, Caml and C allow the creation of variables with an initial value that can never be changed. This type of variable is called a constant variable. A variable that is not constant is called a mutable variable. Java assumes that all variables are mutable unless you specify otherwise. To declare a constant variable in Java, you precede the variable type with the keyword final, for example final int x = 4; y = x + 1; The following statement is not valid, because an attempt is made to alter the value of a constant variable final int x = 4; x = 5; In Caml, to indicate that the variable x is a constant variable, write let x = t in p instead of writing let x = ref t in p. When using constant variables, you do not write !x to express its value, but simply x. So, you can write let x = 4 in y := x + 1, while the statement let x = 4 in x := 5 is invalid. In C, you indicate that a variable is a constant variable by preceding its type with the keyword const.

1.1.3 Sequence A sequence is a construct that allows a single statement to be created out of two statements p1 and p2 . In Java, a sequence is written as {p1 p2 }. The statement {p1 {p2 { ... pn } ...}} can also be written as {p1 p2 ... pn }. To execute the statement {p1 p2 } in the state s, the statement p1 is ﬁrst executed in the state s, which produces a new state s’. Then the statement p2 is executed in the state s’. In Caml, a sequence is written as p1 ; p2 . In C, it is written the same as it is in Java.

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1.1.4 Test A test is a construct that allows the creation of a statement composed of a boolean expression b and two statements p1 and p2 . In Java, this statement is written if (b) p1 else p2 . To execute the statement if (b) p1 else p2 in a state s, the value of expression b is ﬁrst computed in the state s, and depending on whether or not its value is true or false, the statement p1 or p2 is executed in the state s. In Caml, this statement is written if b then p1 else p2 . In C, it is written as it is in Java.

1.1.5 Loop A loop is a construct that allows the creation of a statement composed of a boolean expression b and a statement p. In Java, this statement is written while (b) p. To execute the statement while (b) p in the state s, the value of b is ﬁrst computed in the state s. If this value is false, execution of this statement is terminated. If the value is true, the statement p is executed, and the value of b is recomputed in the new state. If this value is false, execution of this statement is terminated. If the value is true, the statement p is executed, and the value of b is recomputed in the new state... This process continues until b evaluates to false. This construct introduces a new possible behaviour: non-termination. Indeed, if the boolean value b always evaluates to true, the statement p will continue to be executed forever, and the statement while (b) p will never terminate. This is the case with the instruction int x = 1; while (x >= 0) {x = 3;} To understand what is happening, imagine a ﬁctional statement called skip; that performs no action when executed. You can then deﬁne the statement while (b) p as shorthand for the statement if (b) {p if (b) {p if (b) {p if (b) ... else skip;} else skip;} else skip;} else skip; So a loop is one of the ways in which you can express an inﬁnite object using a

1.2 Input and Output

7

ﬁnite expression. And the fact that a loop may fail to terminate is a consequence of the fact that it is an inﬁnite object. In Caml, this statement is written while b do p. In C, it is written as it is in Java.

1.2 Input and Output An input construct allows a language to read values from a keyboard and other input devices, such as a mouse, disk, a network interface card, etc. An output construct allows values to be displayed on a screen and outputted to other peripherals, such as a printer, disk, a network interface card, etc.

1.2.1 Input Input constructs in Java are fairly complex, so we will use an extension of Java created specially for this book: the class Ppl1 . Evaluation of the expression Ppl.readInt() waits for the user to type a number on her/his keyboard, and returns this number as the value of the expression. A typical usage is n = Ppl.readInt();. The class Ppl also contains the construction Ppl.readDouble which allows decimal numbers to be read from the keyboard, and the construction Ppl.readChar which allows characters to be read.

1.2.2 Output Execution of the statement System.out.print(t); outputs the value of expression t to the screen. Execution of the statement System.out.println(); outputs a newline character that moves the cursor to the next line. Execution of the statement System.out.println(t); outputs the value of expression t to the screen, followed by a newline character.

Exercise 1.2 Write a Java program that reads an integer n from the keyboard, computes the value of 2n and outputs it to the screen. 1

The ﬁle Ppl.java is available on the author’s web site. Simply place it in the current directory to use the examples described here.

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Exercise 1.3 Write a Java program that reads an integer n from the keyboard, and outputs a boolean indicating whether the number is prime or not. Graphical constructs that allow drawings to be displayed are fairly complex in Java. But, the class Ppl contains some simple constructions to produce graphics. The statement Ppl.initDrawing(s,x,y,w,h); creates a window with the title s, of width w and of height h, positioned on the screen at coordinates (x,y). The statement Ppl.drawLine(x1,y1,x2,y2); draws a line segment with endpoints (x1,y1) and (x2,y2). The statement Ppl.drawCircle (x,y,r); draws a circle with centre (x,y) and with radius r. The statement Ppl.paintCircle(x,y,r); draws a ﬁlled circle and the statement Ppl.eraseCircle(x,y,r); allows you to erase it.

1.3 The Semantics of the Imperative Core We can, as we have below, express in English what happens when a statement is executed. While this is possible for the simple examples in this chapter, such explanations quickly become complicated and imprecise. Therefore, we shall introduce a theoretical framework that might seem a bit too comprehensive at ﬁrst, but its usefulness will become clear shortly.

1.3.1 The Concept of a State We deﬁne an inﬁnite set Var whose elements are called variables. We also deﬁne the set Val of values which are integers, booleans, etc. A state is a function that associates elements of a ﬁnite subset of Var to elements of the set Val. For example, the state [x = 5, y = 6] associates the value 5 to the variable x and the value 6 to the variable y. On the set of states, we deﬁne an update function + such that the state s + (x = v) is identical to the state s, except for the variable x, which now becomes associated with the value v. This operation is always deﬁned, whether x is originally in the domain of s or not. We can then simply deﬁne a function called Θ, which for each pair (t,s) composed of an expression t and a state s, produces the value of this expression in this state. For example, Θ(x + 3,[x = 5, y = 6]) = 8. This is a partial function, because a state is a function with a ﬁnite domain while the set of variables is inﬁnite. For example, the expression z + 3 has no

1.3 The Semantics of the Imperative Core

9

value in the state [x = 5, y = 6]. In practice, this means that attempting to compute the value of the expression z + 3 in the state [x = 5, y = 6] produces an error. Executing a statement within a state produces another state, and we deﬁne what happens when a statement is executed using a function called Σ. Σ has a statement p, an initial state s and produces a new state, Σ(p,s). This is also a partial function. Σ(p,s) is undeﬁned when executing the statement p in the state s produces an error or does not terminate. In the case of a statement p having the form x = t;, the Σ function is deﬁned as follows Σ(x = t;,s) = s + (x = Θ(t,s)). For example, Σ(x = x + 1;,[x = 5]) = [x = 6]. This is equivalent to saying ‘Executing the statement x = t; loads the memory location x with the value of expression t’.

