MEI - A MODULE SYSTEM FOR MECHANIZED MATHEMATICS SYSTEMS MEI - A MODULE SYSTEM FOR MECHANIZED MATHEMATICS SYSTEMS By JIAN Xu, M.Sc. A Thesis Submitted to the School of Graduate Studies in partial fulfilment of the requirements for the degree of Ph.D. ©Copyright by Jian Xu, January 2008 ii Jian Xu - Ph.D. Thesis - Department of CAS, McMaster University PH.D. (2008) McMaster University (Computer Science) Hamilton, Ontario TITLE: Mei - A Module System for Mechanized Mathematics Systems AUTHOR: Jian Xu, M.Sc. (McMaster University, Canada) SUPERVISOR: Dr. William M. Farmer NUMBER OF PAGES: viii, 194 Abstract This thesis presents several module systems, in particular Mei and DMei, designed for mechanized mathematics systems. Mei is a ..\-calculus style module system that supports higher-order functors in a natural way. The semantics of functor application is based on substitution. A novel coercion mechanism integrates a parameter passing mechanism based on theory interpretations with simple ..\-calculus style higher-order functors. DMei extends Mei by supporting dependent functor types. Mei is the first module system that successfully supports both higher-order functors and a parameter passing mechanism based on theory interpretations. Ill Acknowledgments I would first like to express my sincere thanks and appreciation to my supervisor, Dr. William M. Farmer, for his insight, thoughtful guidance and constant encourage­ ment throughout my study. Thanks to the members of my Supervisor Committee: Dr. Jacques Carette, Dr. Tom Maibaum, and Dr. Jeffery I. Zucker, for many valuable discussions and com­ ments. Thanks to all my other professors for their help in these four years. Thanks to friends in ITC 206 for the time we shared together. Thanks to my brother, Qin, for his encouraging words "You might not be born to be creative, however you can work hard to be creative.". Thanks to my sister, Yanping, and my brother, Jianping for pushing me forward constantly. Last but not least, many thanks to my parents for their support, my daughters, Katie and Kasidy, for the happiness they brought to me, and my wife, Yan Li, for her love and encouragement. I am grateful to have been partially supported by postgraduate scholarships from the Natural Sciences and Engineering Research Council of Canada (NSERC) and Ontario Graduate Scholarships in Science and Technology (OGSST). IV Contents Abstract iii Acknowledgements iv 1 Introduction 1 1.1 Background study ........... 2 1.1.1 ML-family module systems .. 3 1.1.2 Algebraic specification systems 7 1. 2 Design goals . 10 1.3 Overview . 13 1.4 Major contributions . 15 2 A simple module system - Mei Basic 17 2.1 Informal presentation . 17 2 .1.1 Theories . 17 2.1.2 Theory extension, renaming, and union 20 2.1.3 Parameterized theories 24 2.2 Syntax ........ 29 2.3 Rules ............. 31 2.3.1 Rules for types . 31 2.3.2 Rules for typing module expressions . 31 2.4 Semantics . 33 2.4.1 Substitution of module expression . 33 2.4.2 Operational semantics ...... 34 2.4.3 Denotational semantics ...... 42 2.5 Issues about conservative extensions of theories 47 v vi Jian Xu - Ph.D. Thesis - Department of CAS, McMaster University 3 A module system with subtyping and coercion - Mei 49 3.1 Subtyping . 49 3.1.1 Naive subtyping rules .... 51 3.1.2 Algorithmic subtyping rules 51 3.1.3 New typing rule ..... 53 3.1.4 Instantiation of functors 54 3.2 Syntax of Mei Core . 54 3.3 Semantics of Mei Core . 57 3.4 Coercion . 62 3.4.1 Theory interpretation . 64 3.4.2 Views and coercions . 65 3.4.3 Extended module expressions 74 3.4.4 Coercion functor for general theory views . 74 3.4.5 Functor instantiation 75 3.5 Syntax of Mei . 77 3.6 Semantics of Mei 80 3.7 Casting ..... 81 4 A module system with bounded universal types - DMei 84 4.1 Motivation . 84 4.1.1 Type variables and new typing rules 85 4.1.2 Subtyping rules . 88 4.2 Syntax of DMei Core ... 96 4.3 Semantics of DMei Core . 104 4.4 Integration with coercion . 110 4.5 Tradeoff between Mei and DMei 116 4.6 A simplified module system with dependent functor types - SDMei 116 4.6.1 Motivation. 116 4.6.2 Intuition ......... 117 4.6.3 SDMei Core ....... 118 4.6.4 Integration with coercion . 119 5 Comparison of Mei with other module systems 120 5.1 ML-family module systems .... 120 5.2 Algebraic specification languages 121 5.2.1 Maude . 121 5.2.2 Specware 126 CONTENTS vii 5.2.3 CASL ....................... 128 5.2.4 An algebraic framework for higher-order modules. 130 5.3 Theorem provers 133 5.3.1 IMPS 133 5.3.2 PVS .. 135 5.3.3 Isabelle 137 5.3.4 Coq .. 139 5.3.5 Automath 139 5.4 Computer algebra systems 141 5.4.1 Maple .... 141 5.4.2 Mathematica 143 5.4.3 Axiom 144 5.4.4 Aldor 147 5.4.5 Focal . 149 5.4.6 Magma 151 5.5 An expressive language of signatures 152 6 Possible extensions of Mei 154 6.1 Theory definition . 154 6.2 Hiding and revealing 155 6.3 Local theories . 