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178 lines
8.8 KiB
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>1.1. Purpose</TITLE>
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<BODY>
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<meta name="description" value=" Purpose">
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<meta name="keywords" value="clm">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html1555 HREF="node7.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html1553 HREF="node5.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html1547 HREF="node5.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html1557 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html1558 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html1556 HREF="node7.html"> Notational Conventions</A>
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<B>Up:</B> <A NAME=tex2html1554 HREF="node5.html"> Introduction</A>
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<B> Previous:</B> <A NAME=tex2html1548 HREF="node5.html"> Introduction</A>
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<HR> <P>
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<H1><A NAME=SECTION00510000000000000000> 1.1. Purpose</A></H1>
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<P>
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Common Lisp is intended to meet these goals:
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<P>
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<DL COMPACT><DT><i>Commonality</i>
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<DD>
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Common Lisp originated in an attempt to focus the
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work of several implementation groups, each of which was constructing successor
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implementations of MacLisp for different computers. These
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implementations had begun to diverge because of the differences in the
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implementation environments: microcoded personal computers (Zetalisp,
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Spice Lisp), commercial timeshared computers (NIL-the ``New Implementation of
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Lisp''), and supercomputers (S-1 Lisp). While the differences among the several
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implementation environments of necessity will continue to force
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certain incompatibilities among the
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implementations, Common Lisp serves as a common dialect to
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which each implementation makes any necessary extensions.
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<P>
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<DT><i>Portability</i>
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<DD>
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<P>
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Common Lisp intentionally excludes features
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that cannot be implemented easily on a broad class of machines.
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On the one hand, features that are difficult or expensive
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to implement on hardware without special microcode are avoided
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or provided in a more abstract and efficiently implementable form.
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(Examples of this are the invisible forwarding pointers
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and locatives of Zetalisp. Some of the problems that they solve
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are addressed in different ways in Common Lisp.)
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On the other hand, features that are useful only on certain ``ordinary''
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or ``commercial'' processors are avoided or made optional. (An example of
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this is the type declaration facility, which is useful in some
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implementations and completely ignored in others. Type declarations are
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completely optional and for correct programs
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affect only efficiency, not semantics.)
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Common Lisp is designed to make it easy to write programs
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that depend as little as possible on machine-specific
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characteristics, such as word length, while allowing some variety of
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implementation techniques.
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<P><DT><i>Consistency</i>
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<DD>
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Most Lisp implementations are internally inconsistent
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in that by default the interpreter and compiler may assign different
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semantics to correct programs.
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This semantic difference stems primarily from the fact
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that the interpreter assumes all variables to be dynamically scoped,
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whereas the compiler assumes all variables to be local unless explicitly
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directed otherwise. This difference has been the usual practice in Lisp
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for the sake of convenience
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and efficiency but can lead to very subtle bugs. The definition of
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Common Lisp avoids such anomalies by explicitly requiring the interpreter
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and compiler to impose identical semantics on correct programs
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so far as possible.
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<P>
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<DT><i>Expressiveness</i>
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<DD>
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Common Lisp culls what
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experience has shown to be the most useful and understandable constructs
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from not only MacLisp but also
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Interlisp, other Lisp dialects, and other programming languages.
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Constructs judged to be awkward or less useful have been
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excluded. (An example is the <tt>store</tt> construct of MacLisp.)
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<P>
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<DT><i>Compatibility</i>
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<DD>
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Unless there is a good reason to the contrary,
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Common Lisp strives to be compatible with Lisp Machine Lisp, MacLisp, and
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Interlisp, roughly in that order.
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<P>
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<DT><i>Efficiency</i>
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<DD>
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Common Lisp has a number of features designed to
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facilitate the production of high-quality compiled code in those
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implementations whose developers
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care to invest effort in an optimizing compiler.
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One implementation of Common Lisp, namely S-1 Lisp, already has a compiler
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that produces code for numerical computations that is competitive
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in execution speed to that produced by a Fortran compiler [<A HREF="node368.html#S1COMPILER">11</A>].
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The S-1 Lisp compiler
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extends the work done in MacLisp to produce extremely efficient
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numerical code [<A HREF="node368.html#MACLISPBEATSFORTRAN">19</A>].
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<P>
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<DT><i>Power</i>
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<DD>
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Common Lisp is a descendant of MacLisp, which has
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traditionally placed emphasis on providing system-building tools.
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Such tools may in turn be used to build the user-level packages
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such as Interlisp provides; these packages are not, however, part
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of the Common Lisp core specification. It is expected such packages will
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be built on top of the Common Lisp core.
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<P>
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<DT><i>Stability</i>
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<DD>
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It is intended that Common Lisp
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will change only slowly and with due deliberation. The various dialects
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that are supersets of Common Lisp may serve as laboratories within which to
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test language extensions, but such extensions will be added to
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Common Lisp only after careful examination and experimentation.
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<P>
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</DL>
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<P>
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<P>
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The goals of Common Lisp are thus very close to those of Standard Lisp
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[<A HREF="node368.html#STANDARDLISPREPORT">31</A>] and Portable Standard Lisp [<A HREF="node368.html#PSLMANUAL">51</A>].
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Common Lisp differs from Standard Lisp
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primarily in incorporating more features, including a
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richer and more complicated set of data types and more complex
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control structures.
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<P>
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This book is intended to be a language specification
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rather than an implementation specification
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(although implementation notes are scattered throughout the text).
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It defines a set of
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standard language concepts and constructs that may be used
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for communication of data structures and algorithms in the Common Lisp
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dialect. This set of concepts
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and constructs is sometimes referred to as the ``core Common Lisp language''
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because it contains conceptually necessary or important features.
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It is not necessarily implementationally minimal.
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While many features could be defined in terms of others
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by writing Lisp code, and indeed may be implemented that way,
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it was felt that these features should be conceptually primitive
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so that there might be agreement among all users as to their usage.
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(For example, bignums and rational numbers could be implemented as
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Lisp code given operations on fixnums. However, it is important
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to the conceptual integrity of the language that they be regarded
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by the user as primitive, and they are useful enough to warrant
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a standard definition.)
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<P>
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For the most part, this book defines a programming language, not a
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programming environment. A few interfaces are defined for
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invoking such standard programming tools as a compiler, an editor,
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a program trace facility, and a debugger, but very little is said
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about their nature or operation. It is expected that one or more
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extensive programming environments will be built using Common Lisp as a
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foundation, and will be documented separately.
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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There are now many implementations of Common Lisp,
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some programmed by research groups in universities
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and some by companies that sell them commercially,
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and a number of useful
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programming environments have indeed grown up around
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these implementations.
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What is more, all the goals stated above have been achieved,
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most notably that of portability. Moving large bodies
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of Lisp code from one computer to another is now routine.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<BR> <HR><A NAME=tex2html1555 HREF="node7.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html1553 HREF="node5.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html1547 HREF="node5.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html1557 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html1558 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html1556 HREF="node7.html"> Notational Conventions</A>
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<B>Up:</B> <A NAME=tex2html1554 HREF="node5.html"> Introduction</A>
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<B> Previous:</B> <A NAME=tex2html1548 HREF="node5.html"> Introduction</A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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</ADDRESS>
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</BODY>
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