1.3.2 Decomposition of the State A state s is a function that maps a ﬁnite subset of Var to the set Val. It will be helpful for the next chapter if we decompose this function as the composition of two other functions of ﬁnite domains: the ﬁrst is known as the environment, which maps a ﬁnite subset of the set Var to an intermediate set Ref, whose elements are called references and the second, is called the memory state, which maps a ﬁnite subset of the set Ref to the set Val. Var

Ref

e

Val

m

This brings us to propose two inﬁnite sets, Var and Ref, and a set Val of values. The set of environments is deﬁned as the set of functions that map a ﬁnite subset of the set Var to the set Ref. The set of memory states is deﬁned as the set of functions mapping a ﬁnite subset of the set Ref to the set Val. For the set of environments, we deﬁne an update function + such that the environment e + (x = r) is identical to e, except at x, which now becomes associated with

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the reference r. For the set of memory states, we deﬁne an update function + such that the memory state m + (r = v) is identical to m, except at r, which now becomes associated with the value v. However, constant variables complicate things a little bit. For one, the environment must keep track of which variables are constant and which are mutable. So, we deﬁne an environment to be a function mapping a ﬁnite subset of the set Var to the set {constant, mutable} × Ref. We will, however, continue to write e(x) to mean the reference associated to x in the environment e. Then, at the point of execution of the declaration of a constant variable x, we directly associate the variable to a value in the environment, instead of associating it to a reference which is then associated to a value in the memory state. The idea is that the memory state contains information that can be modiﬁed by an assignment, while the environment contains information that cannot. To avoid having a target set for the environment function that is overly complicated, we propose that Ref is a subset of Val, which brings us to propose that the environment is a function that maps a ﬁnite subset of Var to {constant, mutable} × Val and the memory state is a function that maps a ﬁnite subset of Ref to Val. Var

Val

e

m

Ref

1.3.3 A Visual Representation of a State It can be helpful to visualise states with a diagram. Each reference is represented with a box. Two boxes placed in diﬀerent positions always refer to separate references.

Then, we represent the environment by adding one or more labels to certain references.

1.3 The Semantics of the Imperative Core

a

11

x

b

Even though each label is associated with a unique reference, nothing prevents two labels from being associated with the same reference, since an environment is a function, but not necessarily an injective function. Finally, we represent the memory state by ﬁlling each square with a value. a

x

b

4

5

When a variable is associated directly with a value in the environment, we do not draw a box and we put the label directly on the value. x

4

1.3.4 The Value of Expressions The function Θ now associates a value to each triplet composed of an expression, an environment, and a memory state. For example, Θ(x + 3,[x = r1 , y = r2 ],[r1 = 5, r2 = 6]) = 8. For Java, this function is then deﬁned as – Θ(x,e,m) = m(e(x)), if x is a mutable variable in e, – Θ(x,e,m) = e(x), if x is a constant variable in e, – Θ(c,e,m) = c, if c is a constant, such as 4, true, etc., – Θ(t + u,e,m) = Θ(t,e,m) + Θ(u,e,m), – Θ(t - u,e,m) = Θ(t,e,m) - Θ(u,e,m),

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– Θ(t * u,e,m) = Θ(t,e,m) * Θ(u,e,m), – Θ(t / u,e,m) = Θ(t,e,m) / Θ(u,e,m), – Θ(t % u,e,m) = Θ(t,e,m) % Θ(u,e,m), – if Θ(b,e,m) = true then Θ((b) ? t : u,e,m) = Θ(t,e,m), if Θ(b,e,m) = false then Θ((b) ? t : u,e,m) = Θ(u,e,m). At ﬁrst glance, this deﬁnition may seem circular, since to deﬁne the value of an expression of the form t + u, we use the value of expressions t and u. But the size of these expressions is smaller than that of t + u. This deﬁnition is therefore a deﬁnition by induction on the size of expressions. The ﬁrst clause of this deﬁnition indicates that the value of an expression that is a mutable variable is m(e(x)). We apply the function e to the variable x, which produces a reference, and the function m to this reference, which produces a value. If the variable is a constant variable, on the other hand, we ﬁnd its value directly in the environment. The deﬁnition of the function Θ for Caml is identical, except in the case of variables, where we have the unique clause – Θ(x,e,m) = e(x), where the variable x is either mutable or constant. For example, if e is the environment [x = r] and m is the memory state [r = 4] and that the variable x is mutable in e, the value Θ(x,e,m) is 4 in Java, but is r in Caml. Caml also has a construct ! such that – Θ(!t,e,m) = m(Θ(t,e,m)). If x is a variable, then the value of !x is Θ(!x,e,m) = m(Θ(x,e,m)) = m(e(x)) that is the value of x in Java. This explains why we write y := !x + 1 in Caml, where we write y = x + 1; in Java. In Caml, references that can be associated to an integer in memory are of the type int ref. For example, the variable x and the value r from this example are of the type int ref. In contrast to the variable x, the expressions !x, !x + 1, ... are of the type int. The deﬁnition of the function Θ for C is the same as the deﬁnition used for Java.

1.3 The Semantics of the Imperative Core

13

Exercise 1.4 Give the deﬁnition of the function Θ for expressions of the form t & u and t | u. Unlike the boolean operator & that evaluates its two arguments, the operator && evaluates its second argument only if the ﬁrst argument evaluates to true. Give the deﬁnition of the function Θ for expressions of the form t && u. Answer the same question for the boolean operator ||, which only evaluates its second argument if the ﬁrst argument evaluates to false.

1.3.5 Execution of Statements The function Σ now associates memory states to triplets composed of an instruction, an environment, and a memory state. The function Σ in Java is deﬁned below. – When the statement p is a mutable variable declaration of the form {T x = t; q}, the function Σ is deﬁned as follows Σ({T x = t; q},e,m) = Σ(q,e + (x = r),m + (r = Θ(t,e,m))) where r is a new reference that does not appear in e or m. – When the statement p is a constant variable declaration of the form {final T x = t; q}, the function Σ is deﬁned as follows Σ({final T x = t; q},e,m) = Σ(q,e + (x = Θ(t,e,m)),m). – When the statement p is an assignment of the form x = t;, the function is deﬁned as follows Σ(x = t;,e,m) = m + (e(x) = Θ(t,e,m)). – When the statement p is a sequence of the form {p1 p2 }, the function Σ is deﬁned as follows Σ({p1 p2 },e,m) = Σ(p2 ,e,Σ(p1 ,e,m)). – When the statement p is a test of the form if (b) p1 else p2 , the function Σ is deﬁned as follows. If Θ(b,e,m) = true then

