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<HTML
><HEAD
><TITLE
>Frequently Asked Questions for comp.lang.lisp</TITLE
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CLASS="ARTICLE"
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><DIV
CLASS="ARTICLE"
><DIV
CLASS="TITLEPAGE"
><H1
CLASS="TITLE"
><A
NAME="AEN1"
>Frequently Asked Questions for <A
HREF="news:comp.lang.lisp"
TARGET="_top"
>comp.lang.lisp</A
></A
></H1
><DIV
><DIV
CLASS="ABSTRACT"
><A
NAME="AEN4"
></A
><P
></P
><P
>			This document aims to provide answers to questions frequently
			asked on <A
HREF="news:comp.lang.lisp"
TARGET="_top"
>the comp.lang.lisp
			newsgroup</A
>. It is hoped that this will promote a more
			harmonious atmosphere on the newsgroup, while also enabling
			newcomers better to judge the appropriateness or otherwise of
			their putative posts.
		</P
><P
>			This document's source is available from <A
HREF="http://cclan.sourceforge.net/"
TARGET="_top"
>cCLan</A
>'s CVS
			repository, in a module named `faq'; it was derived in part from
			the <A
HREF="http://www.cs.cmu.edu/Groups//AI/html/faqs/lang/lisp/top.html"
TARGET="_top"
>old
			comp.lang.lisp FAQ</A
> maintained by Mark Kantrowitz and
			Barry Margolin, to whom the current maintainers extend their
			thanks. The canonical location for the HTML version of this
			document is currently <A
HREF="http://www-jcsu.jesus.cam.ac.uk/~csr21/lispfaq.html"
TARGET="_top"
>http://www-jcsu.jesus.cam.ac.uk/~csr21/lispfaq.html</A
>.
		</P
><P
>			Corrections, suggestions, and comments to, for and regarding
			this document are welcome, either on <A
HREF="http://sourceforge.net/mail/?group_id=28536"
TARGET="_top"
>cCLan mailing
			lists</A
> or as a followup to the regular posting on <A
HREF="news:comp.lang.lisp"
TARGET="_top"
>comp.lang.lisp</A
>; while this FAQ
			is under development, you may also contact the `primary'
			maintainer directly at <A
HREF="mailto:csr21@cam.ac.uk"
TARGET="_top"
>csr21@cam.ac.uk</A
>.
		</P
><P
></P
></DIV
></DIV
><HR></DIV
><BLOCKQUOTE
CLASS="ABSTRACT"
><DIV
CLASS="ABSTRACT"
><A
NAME="AEN4"
></A
><P
></P
><P
>			This document aims to provide answers to questions frequently
			asked on <A
HREF="news:comp.lang.lisp"
TARGET="_top"
>the comp.lang.lisp
			newsgroup</A
>. It is hoped that this will promote a more
			harmonious atmosphere on the newsgroup, while also enabling
			newcomers better to judge the appropriateness or otherwise of
			their putative posts.
		</P
><P
>			This document's source is available from <A
HREF="http://cclan.sourceforge.net/"
TARGET="_top"
>cCLan</A
>'s CVS
			repository, in a module named `faq'; it was derived in part from
			the <A
HREF="http://www.cs.cmu.edu/Groups//AI/html/faqs/lang/lisp/top.html"
TARGET="_top"
>old
			comp.lang.lisp FAQ</A
> maintained by Mark Kantrowitz and
			Barry Margolin, to whom the current maintainers extend their
			thanks. The canonical location for the HTML version of this
			document is currently <A
HREF="http://www-jcsu.jesus.cam.ac.uk/~csr21/lispfaq.html"
TARGET="_top"
>http://www-jcsu.jesus.cam.ac.uk/~csr21/lispfaq.html</A
>.
		</P
><P
>			Corrections, suggestions, and comments to, for and regarding
			this document are welcome, either on <A
HREF="http://sourceforge.net/mail/?group_id=28536"
TARGET="_top"
>cCLan mailing
			lists</A
> or as a followup to the regular posting on <A
HREF="news:comp.lang.lisp"
TARGET="_top"
>comp.lang.lisp</A
>; while this FAQ
			is under development, you may also contact the `primary'
			maintainer directly at <A
HREF="mailto:csr21@cam.ac.uk"
TARGET="_top"
>csr21@cam.ac.uk</A
>.