157 6.4 Functor composition 158 6.5 View lifting, view union, and view composition . 159 7 A structural implementation of Mei 161 7.1 Structure of the implementation . 161 7.2 Identifiers .......... 163 7.3 Abstract syntax for MMS .. 164 7.4 Abstract syntax for Mei Core 166 7.5 Environment and context ... 168 7.6 Type checking and evaluation of Mei Core 169 7. 7 Theory translation . 173 7.8 Abstract syntax for Mei ......... 174 7. 9 An application over a system of first-order logic 176 8 A module system for multi-logic MMSs 179 viii Jian Xu - Ph.D. Thesis - Department of CAS, McMaster University 9 Conclusion and future work 183 Chapter 1 Introduction A mechanized mathematics system (MMS) is a computer system that supports and improves mathematical processing, by which we mean building mathematical models of the real world, and formulating and reasoning about properties of the real world within the context of the mathematical models. Examples of MMSs are theorem provers and computer algebra systems. Although it is important for an MMS to have an expressive and practical logic, an efficient proof engine, and a friendly user interface, it is barely useful without a powerful library. A powerful library for an MMS should (1) contain sufficient mathematical knowledge to support mathematical activities, and (2) be well organized so that new knowledge can be easily developed from existing knowledge. Currently, more and more mathematical knowledge is being formalized in different systems, which largely achieves the first goal. Thus, a good module system is necessary for MMSs in order to organize this mathematical knowl­ edge. A good modularity mechanism aids the expressivity of MMSs. It helps to build up contexts and allows the user to reuse the theorems developed within one context in other contexts with similar structure. However, the development of the module sys­ tems for MMSs significantly lags behind the development of underlying logics, proof strategies, computational power, algorithms etc. Very few MMSs have a sophisti­ cated module system that supports the development of large pieces of mathematical knowledge. In this thesis, we will present several related module systems designed for MMSs. In this chapter, we will investigate some existing module systems, present our design goals, give an overview of our systems, and state our major contribution. 1 2 Jian Xu - Ph.D. Thesis - Department of CAS, McMaster University 1.1 Background study Most programming languages and specification languages have module systems for organizing large software developments and specifications. In this section we will review two families of module systems, namely, the ML-family module systems and the algebraic specification module systems. Modular programming is a way to structure a large software system consisting of a collection of small pieces of codes. Though modular programming can be done in any programming language with sufficient discipline from programmers [75], many modern languages provide facilities for expressing and automatically checking their modular structures. The module systems of programming languages emphasize the decomposition of a large software development into smaller components. Code reuse and parallel software development are important for them. On the one hand, it is desirable to decompose programs into modules that are as independent as possible. On the other hand, these modules have to interact within some program, which requires an interface to express the essential information for communication. Among many modular mechanisms, the ML-family module systems are of particular interest because it treats parameterized modules as functors, i.e. functions from modules to modules. The module system itself is a small typed functional language, where structures and functors are objects and signatures are types. Hence researchers in this area enjoy a type-theoretic formalism approach to their module system [47, 57]. There are efforts to extend proof assistant systems with an ML-family module system, e.g. in Coq [18]. Algebraic specification languages are another area enjoying modularity. The mod­ ule systems of specification languages stress a refinable way of specification develop­ ments. Theories and axioms are important for them; however, logical reasoning is not one of their concerns. A module of a specification language is an algebraic speci­ fication consisting of a signature and a set of axioms, roughly equal to our definition of a theory. The researchers in this area favour a category-theoretic approach where specification morphism (or simply morphism) plays a central role. A morphism is a homomorphic map from the sorts and operators of one specification to the sorts and operators of another such that axioms of the source specification are translated to axioms (or more generally, theorems) of the target specification.