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Σ(if (b) p1 else p2 ,e,m) = Σ(p1 ,e,m). If Θ(b,e,m) = false then Σ(if (b) p1 else p2 ,e,m) = Σ(p2 ,e,m). – This brings us to the case where the statement p is a loop of the form while (b) q. We have seen that introducing the imaginary statement skip; such that Σ(skip;,e,m) = m, we can deﬁne the statement while (b) q as a shorthand for the inﬁnite statement if (b) {q if (b) {q if (b) {q if (b) ... else skip;} else skip;} else skip;} else skip; When dealing with these types of inﬁnite constructs, we often try to approach them as limits of ﬁnite approximations. We therefore introduce an imaginary statement called giveup; such that the function Σ is never deﬁned on (giveup;,e,m). We can deﬁne a sequence of ﬁnite approximations of the statement while (b) q. p0 = if (b) giveup; else skip; p1 = if (b) {q if (b) giveup; else skip;} else skip; ... pn+1 = if (b) {q pn } else skip;. The statement pn tries to execute the statement while (b) q by completing a maximum of n complete trips through the loop. If, after n loops, it has not terminated on its own, it gives up. If isn’t hard to prove that for every integer n and state e, m, if Σ(pn ,e,m) is deﬁned, then for all n’ greater than n, Σ(pn ,e,m) is also deﬁned, and Σ(pn ,e,m) = Σ(pn ,e,m). This formalises the fact that if the statement while (b) q terminates when the maximum number of loops is n, then it also terminates, and to the same state, when the maximum number of loops is n’. There are therefore two possibilities for the sequence Σ(pn ,e,m): either it is never deﬁned, or it is deﬁned beyond a certain point, and in this case, it is constant over its domain. In the second case, we call the value it takes over its domain the limit of the sequence. In contrast, the sequence does not have

1.3 The Semantics of the Imperative Core

15

a limit if it is never deﬁned. We can now deﬁne the function Σ in the case where the statement p is of the form while (b) q Σ(while (b) q,e,m) = limn Σ(pn ,e,m). Note that the statements pi are not always shorter than p, but if p contains k nested while loops, pi contains k - 1. The deﬁnition of the function Σ is thus a double induction on the number of nested while loops, and on the size of the statement.

Exercise 1.5 What is the memory state Σ(x = 7;,[x = r],[r = 5])? The deﬁnition of the function Σ for Caml is not very diﬀerent from the deﬁnition used for Java. In Caml, any expression that evaluates to a reference can be placed to the left of the sign :=, while in Java, only a variable can appear to the left of the sign =. The value of the function Σ of Caml for the statement t := u is deﬁned below: – Σ(t := u,e,m) = m + (Θ(t,e,m) = Θ(u,e,m)) In the case where the expression t is a variable x, we have Σ(x := u,e,m) = m + (Θ(x,e,m) = Θ(u,e,m)) = m + (e(x) = Θ(u,e,m)) and we end up with the same deﬁnition of Σ used for Java. The deﬁnition of the function Σ for C is not very diﬀerent from the deﬁnition used for Java. The main diﬀerence is in case of variable declaration Σ({T x = t; q},e,m) = (Σ(q,e+(x=r),m + (r = Θ(t,e,m))))|Ref−{r} where r is a new reference that does not appear in e or m, and the notation m|Ref−{r} designates the memory state m in which we have removed the ordered pair r = v if it existed. Thus, if we execute the statement {int x = 4; p} q in the state e, m, we execute the statement p in the state e + (x = r), m + (r = 4) in C as in Java. In contrast, we execute the statement q in the state e, m + (r = 4) in Java and in the state e, m in C. As, in the environment e, there is no variable that allows the reference r to be accessed, the ordered pair r = 4 no longer serves a purpose sitting in memory. Thus, whether it is is left alone, as in Java or Caml, or deleted, as in C, is immaterial. However, we will see, in Exercise 2.17, that this choice in C is a source of diﬃculty when the language contains other constructs.

Exercise 1.6 The incomplete test allows the creation of a statement composed of a boolean expression and a statement. This statement is written if (b) p. The value of the function Σ for this statement is deﬁned as follows. If Θ(b,e,m) = true then

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Σ(if (b) p,e,m) = Σ(p,e,m). If Θ(b,e,m) = false then Σ(if (b) p,e,m) = m. Show that it is possible to deﬁne this construct using a complete test and the statement skip;.

Exercise 1.7 The loop do allows the creation of a statement composed of a boolean expression and a statement. This statement is written do p while (b). This is a shorthand of the statement {p while (b) p}. Give the deﬁnition of the function Σ for this construct.

Exercise 1.8 The loop for allows the creation of a statement composed of three statements and a boolean expression. This statement is written for(p1 ; b; p2 ) p3 . It is a shorthand for the statement p1 ; while (b) {p3 p2 ;}. What does the following statement do? {y = 1; for(x = 1; x

Undergraduate Topics in Computer Science (UTiCS) delivers high-quality instructional content for undergraduates studying in all areas of computing and information science. From core foundational and theoretical material to ﬁnal-year topics and applications, UTiCS books take a fresh, concise, and modern approach and are ideal for self-study or for a one- or two-semester course. The texts are all authored by established experts in their ﬁelds, reviewed by an international advisory board, and contain numerous examples and problems. Many include fully worked solutions.

Also in this series Iain D. Craig Object-Oriented Programming Languages: Interpretation 978-1-84628-773-2 Max Bramer Principles of Data Mining 978-1-84628-765-7 Hanne Riis Nielson and Flemming Nielson Semantics with Applications: An Appetizer 978-1-84628-691-9 Michael Kifer and Scott A. Smolka Introduction to Operating System Design and Implementation: The OSP 2 Approcah 978-1-84628-842-5 Phil Brooke and Richard Paige Practical Distributed Processing 978-1-84628-840-1 Frank Klawonn Computer Graphics with Java 978-1-84628-847-0 David Salomon A Concise Introduction to Data Compression 978-1-84800-071-1 David Makinson Sets, Logic and Maths for Computing 978-1-84628-844-9 Orit Hazzan Agile Software Engineering 978-1-84800-198-5 Pankaj Jalote A Concise Introduction to Software Engineering 978-1-84800-301-9 Alan P. Parkes A Concise Introduction to Languages and Machines 978-1-84800-120-6

Gilles Dowek

Principles of Programming Languages

123

Gilles Dowek École Polytechnique France Series editor Ian Mackie, École Polytechnique, France Advisory board Samson Abramsky, University of Oxford, UK Chris Hankin, Imperial College London, UK Dexter Kozen, Cornell University, USA Andrew Pitts, University of Cambridge, UK Hanne Riis Nielson, Technical University of Denmark, Denmark Steven Skiena, Stony Brook University, USA Iain Stewart, University of Durham, UK David Zhang, The Hong Kong Polytechnic University, Hong Kong

