viii PREFACE
form a complete tutorial on macro technique. By the end of it you will know most
of what an experienced Lisp programmer knows about macros: how they work;
how to define, test, and debug them; when to use macros and when not; the major
types of macros; how to write programs which generate macro expansions; how
macro style differs from Lisp style in general; and how to detect and cure each of
the unique problems that afflict macros.
Following this tutorial, Chapters 11–18 show some of the powerful abstrac-
tions you can build with macros. Chapter 11 shows how to write the classic
macros—those which create context, or implement loops or conditionals. Chap-
ter 12 explains the role of macros in operations on generalized variables. Chap-
ter 13 shows how macros can make programs run faster by shifting computation
to compile-time. Chapter 14 introduces anaphoric macros, which allow you to
use pronouns in your programs. Chapter 15 shows how macros provide a more
convenient interface to the function-builders defined in Chapter 5. Chapter 16
shows how to use macro-defining macros to make Lisp write your programs for
you. Chapter 17 discusses read-macros, and Chapter 18, macros for destructuring.
With Chapter 19 begins the fourth part of the book, devoted to embedded
languages. Chapter 19 introduces the subject by showing the same program, a
program to answer queries on a database, implemented first by an interpreter
and then as a true embedded language. Chapter 20 shows how to introduce
into Common Lisp programs the notion of a continuation, an object representing
the remainder of a computation. Continuations are a very powerful tool, and
can be used to implement both multiple processes and nondeterministic choice.
Embedding these control structures in Lisp is discussed in Chapters 21 and 22,
respectively. Nondeterminism, which allows you to write programs as if they
had foresight, sounds like an abstraction of unusual power. Chapters 23 and 24
present two embedded languages which show that nondeterminism lives up to its
promise: a complete
ATN parser and an embedded Prolog which combined total
about 200 lines of code.◦
Thefactthat theseprogramsare shortmeansnothing initself. If youresortedto
writing incomprehensible code, there’s no telling what you could do in 200 lines.
The point is, these programs are not short because they depend on programming
tricks, but because they’re written using Lisp the way it’s meant to be used. The
point of Chapters 23 and 24 is not how to implement
ATNs in one page of code
or Prolog in two, but to show that these programs, when given their most natural
Lisp implementation, simply are that short. The embedded languages in the latter
chapters provide a proof by example of the twin points with which I began: that
Lisp is a natural language for bottom-up design, and that bottom-up design is a
natural way to use Lisp.
The book concludes with a discussion of object-oriented programming, and
particularly
CLOS, the Common Lisp Object System. By saving this topic till
