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* doc/*: Revert to Docbook 4.1 DOCTYPE for compatibility with older systems such as Debian Woody. Add catalog for Darwin's fink and for partially for Redhat. Need to add OS detection for fink in Makefile and stylesheet location for RedHat in catalog-redhat.xml
217 lines
11 KiB
XML
217 lines
11 KiB
XML
<?xml version="1.0" encoding="iso-8859-1"?>
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1//EN"
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"http://www.oasis-open.org/docbook/xml/4.1/docbookx.dtd" [
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<!ENTITY % myents SYSTEM "entities.inc">
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%myents;
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]>
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<chapter id="intro"><title>Introduction</title>
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<para>&SBCL; is a mostly-conforming implementation of the &ANSI;
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&CommonLisp; standard. This manual focuses on behavior which is
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specific to &SBCL;, not on behavior which is common to all
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implementations of &ANSI; &CommonLisp;.</para>
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<sect1 id="more-cl-info">
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<title>Where To Go For More Information about &CommonLisp; in General</title>
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<para>Regardless of your ability level, two very useful resources
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for working with any implementation of
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&CommonLisp; are the
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<ulink url="http://ilisp.cons.org"><application>ILISP</application></ulink>
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package for <application>Emacs</application> and
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<ulink url="http://www.harlequin.com/books/HyperSpec">the &CommonLisp;
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HyperSpec</ulink>.</para>
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<para>If you're not a programmer and you're trying to learn,
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many introductory Lisp books are available. However, we don't have any
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standout favorites. If you can't decide, try checking the Usenet
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comp.lang.lisp FAQ for recent recommendations.</para>
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<para>If you are an experienced programmer in other languages
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but need to learn about Lisp, three books stand out.
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<itemizedlist>
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<listitem><para><emphasis>ANSI Common Lisp</emphasis>, by Paul Graham,
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will teach you about most of the language. (And later it might
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also be worth checking out <emphasis>On Lisp</emphasis>, by the same
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author.)</para></listitem>
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<listitem><para><emphasis>Paradigms Of Artificial Intelligence
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Programming</emphasis>, by Peter Norvig, also has some good information
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on general &CommonLisp; programming, and many nontrivial examples.
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Whether or not your work is AI, it's a very good book to look
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at.</para></listitem>
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<listitem><para>
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Neither of the books above emphasizes CLOS, but
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<emphasis>Object-Oriented Programming In Common Lisp</emphasis> by Sonya Keene
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does. Even if you're very knowledgeable about object oriented
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programming in the abstract, it's worth looking at this book
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if you want to do any OO in &CommonLisp;. Some abstractions
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in CLOS (especially multiple dispatch) go beyond anything
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you'll see in most OO systems, and there are a number of lesser
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differences as well. This book tends to help with the culture shock.
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</para></listitem>
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</itemizedlist>
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</para>
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</sect1>
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<sect1 id="where-more">
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<title>Where To Go For More Information About &SBCL;</title>
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<para>Before you read this user manual, you should probably read
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two other things.
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<itemizedlist>
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<listitem><para>You should know how to program in &CommonLisp;.
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If you don't already know how, you should probably read a
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<link linkend="more-cl-info">book on it</link>.</para></listitem>
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<listitem><para>The Unix <quote>man page</quote> for &SBCL; will tell you
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how to start the &SBCL; environment, so you can get to the
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classic <quote>hello, world</quote> level of knowledge. It's the file
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called <filename>sbcl.1</filename> in the &SBCL; distribution. If &SBCL; is
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installed on your system, you can read a formatted copy by
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executing the command <command>man sbcl</command>.</para></listitem>
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</itemizedlist>
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</para>
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<para>Besides this user manual and the Unix man page, some
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other &SBCL;-specific information is available:
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<itemizedlist>
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<listitem><para>The <ulink url="http://sbcl.sourceforge.net/">
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&SBCL; home page</ulink> has some general
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information, plus links to mailing lists devoted to &SBCL;,
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and to archives of these mailing lists.</para></listitem>
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<listitem><para>Documentation for non-&ANSI; extensions for
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various commands is available online from the &SBCL; executable
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itself. The extensions for functions which have their own
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command prompts (e.g. the debugger, and <function>inspect</function>)
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are documented in text available by typing <userinput>help</userinput>
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at their command prompts. The extensions for functions which
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don't have their own command prompt (like <function>trace</function>
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does) are described in their documentation strings,
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unless your &SBCL; was compiled with an option not
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to include documentation strings, in which case the doc strings
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are only readable in the source code.</para></listitem>
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<listitem><para>Some low-level information describing the
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programming details of the conversion from &CMUCL; to &SBCL;
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is available in the <filename>doc/FOR-CMUCL-DEVELOPERS</filename>
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file in the &SBCL; distribution.</para></listitem>
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</itemizedlist>
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</para>
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</sect1>
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<sect1 id="implementation">
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<title>Overview Of SBCL, How It Works And Where It Came From</title>
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<para>You can work productively with SBCL without knowing anything
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understanding anything about where it came from, how it is implemented,
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or how it extends the &ANSI; &CommonLisp; standard. However,
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a little knowledge can be helpful in order to understand error
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messages, to troubleshoot problems, to understand why some parts of
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the system are better debugged than others, and to anticipate which
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known bugs, known performance problems, and missing extensions are
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likely to be fixed, tuned, or added. </para>
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<para>&SBCL; is descended from &CMUCL;, which is itself descended from
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Spice Lisp, including early implementations for the Mach operating
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system on the IBM RT, back in the 1980s. Design decisions from that
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time are still reflected in the current implementation:
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<itemizedlist>
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<listitem><para>The system expects to be loaded into a
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fixed-at-compile-time location in virtual memory, and also expects
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the location of all of its heap storage to be specified
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at compile time.</para></listitem>
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<listitem><para>The system overcommits memory, allocating large
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amounts of address space from the system (often more than
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the amount of virtual memory available) and then failing
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if ends up using too much of the allocated storage.</para></listitem>
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<listitem><para>A word is a 32-bit quantity. The system has been
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ported to many processor architectures without altering this
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basic principle. Some hacks allow the system to run on the Alpha
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chip (a 64-bit architecture) but even there 32-bit words are
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used. The assumption that a word is
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32 bits wide is implicit in hundreds of places in the
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system.</para></listitem>
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<listitem><para>The system is implemented as a C program which is
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responsible for supplying low-level services and loading a
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Lisp <quote>.core</quote> file.