Undergraduate Topics in Computer Science ISSN 1863-7310 ISBN: 978-1-84882-031-9 e-ISBN: 978-1-84882-032-6 DOI: 10.1007/978-1-84882-032-6 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Control Number: 2008943965 Based on course notes by Gilles Dowek published in 2006 by L’Ecole Polytechnique with the following title: “Les principes des langages de programmation.” c Springer-Verlag London Limited 2009 Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licenses issued by the Copyright Licensing Agency. Enquiries concerning reproduction outside those terms should be sent to the publishers. The use of registered names, trademarks, etc., in this publication does not imply, even in the absence of a speciﬁc statement, that such names are exempt from the relevant laws and regulations and therefore free for general use. The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made. Printed on acid-free paper Springer Science+Business Media springer.com

The author wants to thank François Pottier, Philippe Baptiste, Julien Cervelle, Albert Cohen, Olivier Delande, Olivier Hermant, Ian Mackie, François Morain, Jean-Marc Steyaert and Paul Zimmermann for their remarks on a ﬁrst version of this book.

Preface

We’ve known about algorithms for millennia, but we’ve only been writing computer programs for a few decades. A big diﬀerence between the Euclidean or Eratosthenes age and ours is that since the middle of the twentieth century, we express the algorithms we conceive using formal languages: programming languages. Computer scientists are not the only ones who use formal languages. Optometrists, for example, prescribe eyeglasses using very technical expressions, such as “OD: -1.25 (-0.50) 180◦ OS: -1.00 (-0.25) 180◦ ”, in which the parentheses are essential. Many such formal languages have been created throughout history: musical notation, algebraic notation, etc. In particular, such languages have long been used to control machines, such as looms and cathedral chimes. However, until the appearance of programming languages, those languages were only of limited importance: they were restricted to specialised ﬁelds with only a few specialists and written texts of those languages remained relatively scarce. This situation has changed with the appearance of programming languages, which have a wider range of applications than the prescription of eyeglasses or the control of a loom, are used by large communities, and have allowed the creation of programs of many hundreds of thousands of lines. The appearance of programming languages has allowed the creation of artiﬁcial objects, programs, of a complexity incomparable to anything that has come before, such as steam engines or radios. These programs have, in return, allowed the creation of other complex objects, such as integrated circuits made of millions of transistors, or mathematical proofs that are hundreds of thousands of pages long. It is very surprising that we have succeeded in writing such complex programs in languages comprising such a small number of constructs — assignment, loops, etc. — that is to say in languages barely more sophisticated than the language of prescription eyeglasses. vii

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Programs written in these programming languages have the novelty of not only being understandable by humans, which brings them closer to the scores used by organists, but also readable by machines, which brings them closer to the punch cards used in Barbarie organs. The appearance of programming languages has therefore profoundly impacted our relationship with language, complexity, and machines. This book is an introduction to the principles of programming languages. It uses the Java language for support. It is intended for students who already have some experience with computer programming. It is assumed that they have learned some programming empirically, in a single programming language, other than Java. The ﬁrst objective of this book will then be to learn the fundamentals of the Java programming language. However, knowing a single programming language is not suﬃcient to be a good programmer. For this, you must not only know several languages, but be able to easily learn new ones. This requires that you understand universal concepts like functions or cells, which exist in one form or another in all programming languages. This can only be done by comparing two or more languages. In this book, two comparison languages have been chosen: Caml and C. Therefore, the goal is not for the students to learn three programming languages simultaneously, but that with the comparison with Caml and C, they can learn the principles around which programming languages are created. This understanding will allow them to develop, if they wish, a real competence in Caml or in C, or in any other programming language. Another objective of this book is for the students to begin acquiring the tools which permit them to precisely deﬁne the meaning of the program. This precision is, indeed, the only means to clearly understand what happens when a program is executed, and to reason in situations where complexity deﬁes intuition. The idea is to describe the meaning of a statement by a function operating on a set of states. However, our expectations of this objective remain modest: students wishing to pursue this goal will have to do so elsewhere. The ﬁnal objective of this course is to learn basic algorithms for lists and trees. Here too, our expectations remain modest: students wishing to pursue this will also have to look elsewhere.

Contents

1.

Imperative Core . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Five Constructs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.1 Assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.2 Variable Declaration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.3 Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1.4 Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1.5 Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2 Input and Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.2.1 Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.2.2 Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.3 The Semantics of the Imperative Core . . . . . . . . . . . . . . . . . . . . . . . 8 1.3.1 The Concept of a State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.3.2 Decomposition of the State . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.3.3 A Visual Representation of a State . . . . . . . . . . . . . . . . . . . 10 1.3.4 The Value of Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 1.3.5 Execution of Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.

Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 The Concept of Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Avoiding Repetition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3 Return Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4 The return Construct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.5 Functions and Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.6 Global Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.7 The Main Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

19 19 19 21 22 23 24 25 25 ix

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2.1.8 Global Variables Hidden by Local Variables . . . . . . . . . . . . 2.1.9 Overloading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 The Semantics of Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 The Value of Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Execution of Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Order of Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Caml . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5 C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Expressions as Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Passing Arguments by Value and Reference . . . . . . . . . . . . . . . . . . 2.4.1 Pascal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Caml . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3 C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.4 Java . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

27 28 29 30 31 34 34 36 37 37 39 40 41 45

3.

Recursion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Calling a Function from Inside the Body of that Function . . . . . 3.2 Recursive Deﬁnitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Recursive Deﬁnitions and Circular Deﬁnitions . . . . . . . . . . 3.2.2 Recursive Deﬁnitions and Deﬁnitions by Induction . . . . . . 3.2.3 Recursive Deﬁnitions and Inﬁnite Programs . . . . . . . . . . . . 3.2.4 Recursive Deﬁnitions and Fixed Point Equations . . . . . . . 3.3 Caml . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Programming Without Assignment . . . . . . . . . . . . . . . . . . . . . . . . .

47 47 48 48 49 49 51 53 54 55

4.

Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Tuples with Named Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 The Deﬁnition of a Record Type . . . . . . . . . . . . . . . . . . . . . 4.1.2 Allocation of a Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3 Accessing Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.4 Assignment of Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.5 Constructors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.6 The Semantics of Records . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Sharing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Sharing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Equality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 Wrapper Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Caml . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Deﬁnition of a Record Type . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Creating a Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3 Accessing Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

59 59 60 60 62 62 64 65 66 66 68 68 73 73 73 74

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4.3.4 Assigning to Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Deﬁnition of a Record Type . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Creating a Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.3 Accessing Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.4 Assigning to Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1 Array Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2 Allocation of an Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.3 Accessing and Assigning to Fields . . . . . . . . . . . . . . . . . . . . 4.5.4 Arrays of Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.5 Arrays in Caml . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.6 Arrays in C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xi

74 76 76 76 77 77 79 79 80 80 82 83 84

5.

