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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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<TITLE>Preface SECOND EDITION</TITLE>
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<meta name="description" value=" Preface
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SECOND EDITION">
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html1498 HREF="node3.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html1496 HREF="clm.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html1490 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html1500 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html1501 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html1499 HREF="node3.html"> Acknowledgments
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SECOND EDITION</A>
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<B>Up:</B> <A NAME=tex2html1497 HREF="clm.html">Common Lisp the Language</A>
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<B> Previous:</B> <A NAME=tex2html1491 HREF="node1.html">Contents</A>
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<HR> <P>
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<H1><A NAME=SECTION00200000000000000000> Preface </A></H1>
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SECOND EDITION <br>
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<P>
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Common Lisp has succeeded. Since publication of the first edition of
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this book in 1984, many implementors have used it as a <i>de facto</i>
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standard for Lisp implementation. As a result, it is now much easier
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to port large Lisp programs from one implementation to another.
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Common Lisp has proved to be a useful and stable platform for
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rapid prototyping and
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systems delivery in artificial intelligence and other areas.
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WIth experience gained in using Common Lisp for so many
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applications, implementors found no shortage of opportunities for
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innovation.
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One of the important
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characteristics of Lisp is its good support for experimental extension
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of the language; while Common Lisp has been stable, it has not stagnated.
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<P>
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The 1984 definition of Common Lisp was imperfect and incomplete.
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In some cases this was inadvertent: some odd boundary situation was
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overlooked and its consequences not specified, or different
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passages in were conflict, or some property of Lisp was so well-known
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and traditionally relied upon that I forgot to write it down.
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In other cases the informal committee that was defining
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Common Lisp could not settle on a solution, and therefore agreed to leave
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some important aspect of the language unspecified rather than
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choose a less than satisfactory definition. An example
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is error handling; 1984 Common Lisp had plenty of ways to signal
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errors but no way for a program to trap or process them.
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<P>
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Over the next year I collected reports of errors in the book and gaps in the
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language. In December 1985, a group of implementors and users met in
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Boston to discuss the state of Common Lisp. I prepared
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two lists for this meeting, one of errata and clarifications
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that I thought would be relatively uncontroversial (boy, was I wrong!)
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and one of more substantial changes I thought should be considered and
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perhaps voted upon. Others also brought proposals to discuss.
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It became clear to everyone that there was now enough interest in Common Lisp,
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and dependence on its stability, that a more formal mechanism was needed for
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managing changes to the language.
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<P>
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This realization led to the formation of X3J13, a subcommittee of
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ANSI committee X3, to produce a formal American National Standard
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for Common Lisp. That process is nearing completion. X3J13 has
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completed the bulk of its technical work in rectifying the 1984
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definition and codifying extensions to that definition that have
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received widespread use and approval. A draft standard is now
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being prepared; it will probably be available
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in 1990. There will then be a period (required by ANSI) for
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public review. X3J13 must then consider the comments it receives
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and respond appropriately. If the comments result
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in substantial changes to the draft standard, multiple public review
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periods may be required before the draft can be approved as an American
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National Standard.
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<P>
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Fortunately, X3J13 has done an outstanding job of documenting its work.
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For every change that came to a formal vote, a document was prepared
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that described the problem to be solved and one or more solutions.
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For each solution there is a detailed proposal for changing the
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language; a rationale; test cases that distinguish the proposal
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from the status quo or from other proposals for solving that problem;
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discussions of current practice, cost to implementors, cost to users,
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cost of not adopting the proposal, benefits of adoption,
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aesthetic criteria; and any relevant informal discussion that may have
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preceded creation of the formal proposal. All of these proposal
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documents were made available on-line as well as in paper form.
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By my count, by June 1989 some
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186 such proposals were approved as language changes.
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(This count does not include many proposals that came before the committee
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but were rejected.)
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<P>
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The purpose of this second edition is to bridge the gap between the
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first edition and the forthcoming ANSI standard for Common Lisp.
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Because of the requirement for formal public review,
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it will be some time yet before the ANSI standard is final.
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This book in no way resembles the forthcoming standard (which
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is being written independently
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by Kathy Chapman of Digital Equipment Corporation with assistance
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from the X3J13 Drafting Subcommittee).
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<P>
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I have incorporated into this second edition
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a great deal of material based on the votes of X3J13,
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in order to give the reader a picture of where the language is heading.
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My purpose here is not simply to quote the X3J13 documents verbatim
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but to paraphrase them and relate them to the structure of the first
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edition. A single vote by X3J13 may be discussed in many parts of this book,
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and a single passage of this book may be affected by many of the votes.
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<P>
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I wish to be very clear: this book is not an official document
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of X3J13, though it is based on publicly available material
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produced by X3J13. In no way does this book constitute a definitive
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description of the forthcoming ANSI standard. The
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committee's decisions have been remarkably stable (it has rescinded
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earlier decisions only two or three times), and I do not
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expect radical changes in direction.
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Nevertheless, it is quite probable
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that the draft standard will be substantively revised in response to
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editorial review or public comment.
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I have therefore reported here on the actions of X3J13 not to
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inscribe them in stone, but to make clear how the language
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of the first edition is likely to change.