		</P
><P
></P
></DIV
></BLOCKQUOTE
><DIV
CLASS="QANDASET"
><DL
><DT
>1. <A
HREF="lispfaq.html#AEN16"
>This newsgroup</A
></DT
><DD
><DL
><DT
>1.1. <A
HREF="lispfaq.html#AEN19"
>What is the purpose of comp.lang.lisp?</A
></DT
><DT
>1.2. <A
HREF="lispfaq.html#AEN24"
>What is on-topic on comp.lang.lisp?</A
></DT
><DT
>1.3. <A
HREF="lispfaq.html#AEN31"
>What is off-topic on comp.lang.lisp?</A
></DT
><DT
>1.4. <A
HREF="lispfaq.html#AEN36"
>What do all those abbreviations mean?</A
></DT
><DT
>1.5. <A
HREF="lispfaq.html#AEN39"
>Is Scheme a lisp?</A
></DT
><DT
>1.6. <A
HREF="lispfaq.html#AEN46"
>Can you help me with my homework?</A
></DT
><DT
>1.7. <A
HREF="lispfaq.html#AEN54"
>What should I do if I feel victimized?</A
></DT
></DL
></DD
><DT
>2. <A
HREF="lispfaq.html#AEN56"
>Resources</A
></DT
><DD
><DL
><DT
>2.1. <A
HREF="lispfaq.html#AEN59"
>Where can I find free lisp compilers or interpreters?</A
></DT
><DT
>2.2. <A
HREF="lispfaq.html#AEN106"
>Where can I buy a professional lisp system?</A
></DT
><DT
>2.3. <A
HREF="lispfaq.html#AEN136"
>What books should I read to learn more about
		lisp?</A
></DT
><DT
>2.4. <A
HREF="lispfaq.html#AEN139"
>What online resources are there for lisp users?</A
></DT
></DL
></DD
><DT
>3. <A
HREF="lispfaq.html#AEN158"
>Terminology</A
></DT
><DD
><DL
><DT
>3.1. <A
HREF="lispfaq.html#AEN161"
>I've heard the terms "Lisp-1" and "Lisp-2" &#8211; what do
		they mean?</A
></DT
></DL
></DD
><DT
>4. <A
HREF="lispfaq.html#AEN163"
>Programming questions</A
></DT
><DD
><DL
><DT
>4.1. <A
HREF="lispfaq.html#AEN166"
>How do I write a "Hello, World!" programme in Lisp?"</A
></DT
><DT
>4.2. <A
HREF="lispfaq.html#AEN173"
>Does anyone actually use flet?</A
></DT
><DT
>4.3. <A
HREF="lispfaq.html#AEN182"
>How do I split a string?</A
></DT
><DT
>4.4. <A
HREF="lispfaq.html#AEN198"
>Why doesn't Common Lisp have continuations?</A
></DT
><DT
>4.5. <A
HREF="lispfaq.html#AEN205"
>How does (defun car (x) (car x)) work?</A
></DT
><DT
>4.6. <A
HREF="lispfaq.html#AEN210"
>			I want an array of foos, but (make-array 10 :initial-element
			(make-foo)) causes strange bugs. Why?
		</A
></DT
><DT
>4.7. <A
HREF="lispfaq.html#AEN215"
>How do I make an executable from my programme?</A
></DT
><DT
>4.8. <A
HREF="lispfaq.html#AEN218"
>How do I run my programme as a script, then?</A
></DT
><DT
>4.9. <A
HREF="lispfaq.html#AEN221"
>			What is the Lisp equivalent of the __FILE__ and __LINE__ ANSI C
			preprocessor macros? How do I find out where an error occurred?
		</A
></DT
></DL
></DD
><DT
>5. <A
HREF="lispfaq.html#AEN230"
>Common programming pitfalls</A
></DT
><DD
><DL
><DT
>5.1. <A
HREF="lispfaq.html#AEN233"
>			The read-eval-print loop is just sitting there after I've
			typed in my form. What's happening?
		</A
></DT
><DT
>5.2. <A
HREF="lispfaq.html#AEN240"
>Why does (read-from-string "foobar" :start 3) return
   FOOBAR instead of BAR?</A
></DT
><DT
>5.3. <A
HREF="lispfaq.html#AEN255"
>Why can't I apply <TT
CLASS="FUNCTION"
>#'AND</TT
> or
		<TT
CLASS="FUNCTION"
>#'OR</TT
>?</A
></DT
><DT
>5.4. <A
HREF="lispfaq.html#AEN284"
>Why are my structure contents wrong?</A
></DT
><DT
>5.5. <A
HREF="lispfaq.html#AEN290"
>			Why isn't there a DEEP-COPY function in the language?