Dynamic Data Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 5.1 Recursive Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 5.1.1 Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 5.1.2 The null Value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 5.1.3 An Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 5.1.4 Recursive Deﬁnitions and Fixed Point Equations . . . . . . . 88 5.1.5 Inﬁnite Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 5.2 Disjunctive Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 5.3 Dynamic Data Types and Computability . . . . . . . . . . . . . . . . . . . . 92 5.4 Caml . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 5.5 C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 5.6 Garbage Collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 5.6.1 Inaccessible Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 5.6.2 Programming without Garbage Collection . . . . . . . . . . . . . 98 5.6.3 Global Methods of Memory Management . . . . . . . . . . . . . . 100 5.6.4 Garbage Collection and Functions . . . . . . . . . . . . . . . . . . . . 102

6.

Programming with Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 6.1 Finite Sets and Functions of a Finite Domain . . . . . . . . . . . . . . . . 103 6.1.1 Membership . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 6.1.2 Association Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 6.2 Concatenation: Modify or Copy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 6.2.1 Modify . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 6.2.2 Copy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 6.2.3 Using Recursion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 6.2.4 Chemical Reactions and Mathematical Functions . . . . . . . 111 6.3 List Inversion: an Extra Argument . . . . . . . . . . . . . . . . . . . . . . . . . . 112 6.4 Lists and Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

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6.5 Stacks and Queues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 6.5.1 Stacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 6.5.2 Queues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 6.5.3 Priority Queues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 7.

Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 7.1 Exceptional Circumstances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 7.2 Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 7.3 Catching Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 7.4 The Propagation of Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 7.5 Error Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 7.6 The Semantics of Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 7.7 Caml . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

8.

Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 8.1 Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 8.1.1 Functions as Part of a Type . . . . . . . . . . . . . . . . . . . . . . . . . 127 8.1.2 The Semantics of Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 8.2 Dynamic Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 8.3 Methods and Functional Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 8.4 Static Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 8.5 Static Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 8.6 Inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 8.7 Caml . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

9.

Programming with Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 9.1 Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 9.2 Traversing a Tree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 9.2.1 Depth First Traversal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 9.2.2 Breadth First Traversal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 9.3 Search Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 9.3.1 Membership . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 9.3.2 Balanced Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 9.3.3 Dictionaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 9.4 Priority Queues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 9.4.1 Partially Ordered Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 9.4.2 Partially Ordered Balanced Trees . . . . . . . . . . . . . . . . . . . . . 153

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

1 Imperative Core

1.1 Five Constructs Most programming languages have, among others, ﬁve constructs: assignment, variable declaration, sequence, test, and loop. These constructs form the imperative core of the language.

1.1.1 Assignment The assignment construct allows the creation of a statement with a variable x and an expression t. In Java, this statement is written as x = t;. Variables are identiﬁers which are written as one of more letters. Expressions are composed of variables and constants with operators, such as +, -, *, / — division — and % — modulo. Therefore, the following statements x = y % 3; x = y; y = 3; x = x + 1; G. Dowek, Principles of Programming Languages, Undergraduate Topics in Computer Science, DOI 10.1007/978-1-84882-032-6_1, c Springer-Verlag London Limited 2009

1

2

1. Imperative Core

are all proper Java statements, while y + 3 = x; x + 2 = y + 5; are not. To understand what happens when you execute the statement x = t; suppose that within the recesses of your computer’s memory, there is a compartment labelled x. Executing the statement x = t; consists of ﬁlling this compartment with the value of the expression t. The value previously contained in compartment x is erased. If the expression t is a constant, for example 3, its value is the same constant. If it is an expression with no variables, such as 3 + 4, its value is obtained by carrying out mathematical operations, in this case, addition. If expression t contains variables, the values of these variables must be looked up in the computer’s memory. The whole of the contents of the computer’s memory is called a state. Let us consider, initially, that expressions, such as x + 3, and statements, such as y = x + 3;, form two disjoint categories. Later, however, we shall be brought to revise this premise. In these examples, the values of expressions are integers. Computers can only store integers within a ﬁnite interval. In Java, integers must be between -231 and 231 - 1, so there are 232 possible values. When a mathematical operation produces a value outside of this interval, the result is kept within the interval by taking its modulo 232 remainder. Thus, by adding 1 to 231 - 1, that is to say 2147483647, we leave the interval and then return to it by removing 232 , which gives -231 or -2147483648.

Exercise 1.1 What is the value of the variable x after executing the following statement? x = 2 * 1500000000; In Caml, assignment is written x := t. In the expression t, we designate the value of x, not with the expression x itself, but with the expression !x. Thus, in Caml we write y := !x + 1 while in Java we write y = x + 1;. In C, assignment is written as it is in Java.

1.1 Five Constructs

3

1.1.2 Variable Declaration Before being able to assign values to a variable x, it must be declared, which associates the name x to a location in the computer’s memory. Variable declaration is a construct that allows the creation of a statement composed of a variable, an expression, and a statement. In Java, this statement is written {int x = t; p} where p is a statement, for example {int x = 4; x = x + 1;}. The variable x can then be used in the statement p, which is called the scope of variable x. It is also possible to declare a variable without giving it an initial value, for example, {int x; x = y + 4;}. We must of course be careful not to use a variable which has been declared without an initial value and that has not been assigned a value. This produces an error. Apart from the int type, Java has three other integer types that have diﬀerent intervals. These types are deﬁned in Table 1.1. When a mathematical operation produces a value outside of these intervals, the result is returned to the interval by taking its remainder, modulo the size of the interval. In Java, there are also other scalar types for decimal numbers, booleans, and characters. These types are deﬁned in Table 1.1. Operations allowed in the construction of expressions for each of these types are described in Table 1.2. Variables can also contain objects that are of composite types, like arrays and character strings, which we will address later. Because we will need them shortly, character strings are described brieﬂy in Table 1.3. The integers are of type byte, short, int or long corresponding to the intervals [-27 , 27 - 1], [-215 , 215 - 1], [-231 , 231 - 1] and [-263 , 263 - 1], Respectively. Constants are written in base 10, for example, -666. Decimal numbers are of type float or double. Constants are written in scientiﬁc notation, for example 3.14159, 666 or 6.02E23. Booleans are of type boolean. Constants are written as false and true. Characters are of type char. Constants are written between apostrophes, for example ‘b’. Table 1.1 Scalars types in Java

To declare a variable of type T, replace the type int with T. The general form of a declaration is thus {T x = t; p}.

