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Q stands for "equational", so Q, in a nutshell, is a programming language which lets you "program by equations". You specify a system of equations which the interpreter uses as "rewrite rules" to reduce expressions to "normal form". This allows you to formulate your programs in a high-level, concise and declarative style. Q's development started out in the 1990s, motivated by the author's research on term pattern matching [Gräf 1991] and inspired by the pioneering work on equational programming by Michael O'Donnell and others [O'Donnell 1985], as an attempt to show that term rewriting would provide a feasible basis for a practical programming language. I think that this goal has been achieved, and that the present interpreter is efficient and robust enough for practical purposes. Q has been ported to a variety of operating systems, such as BeOS, FreeBSD, Linux, MacOS X, Solaris and Windows. Porting to other modern UNIX/POSIX environments should be a piece of cake. Thus Q can be used on most modern computer systems, and Q scripts written on one system will usually run on all supported platforms.
The Q language supports a rich variety of built-in types, like arbitrary precision integers, floating point numbers (double precision 64 bit), truth values, strings, tuples, lists, streams (a "lazy" list data structure with call-by-need evaluation), lambda functions and files. It also provides primitives for exception handling and multithreaded execution. Q scripts can be broken down into "modules", each with their separate namespace, and Q gives you full control over which symbols (function, variable and type names) are exported by each module. This makes it easy to organize large scripts in a modular fashion. Q also allows you to interface to "external" modules written in the C programming language, which provides a means to access functions in C and C++ libraries and employ C's higher processing speed for time-critical tasks. Conversely, Q scripts can also be executed from C and C++, which allows Q to be used as an embedded language or term rewriting engine in C/C++ applications.
As a practical programming language, Q comes with "batteries included". The standard library, which is mostly written in Q itself, provides rational and complex numbers, a lot of useful tuple, list and stream processing functions (including comprehensions), some common container data structures (dictionaries, sets, etc.), and an interface to the PostScript language. It also includes an extensive system interface which offers services such as binary and C-style formatted I/O, BSD socket I/O, process management, POSIX threads, regular expression matching and internationalization features. Additional extension modules provide interfaces to a number of other third party libraries, which turns Q into a practical tool for a variety of application areas.
In difference to other functional languages, Q is entirely based on the notions of rewrite rules, reductions and irreducible expressions (also known as normal forms) pertaining to the term rewriting calculus. A Q "program", called a script, consists of equations which are treated as rewrite rules and are used to reduce expressions to normal form. The normal form of an expression denotes its value, which can itself be a compound expression. Q has no rigid distinction between "constructor" and "defined" function symbols and it also allows you to evaluate expressions containing "free" variables. Basically, both sides of an equation may involve arbitrary expressions. Therefore Q can also be used as a tool for symbolic expression evaluation.
On the surface, Q looks very much like contemporary functional languages such as Miranda or Haskell. In fact, the syntax of the language has largely been inspired by the first edition of Introduction to Functional Programming by Richard Bird and Philip Wadler. However, Q is an interpreted language with dynamic typing and eager (leftmost-innermost) evaluation, which is more in line with classical functional languages such as Lisp. For the sake of efficiency, Q scripts are first translated into "bytecode" (an intermediate binary format) which is executed on a virtual stack machine. The interpreter automatically optimizes tail recursion, such that "iterative" algorithms can be realized in constant stack space. Besides the built-in I/O operations, the language is free of side-effects; in particular, Q itself does not have mutable variables (although the standard library provides such imperative features for those who need them). Q also provides two novel and (IMHO) interesting features: a notion of special forms which allows to handle both macro-like functions and lazy evaluation in a uniform setting without having to give up the basic eager evaluation strategy; and a notion of type guards which provides a means to cope with hierarchies of abstract data types (similar to the notion of classes with single inheritance in object-oriented languages) in the context of a term rewriting language.
Using Q is supposed to be fairly simple: you throw together some equations, start the interpreter and then type in the expressions you wish to evaluate. All this can be done with a few keystrokes, if you use the Emacs Q mode supplied with the package. A graphical user interface for Windows is also available. Of course, you can also run the Q programming utilities from the command line if you prefer that.
The manual is organized as follows. In Getting Started, we start out with a brief and informal introduction to the main features of the language, and a glimpse of how Q scripts look like. Lexical Matters, describes the lexical part of the Q language. In Scripts and Modules, we discuss how declarations, definitions and equations are put together to form a script, and how to break down larger scripts into a collection of smaller modules which can be managed separately. Declarations, discusses how certain entities like types, variables and function symbols can be declared in a Q script. Expressions, treats the syntax of Q expressions, and describes the built-in operators provided by the Q language. Equations and Expression Evaluation, is about equations and variable definitions, and how they are used in the evaluation process. Types, and Special Forms, describe the facilities provided by the Q language for dealing with abstract data types and deferred evaluation. Built-In Functions, and The Standard Library, discuss the built-in and standard library functions of the Q language. Clib, describes Q's "system module" which provides access to some important functions from the C library. The appendix gives additional information about some aspects of the language and its implementation. Q Language Grammar, contains a summary of the Q language syntax in BNF. Using Q, provides a description of the programming tools included in the Q programming system, C Language Interface, describes Q's interface to the C programming language, and Debugging, is a brief introduction to the symbolic debugger built into the Q interpreter. Finally, Running Scripts in Emacs, discusses how Q scripts can be edited and run using the Emacs editor.
This project has become much further reaching and took a lot longer than I anticipated when I started out working on it, and I wouldn't have been able to keep it going without the patience and support of my beloved wife Evelyn and children Sebastian, Janosch, Yannic and Miriam.
Next I have to acknowledge the support and help provided by colleagues at Mainz and Bremen (Germany) and at INRIA Lorraine (France) during the initial phases of this project. In 1992 Dieter Hofbauer invited me to the term rewriting group at INRIA Lorraine to discuss the design of the (then still nascent) Q with his colleagues and friends. During the following years, I also collaborated with Frank Drewes at Bremen (now at Umea, Sweden) who was involved in the initiation of this project, and Klaus Barthelmann at Mainz who provided many comments which helped to improve the early versions of this manual. Frank and Klaus tested various early beta versions of the Q interpreter, and contributed sample scripts as well as substantial parts of the standard Q library.
I'd also like to express my gratitude to the growing community of users of the Q language all over the world, in particular the members of the Q mailing list, for the lively discussions which helped to improve the language and its implementation, and for the contributed Q modules and applications. Special thanks are due to John Cowan for proofreading the manual, for his valuable suggestions concerning design and implementation issues and for his Q-Chicken interface, and to Rob Hubbard for his comprehensive rational number and polynomial modules which are now part of the distribution.
Last but not least, thanks are also due to the developers of ML and Haskell. While these people have not been involved with Q in any way, their work, which has changed the landscape of functional programming, has been a constant source of inspiration for me.
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