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I have tried to be careful
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in my wording to avoid saying ``the language has been changed''
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and to state simply that
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``X3J13 voted at such-and-so time to make the following change.''
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<P>
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Until the day when an official ANSI Common Lisp standard emerges,
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it is likely that the 1984 definition of Common Lisp will
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continue to be used widely. This book has been designed
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to be used as a reference both to the 1984 definition
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and to the language as modified by the actions of X3J13.
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<P>
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It contains the entire text of the first edition
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of <i>Common Lisp: The Language</i>, with corrections
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and minor editorial changes;
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however, more than half of the material in this edition is new.
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All new material is
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identified by solid lines
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in the left margin.
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Dotted lines in the left margin indicate material from the first edition
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that applies to the 1984 definition but that has been modified
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by a vote of X3J13. Modifications to these outmoded
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passages are explained by preceding or following text (which will
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have a solid line in the margin).
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In summary:
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<UL><LI> To use the 1984 language definition, read all material that does not
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have a solid line in the margin.
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<LI> To use the updated language definition, read everything, but
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be wary of material with a dotted line in the margin.
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</UL>
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<P>
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At the end of the book is an index of the X3J13 votes, ordered
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by the committee's internal code names (included to ease cross-reference
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to the X3J13 documents, which may be useful during the public review
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periods). References to this list of votes appear as numbers
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in angle brackets; thus
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``<14>'' refers to the vote on issue number 14, whereas
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``[14]'' refers to reference 14 in the bibliography.
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<P>
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I have kept changes to the wording of the first-edition material to a minimum.
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Obvious spelling and typographical errors have been corrected,
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and the entire text has been edited to a uniform style of
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spelling and punctuation. (Note in particular that the first edition
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used the spelling ``signalling'' but this edition,
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in deference to the style decision of the X3J13 Drafting
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Subcommittee, uses ``signaling.'') A few minor
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changes were made to accommodate typographical or layout constraints.
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(For example, the word ``also'' has been deleted from the first
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sentence of chapter 1, partly to make that paragraph look better
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and partly to allow a better page break at the bottom of page 2.)
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In a very few cases the first edition contained substantive errors
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that I could not in good conscience correct silently; these have
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been flagged by paragraphs beginning with the phrase
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<i>Notice of correction</i>.
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<P>
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The chapter and section numbering of this edition matches that
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of the first edition, with the exception that a new
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section <A HREF="node92.html#STRUCTURETRAVERSALSECTION">7.9</A>
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has been interpolated.
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Four new chapters (<A HREF="node235.html#LOOP">26</A>-<A HREF="node312.html#CONDITION">29</A>)
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describe substantial changes approved by X3J13: an extended
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<tt>loop</tt> macro, a pretty printer interface, the Common Lisp
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Object System, and the Common Lisp Condition System.
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<P>
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X3J13, in the course of its work, formed a subcommittee to study
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whether additional means of iteration
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should be standardized for use in Common Lisp, for a great
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deal of existing practice in this area was not included in the
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first edition because of lack of agreement in 1984.
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The X3J13 Iteration Subcommittee produced reports on three possible facilities.
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One (<tt>loop</tt>) was approved for inclusion in the forthcoming draft standard
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and is described in chapter <A HREF="node235.html#LOOP">26</A>.
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<P>
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X3J13 expressed interest in the other two approaches (series and generators),
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but the consensus as of January 1989
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was that these other approaches were not yet sufficiently mature or
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in sufficiently widespread use to warrant inclusion in the draft Common Lisp
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standard at that time. However, the subcommittee was directed to continue work
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on these approaches and X3J13 is open to the possibility of standardizing
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them at a later date.
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Please note that I do not wish the prejudge the
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question of whether X3J13 will ever choose to make the other two proposals the
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subject of standardization. Nevertheless,
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I have chosen to include them in the second edition,
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in cooperation with Dr. Richard C. Waters,
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as appendices <A HREF="node347.html#SERIES">A</A> and <A HREF="node362.html#GENERATORS">B</A>,
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in order to make these ideas
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available to the Lisp community. In my judgement these proposals
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address an area of language design not otherwise covered by Common Lisp
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and are likely to have practical value even if they are never
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adopted as part of a formal standard.
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<P>
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Some new material in this book has nothing to do with the work of X3J13.
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In many places I have added explanations, clarifications, new examples,
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warnings, and tips on writing portable code.
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Appendix <A HREF="node367.html#BACKQUOTESIMULATOR">C</A> contains a piece of code
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that may help in understanding the backquote syntax.
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<P>
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This second edition,
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unlike the first edition, also includes a few diagrams to pep up the text.
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However, there are absolutely no new jokes, and very few outright lies.
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<P>
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<BR> <HR><A NAME=tex2html1498 HREF="node3.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html1496 HREF="clm.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html1490 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html1500 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html1501 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html1499 HREF="node3.html"> Acknowledgments
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SECOND EDITION</A>
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<B>Up:</B> <A NAME=tex2html1497 HREF="clm.html">Common Lisp the Language</A>
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<B> Previous:</B> <A NAME=tex2html1491 HREF="node1.html">Contents</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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