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The basic operations that allow for arithmetical expressions are +, -, *, / — division — and % — modulo. When one of the numbers a or b is negative, the number a / b is the quotient rounded towards 0. So the result of a / b is the quotient of the absolute values of a and b, and is positive when a and b have the same sign, and negative if they have diﬀerent signs. The number a % b is a - b * (a / b). So (-29) / 4 equals -7 and (-29) % 4 equals -1. The operations for decimal numbers are +, -, *, /, along with some transcendental functions: Math.sin, Math.cos, ... The operations allowed in boolean expressions are ==, != — diﬀerent —, , =, & — and —, &&, | — or —, || and ! — not. For all data types, the expression (b) ? t : u evaluates to the value of t if the boolean expression b has the value true, and evaluates to the value of u if the boolean expression b has the value false. Table 1.2 Expressions in Java Character strings are of type String. Constants are written inside quotation marks, for example "Principles of Programming Languages". Table 1.3 Character strings in Java In Caml, variable declaration is written as let x = ref t in p and it isn’t necessary to explicitly declare the variable’s type. It is not possible in Caml to declare a variable without giving it an initial value. In C, like in Java, declaration is written {T x = t; p}. It is possible to declare a variable without giving it an initial value, and in this case, it could have any value. In Java and in C, it is impossible to declare the same variable twice, and the following program is not valid. int int int y =

y = 4; x = 5; x = 6; x;

In contrast, nothing in Caml stops you from writing

1.1 Five Constructs

5

let y = ref 4 in let x = ref 5 in let x = ref 6 in y := !x and this program assigns the value 6 to the variable y, so it is the most recent declaration of x that is used. We say that the ﬁrst declaration of x is hidden by the second. Java, Caml and C allow the creation of variables with an initial value that can never be changed. This type of variable is called a constant variable. A variable that is not constant is called a mutable variable. Java assumes that all variables are mutable unless you specify otherwise. To declare a constant variable in Java, you precede the variable type with the keyword final, for example final int x = 4; y = x + 1; The following statement is not valid, because an attempt is made to alter the value of a constant variable final int x = 4; x = 5; In Caml, to indicate that the variable x is a constant variable, write let x = t in p instead of writing let x = ref t in p. When using constant variables, you do not write !x to express its value, but simply x. So, you can write let x = 4 in y := x + 1, while the statement let x = 4 in x := 5 is invalid. In C, you indicate that a variable is a constant variable by preceding its type with the keyword const.

1.1.3 Sequence A sequence is a construct that allows a single statement to be created out of two statements p1 and p2 . In Java, a sequence is written as {p1 p2 }. The statement {p1 {p2 { ... pn } ...}} can also be written as {p1 p2 ... pn }. To execute the statement {p1 p2 } in the state s, the statement p1 is ﬁrst executed in the state s, which produces a new state s’. Then the statement p2 is executed in the state s’. In Caml, a sequence is written as p1 ; p2 . In C, it is written the same as it is in Java.

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1.1.4 Test A test is a construct that allows the creation of a statement composed of a boolean expression b and two statements p1 and p2 . In Java, this statement is written if (b) p1 else p2 . To execute the statement if (b) p1 else p2 in a state s, the value of expression b is ﬁrst computed in the state s, and depending on whether or not its value is true or false, the statement p1 or p2 is executed in the state s. In Caml, this statement is written if b then p1 else p2 . In C, it is written as it is in Java.

1.1.5 Loop A loop is a construct that allows the creation of a statement composed of a boolean expression b and a statement p. In Java, this statement is written while (b) p. To execute the statement while (b) p in the state s, the value of b is ﬁrst computed in the state s. If this value is false, execution of this statement is terminated. If the value is true, the statement p is executed, and the value of b is recomputed in the new state. If this value is false, execution of this statement is terminated. If the value is true, the statement p is executed, and the value of b is recomputed in the new state... This process continues until b evaluates to false. This construct introduces a new possible behaviour: non-termination. Indeed, if the boolean value b always evaluates to true, the statement p will continue to be executed forever, and the statement while (b) p will never terminate. This is the case with the instruction int x = 1; while (x >= 0) {x = 3;} To understand what is happening, imagine a ﬁctional statement called skip; that performs no action when executed. You can then deﬁne the statement while (b) p as shorthand for the statement if (b) {p if (b) {p if (b) {p if (b) ... else skip;} else skip;} else skip;} else skip; So a loop is one of the ways in which you can express an inﬁnite object using a

1.2 Input and Output

7

ﬁnite expression. And the fact that a loop may fail to terminate is a consequence of the fact that it is an inﬁnite object. In Caml, this statement is written while b do p. In C, it is written as it is in Java.

1.2 Input and Output An input construct allows a language to read values from a keyboard and other input devices, such as a mouse, disk, a network interface card, etc. An output construct allows values to be displayed on a screen and outputted to other peripherals, such as a printer, disk, a network interface card, etc.

1.2.1 Input Input constructs in Java are fairly complex, so we will use an extension of Java created specially for this book: the class Ppl1 . Evaluation of the expression Ppl.readInt() waits for the user to type a number on her/his keyboard, and returns this number as the value of the expression. A typical usage is n = Ppl.readInt();. The class Ppl also contains the construction Ppl.readDouble which allows decimal numbers to be read from the keyboard, and the construction Ppl.readChar which allows characters to be read.

1.2.2 Output Execution of the statement System.out.print(t); outputs the value of expression t to the screen. Execution of the statement System.out.println(); outputs a newline character that moves the cursor to the next line. Execution of the statement System.out.println(t); outputs the value of expression t to the screen, followed by a newline character.

Exercise 1.2 Write a Java program that reads an integer n from the keyboard, computes the value of 2n and outputs it to the screen. 1

The ﬁle Ppl.java is available on the author’s web site. Simply place it in the current directory to use the examples described here.

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Exercise 1.3 Write a Java program that reads an integer n from the keyboard, and outputs a boolean indicating whether the number is prime or not. Graphical constructs that allow drawings to be displayed are fairly complex in Java. But, the class Ppl contains some simple constructions to produce graphics. The statement Ppl.initDrawing(s,x,y,w,h); creates a window with the title s, of width w and of height h, positioned on the screen at coordinates (x,y). The statement Ppl.drawLine(x1,y1,x2,y2); draws a line segment with endpoints (x1,y1) and (x2,y2). The statement Ppl.drawCircle (x,y,r); draws a circle with centre (x,y) and with radius r. The statement Ppl.paintCircle(x,y,r); draws a ﬁlled circle and the statement Ppl.eraseCircle(x,y,r); allows you to erase it.

