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14.1.1 Preface SECOND EDITION

Common Lisp has succeeded.

Common Lisp has proved to be a useful and stable platform for rapid prototyping and systems delivery in artificial intelligence and other areas.

One of the important characteristics of Lisp is its good support for experimental extension of the language; while Common Lisp has been stable, it has not stagnated.

The 1984 definition of Common Lisp was imperfect and incomplete.

1984 Common Lisp had plenty of ways to signal errors but no way for a program to trap or process them.

This realization led to the formation of X3J13, a subcommittee of ANSI committee X3, to produce a formal American National Standard for Common Lisp.

The purpose of this second edition is to bridge the gap between the first edition and the forthcoming ANSI standard for Common Lisp. Because of the requirement for formal public review, it will be some time yet before the ANSI standard is final. This book in no way resembles the forthcoming standard (which is being written independently by Kathy Chapman of Digital Equipment Corporation with assistance from the X3J13 Drafting Subcommittee).

I have incorporated into this second edition a great deal of material based on the votes of X3J13, in order to give the reader a picture of where the language is heading. My purpose here is not simply to quote the X3J13 documents verbatim but to paraphrase them and relate them to the structure of the first edition.

I wish to be very clear: this book is not an official document of X3J13, though it is based on publicly available material produced by X3J13. In no way does this book constitute a definitive description of the forthcoming ANSI standard.

Until the day when an official ANSI Common Lisp standard emerges, it is likely that the 1984 definition of Common Lisp will continue to be used widely. This book has been designed to be used as a reference both to the 1984 definition and to the language as modified by the actions of X3J13.

It contains the entire text of the first edition of Common Lisp: The Language, with corrections and minor editorial changes; however, more than half of the material in this edition is new. All new material is identified by solid lines in the left margin. Dotted lines in the left margin indicate material from the first edition that applies to the 1984 definition but that has been modified by a vote of X3J13. Modifications to these outmoded passages are explained by preceding or following text (which will have a solid line in the margin).

The chapter and section numbering of this edition matches that of the first edition, with the exception that a new section 7.9 has been interpolated. Four new chapters (26-29) describe substantial changes approved by X3J13:

X3J13, in the course of its work, formed a subcommittee to study whether additional means of iteration should be standardized for use in Common Lisp, for a great deal of existing practice in this area was not included in the first edition because of lack of agreement in 1984. The X3J13 Iteration Subcommittee produced reports on three possible facilities. One (loop) was approved for inclusion in the forthcoming draft standard and is described in chapter 26.

X3J13 expressed interest in the other two approaches (series and generators), but the consensus as of January 1989 was that these other approaches were not yet sufficiently mature or in sufficiently widespread use to warrant inclusion in the draft Common Lisp standard at that time. However, the subcommittee was directed to continue work on these approaches and X3J13 is open to the possibility of standardizing them at a later date. Please note that I do not wish the prejudge the question of whether X3J13 will ever choose to make the other two proposals the subject of standardization. Nevertheless, I have chosen to include them in the second edition, in cooperation with Dr. Richard C. Waters, as appendices A and B, in order to make these ideas available to the Lisp community. In my judgement these proposals address an area of language design not otherwise covered by Common Lisp and are likely to have practical value even if they are never adopted as part of a formal standard.

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