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</para></listitem>
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</itemizedlist>
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</para>
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<para>&SBCL; also inherited some newer architectural features from
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&CMUCL;. The most important is that it has a generational garbage
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collector (<quote>GC</quote>), which has various implications (mostly good)
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for performance. These are discussed in <link linkend="efficiency">
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another chapter</link>.</para>
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<para>&SBCL; has diverged from &CMUCL; in that &SBCL; is now
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essentially a <quote>compiler-only implementation</quote> of
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&CommonLisp;. A &CommonLisp; implementation is permitted to implement
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both a compiler and an interpreter, and there's some special support
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in the standard (e.g. the distinction between <function>functionp</function>
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and <function>compiled-function-p</function>) to help support that. But &SBCL;
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has only a vestigial, rudimentary true interpreter. In &SBCL;, the
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<function>eval</function> function only truly <quote>interprets</quote> a few
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special classes of forms, such as symbols which are
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<function>boundp</function>. More complicated forms are evaluated by calling
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<function>compile</function> and then calling <function>funcall</function> on the
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returned result.
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</para>
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<para>The direct ancestor of &SBCL; is the X86 port of &CMUCL;. This
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port was in some ways the most cobbled-together of all the &CMUCL;
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ports, since a number of strange changes had to be made to support the
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register-poor X86 architecture. Some things (like tracing and
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debugging) do not work particularly well there. &SBCL; should be able
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to improve in these areas (and has already improved in some other
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areas), but it takes a while.</para>
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<para>On the x86, &SBCL; like the X86 port of &CMUCL;, uses a
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<emphasis>conservative</emphasis> GC. This means that it doesn't maintain a
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strict separation between tagged and untagged data, instead treating
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some untagged data (e.g. raw floating point numbers) as
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possibly-tagged data and so not collecting any Lisp objects that they
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point to. This has some negative consequences for average time
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efficiency (though possibly no worse than the negative consequences of
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trying to implement an exact GC on a processor architecture as
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register-poor as the X86) and also has potentially unlimited
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consequences for worst-case memory efficiency. In practice,
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conservative garbage collectors work reasonably well, not getting
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anywhere near the worst case. But they can occasionally cause
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odd patterns of memory usage.</para>
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<para>The fork from &CMUCL; was based on a major rewrite of the system
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bootstrap process. &CMUCL; has for many years tolerated a very unusual
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<quote>build</quote> procedure which doesn't actually build the complete
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system from scratch, but instead progressively overwrites parts of a
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running system with new versions. This quasi-build procedure can cause
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various bizarre bootstrapping hangups, especially when a major change
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is made to the system. It also makes the connection between the
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current source code and the current executable more tenuous than in
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other software systems -- it's easy to accidentally
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<quote>build</quote> a &CMUCL; system containing characteristics not
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reflected in the current version of the source code.</para>
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<para>Other major changes since the fork from &CMUCL; include
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<itemizedlist>
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<listitem><para>&SBCL; has dropped support for many &CMUCL; extensions,
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(e.g. IP networking, remote procedure call, Unix system interface, and X11
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interface). Some of these are now available as contributed or
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third-party modules.</para></listitem>
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<listitem><para>&SBCL; has deleted or deprecated
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some nonstandard features and code complexity which helped
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efficiency at the price of maintainability. For example, the
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&SBCL; compiler no longer implements memory pooling internally
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(and so is simpler and more maintainable, but generates more
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garbage and runs more slowly), and various block-compilation
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efficiency-increasing extensions to the language have been
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deleted or are no longer used in the implementation of &SBCL;
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itself.</para></listitem>
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</itemizedlist>
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</para>
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</sect1>
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</chapter>
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