1.3 The Semantics of the Imperative Core We can, as we have below, express in English what happens when a statement is executed. While this is possible for the simple examples in this chapter, such explanations quickly become complicated and imprecise. Therefore, we shall introduce a theoretical framework that might seem a bit too comprehensive at ﬁrst, but its usefulness will become clear shortly.

1.3.1 The Concept of a State We deﬁne an inﬁnite set Var whose elements are called variables. We also deﬁne the set Val of values which are integers, booleans, etc. A state is a function that associates elements of a ﬁnite subset of Var to elements of the set Val. For example, the state [x = 5, y = 6] associates the value 5 to the variable x and the value 6 to the variable y. On the set of states, we deﬁne an update function + such that the state s + (x = v) is identical to the state s, except for the variable x, which now becomes associated with the value v. This operation is always deﬁned, whether x is originally in the domain of s or not. We can then simply deﬁne a function called Θ, which for each pair (t,s) composed of an expression t and a state s, produces the value of this expression in this state. For example, Θ(x + 3,[x = 5, y = 6]) = 8. This is a partial function, because a state is a function with a ﬁnite domain while the set of variables is inﬁnite. For example, the expression z + 3 has no

1.3 The Semantics of the Imperative Core

9

value in the state [x = 5, y = 6]. In practice, this means that attempting to compute the value of the expression z + 3 in the state [x = 5, y = 6] produces an error. Executing a statement within a state produces another state, and we deﬁne what happens when a statement is executed using a function called Σ. Σ has a statement p, an initial state s and produces a new state, Σ(p,s). This is also a partial function. Σ(p,s) is undeﬁned when executing the statement p in the state s produces an error or does not terminate. In the case of a statement p having the form x = t;, the Σ function is deﬁned as follows Σ(x = t;,s) = s + (x = Θ(t,s)). For example, Σ(x = x + 1;,[x = 5]) = [x = 6]. This is equivalent to saying ‘Executing the statement x = t; loads the memory location x with the value of expression t’.

1.3.2 Decomposition of the State A state s is a function that maps a ﬁnite subset of Var to the set Val. It will be helpful for the next chapter if we decompose this function as the composition of two other functions of ﬁnite domains: the ﬁrst is known as the environment, which maps a ﬁnite subset of the set Var to an intermediate set Ref, whose elements are called references and the second, is called the memory state, which maps a ﬁnite subset of the set Ref to the set Val. Var

Ref

e

Val

m

This brings us to propose two inﬁnite sets, Var and Ref, and a set Val of values. The set of environments is deﬁned as the set of functions that map a ﬁnite subset of the set Var to the set Ref. The set of memory states is deﬁned as the set of functions mapping a ﬁnite subset of the set Ref to the set Val. For the set of environments, we deﬁne an update function + such that the environment e + (x = r) is identical to e, except at x, which now becomes associated with

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the reference r. For the set of memory states, we deﬁne an update function + such that the memory state m + (r = v) is identical to m, except at r, which now becomes associated with the value v. However, constant variables complicate things a little bit. For one, the environment must keep track of which variables are constant and which are mutable. So, we deﬁne an environment to be a function mapping a ﬁnite subset of the set Var to the set {constant, mutable} × Ref. We will, however, continue to write e(x) to mean the reference associated to x in the environment e. Then, at the point of execution of the declaration of a constant variable x, we directly associate the variable to a value in the environment, instead of associating it to a reference which is then associated to a value in the memory state. The idea is that the memory state contains information that can be modiﬁed by an assignment, while the environment contains information that cannot. To avoid having a target set for the environment function that is overly complicated, we propose that Ref is a subset of Val, which brings us to propose that the environment is a function that maps a ﬁnite subset of Var to {constant, mutable} × Val and the memory state is a function that maps a ﬁnite subset of Ref to Val. Var

Val

e

m

Ref

1.3.3 A Visual Representation of a State It can be helpful to visualise states with a diagram. Each reference is represented with a box. Two boxes placed in diﬀerent positions always refer to separate references.

Then, we represent the environment by adding one or more labels to certain references.

1.3 The Semantics of the Imperative Core

a

11

x

b

Even though each label is associated with a unique reference, nothing prevents two labels from being associated with the same reference, since an environment is a function, but not necessarily an injective function. Finally, we represent the memory state by ﬁlling each square with a value. a

x

b

4

5

When a variable is associated directly with a value in the environment, we do not draw a box and we put the label directly on the value. x

4

1.3.4 The Value of Expressions The function Θ now associates a value to each triplet composed of an expression, an environment, and a memory state. For example, Θ(x + 3,[x = r1 , y = r2 ],[r1 = 5, r2 = 6]) = 8. For Java, this function is then deﬁned as – Θ(x,e,m) = m(e(x)), if x is a mutable variable in e, – Θ(x,e,m) = e(x), if x is a constant variable in e, – Θ(c,e,m) = c, if c is a constant, such as 4, true, etc., – Θ(t + u,e,m) = Θ(t,e,m) + Θ(u,e,m), – Θ(t - u,e,m) = Θ(t,e,m) - Θ(u,e,m),

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– Θ(t * u,e,m) = Θ(t,e,m) * Θ(u,e,m), – Θ(t / u,e,m) = Θ(t,e,m) / Θ(u,e,m), – Θ(t % u,e,m) = Θ(t,e,m) % Θ(u,e,m), – if Θ(b,e,m) = true then Θ((b) ? t : u,e,m) = Θ(t,e,m), if Θ(b,e,m) = false then Θ((b) ? t : u,e,m) = Θ(u,e,m). At ﬁrst glance, this deﬁnition may seem circular, since to deﬁne the value of an expression of the form t + u, we use the value of expressions t and u. But the size of these expressions is smaller than that of t + u. This deﬁnition is therefore a deﬁnition by induction on the size of expressions. The ﬁrst clause of this deﬁnition indicates that the value of an expression that is a mutable variable is m(e(x)). We apply the function e to the variable x, which produces a reference, and the function m to this reference, which produces a value. If the variable is a constant variable, on the other hand, we ﬁnd its value directly in the environment. The deﬁnition of the function Θ for Caml is identical, except in the case of variables, where we have the unique clause – Θ(x,e,m) = e(x), where the variable x is either mutable or constant. For example, if e is the environment [x = r] and m is the memory state [r = 4] and that the variable x is mutable in e, the value Θ(x,e,m) is 4 in Java, but is r in Caml. Caml also has a construct ! such that – Θ(!t,e,m) = m(Θ(t,e,m)). If x is a variable, then the value of !x is Θ(!x,e,m) = m(Θ(x,e,m)) = m(e(x)) that is the value of x in Java. This explains why we write y := !x + 1 in Caml, where we write y = x + 1; in Java. In Caml, references that can be associated to an integer in memory are of the type int ref. For example, the variable x and the value r from this example are of the type int ref. In contrast to the variable x, the expressions !x, !x + 1, ... are of the type int. The deﬁnition of the function Θ for C is the same as the deﬁnition used for Java.

1.3 The Semantics of the Imperative Core

13

Exercise 1.4 Give the deﬁnition of the function Θ for expressions of the form t & u and t | u. Unlike the boolean operator & that evaluates its two arguments, the operator && evaluates its second argument only if the ﬁrst argument evaluates to true. Give the deﬁnition of the function Θ for expressions of the form t && u. Answer the same question for the boolean operator ||, which only evaluates its second argument if the ﬁrst argument evaluates to false.

1.3.5 Execution of Statements The function Σ now associates memory states to triplets composed of an instruction, an environment, and a memory state. The function Σ in Java is deﬁned below. – When the statement p is a mutable variable declaration of the form {T x = t; q}, the function Σ is deﬁned as follows Σ({T x = t; q},e,m) = Σ(q,e + (x = r),m + (r = Θ(t,e,m))) where r is a new reference that does not appear in e or m. – When the statement p is a constant variable declaration of the form {final T x = t; q}, the function Σ is deﬁned as follows Σ({final T x = t; q},e,m) = Σ(q,e + (x = Θ(t,e,m)),m). – When the statement p is an assignment of the form x = t;, the function is deﬁned as follows Σ(x = t;,e,m) = m + (e(x) = Θ(t,e,m)). – When the statement p is a sequence of the form {p1 p2 }, the function Σ is deﬁned as follows Σ({p1 p2 },e,m) = Σ(p2 ,e,Σ(p1 ,e,m)). – When the statement p is a test of the form if (b) p1 else p2 , the function Σ is deﬁned as follows. If Θ(b,e,m) = true then

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Σ(if (b) p1 else p2 ,e,m) = Σ(p1 ,e,m). If Θ(b,e,m) = false then Σ(if (b) p1 else p2 ,e,m) = Σ(p2 ,e,m). – This brings us to the case where the statement p is a loop of the form while (b) q. We have seen that introducing the imaginary statement skip; such that Σ(skip;,e,m) = m, we can deﬁne the statement while (b) q as a shorthand for the inﬁnite statement if (b) {q if (b) {q if (b) {q if (b) ... else skip;} else skip;} else skip;} else skip; When dealing with these types of inﬁnite constructs, we often try to approach them as limits of ﬁnite approximations. We therefore introduce an imaginary statement called giveup; such that the function Σ is never deﬁned on (giveup;,e,m). We can deﬁne a sequence of ﬁnite approximations of the statement while (b) q. p0 = if (b) giveup; else skip; p1 = if (b) {q if (b) giveup; else skip;} else skip; ... pn+1 = if (b) {q pn } else skip;. The statement pn tries to execute the statement while (b) q by completing a maximum of n complete trips through the loop. If, after n loops, it has not terminated on its own, it gives up. If isn’t hard to prove that for every integer n and state e, m, if Σ(pn ,e,m) is deﬁned, then for all n’ greater than n, Σ(pn ,e,m) is also deﬁned, and Σ(pn ,e,m) = Σ(pn ,e,m). This formalises the fact that if the statement while (b) q terminates when the maximum number of loops is n, then it also terminates, and to the same state, when the maximum number of loops is n’. There are therefore two possibilities for the sequence Σ(pn ,e,m): either it is never deﬁned, or it is deﬁned beyond a certain point, and in this case, it is constant over its domain. In the second case, we call the value it takes over its domain the limit of the sequence. In contrast, the sequence does not have

1.3 The Semantics of the Imperative Core

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a limit if it is never deﬁned. We can now deﬁne the function Σ in the case where the statement p is of the form while (b) q Σ(while (b) q,e,m) = limn Σ(pn ,e,m). Note that the statements pi are not always shorter than p, but if p contains k nested while loops, pi contains k - 1. The deﬁnition of the function Σ is thus a double induction on the number of nested while loops, and on the size of the statement.

Exercise 1.5 What is the memory state Σ(x = 7;,[x = r],[r = 5])? The deﬁnition of the function Σ for Caml is not very diﬀerent from the deﬁnition used for Java. In Caml, any expression that evaluates to a reference can be placed to the left of the sign :=, while in Java, only a variable can appear to the left of the sign =. The value of the function Σ of Caml for the statement t := u is deﬁned below: – Σ(t := u,e,m) = m + (Θ(t,e,m) = Θ(u,e,m)) In the case where the expression t is a variable x, we have Σ(x := u,e,m) = m + (Θ(x,e,m) = Θ(u,e,m)) = m + (e(x) = Θ(u,e,m)) and we end up with the same deﬁnition of Σ used for Java. The deﬁnition of the function Σ for C is not very diﬀerent from the deﬁnition used for Java. The main diﬀerence is in case of variable declaration Σ({T x = t; q},e,m) = (Σ(q,e+(x=r),m + (r = Θ(t,e,m))))|Ref−{r} where r is a new reference that does not appear in e or m, and the notation m|Ref−{r} designates the memory state m in which we have removed the ordered pair r = v if it existed. Thus, if we execute the statement {int x = 4; p} q in the state e, m, we execute the statement p in the state e + (x = r), m + (r = 4) in C as in Java. In contrast, we execute the statement q in the state e, m + (r = 4) in Java and in the state e, m in C. As, in the environment e, there is no variable that allows the reference r to be accessed, the ordered pair r = 4 no longer serves a purpose sitting in memory. Thus, whether it is is left alone, as in Java or Caml, or deleted, as in C, is immaterial. However, we will see, in Exercise 2.17, that this choice in C is a source of diﬃculty when the language contains other constructs.

Exercise 1.6 The incomplete test allows the creation of a statement composed of a boolean expression and a statement. This statement is written if (b) p. The value of the function Σ for this statement is deﬁned as follows. If Θ(b,e,m) = true then

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1. Imperative Core

Σ(if (b) p,e,m) = Σ(p,e,m). If Θ(b,e,m) = false then Σ(if (b) p,e,m) = m. Show that it is possible to deﬁne this construct using a complete test and the statement skip;.

Exercise 1.7 The loop do allows the creation of a statement composed of a boolean expression and a statement. This statement is written do p while (b). This is a shorthand of the statement {p while (b) p}. Give the deﬁnition of the function Σ for this construct.

Exercise 1.8 The loop for allows the creation of a statement composed of three statements and a boolean expression. This statement is written for(p1 ; b; p2 ) p3 . It is a shorthand for the statement p1 ; while (b) {p3 p2 ;}. What does the following statement do? {y = 1; for(x = 1; x

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