		</A
></DT
></DL
></DD
><DT
>6. <A
HREF="lispfaq.html#AEN298"
>Debunking myths</A
></DT
><DD
><DL
><DT
>6.1. <A
HREF="lispfaq.html#AEN301"
>When did LISP die?</A
></DT
><DT
>6.2. <A
HREF="lispfaq.html#AEN304"
>Why is LISP slow?</A
></DT
><DT
>6.3. <A
HREF="lispfaq.html#AEN307"
>How do I manipulate symbols with IMPLODE/EXPLODE?</A
></DT
><DT
>6.4. <A
HREF="lispfaq.html#AEN314"
>Why doesn't lisp have structures or arrays?</A
></DT
></DL
></DD
></DL
><DIV
CLASS="QANDADIV"
><H3
><A
NAME="AEN16"
></A
>1. This newsgroup</H3
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN19"
></A
><B
>1.1. </B
>What is the purpose of comp.lang.lisp?</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>To exist.</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN24"
></A
><B
>1.2. </B
>What is on-topic on comp.lang.lisp?</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			Disscussion of the language defined by the ANSI Common Lisp
			standard is definitely on-topic on comp.lang.lisp. Unlike
			comp.lang.c, we do not restrict our discussions to the standard,
			but also actively discuss the differences between
			implementations and implementation-specific extensions.
		</P
><P
>			Since the Lisp community is remarkably long-lived, discussion of
			the history and evolution of Lisp tends also to be welcomed, or
			at least tolerated; discussion of non-standard lisps (though
			generally not Scheme or Emacs Lisp) is also accepted. Though
			CLOS (the Common Lisp Object System) has <A
HREF="news:comp.lang.lisp.clos"
TARGET="_top"
>its own newsgroup</A
> it is
			also part of ANSI CL, and so is a valid topic for discussion.
		</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN31"
></A
><B
>1.3. </B
>What is off-topic on comp.lang.lisp?</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			Questions about Scheme, Emacs Lisp and AutoLisp tend not to be
			terribly welcome, as they have their own fora in the 
			comp.lang.scheme, comp.emacs and comp.cad.autocad newsgroups.
		</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN36"
></A
><B
>1.4. </B
>What do all those abbreviations mean?</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN39"
></A
><B
>1.5. </B
>Is Scheme a lisp?</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			Scheme is a member of the greater family of Lisp languages,
			assuming that is considered to include others like Dylan and
			Emacs Lisp.  The design of Scheme predates the ANSI Common Lisp
			standard, and some CL features such as lexical scoping may be
			considered to have been derived from Scheme.
		</P
><P
>			More detailed comparative discussions don't generally prove very
			productive; those that are interested in discussing Scheme
			should first consider discussing it in <A
HREF="news:comp.lang.scheme"
TARGET="_top"
>comp.lang.scheme</A
>, where
			discussion would be much more welcome and appropriate.
		</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN46"
></A
><B
>1.6. </B
>Can you help me with my homework?</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>But of course!</P
><P
>However, we will need the e-mail address of your lecturer or
			teaching instructor to best aid you; for it is only fair that if
			we do the homework we should get the course
			credits. Alternatively, should you wish to hire a consultant from
			the group, competitive rates can be arranged.</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>Post the assignment, and your attempt so far; state
			explicitly that it is homework. You may find a kind soul on the
			group to explain some point that you are missing; this is more
			likely the more visible your own work is. Posts of the form "my
			assignment is due tomorrow please hlep!" are unlikely to engender
			much sympathy.
		</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN54"
></A
><B
>1.7. </B
>What should I do if I feel victimized?</P
></DIV
></DIV
></DIV
><DIV
CLASS="QANDADIV"
><H3
><A
NAME="AEN56"
></A
>2. Resources</H3
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN59"
></A
><B
>2.1. </B
>Where can I find free lisp compilers or interpreters?</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			There are a number of free (in both the `gratis' and the
			`libre' senses) lisp environments:
		</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
HREF="http://clisp.cons.org/"
TARGET="_top"
>CLISP</A
></DT
><DD
><P
>						A lightweight common lisp interpreter and compiler, which
						compiles to bytecode, and runs on Windows, AmigaOS,
						Acorns, OS/2 and most flavours of Unix. CLISP's
						implementation of CLOS is currently <A
HREF="http://clisp.cons.org/impnotes.html#clos-diff"
TARGET="_top"
>not
						quite complete</A
>. CLISP is licensed under the GNU
						GPL.
					</P
></DD
><DT
><A
HREF="http://www.cons.org/cmucl/"
TARGET="_top"
>CMUCL</A
></DT
><DD
><P
>						An interpreter and optimizing compiler to native code
						running on a few flavours of Unix (including x86/FreeBSD,
						x86/Linux and sparc/Solaris). CMUCL can be difficult to
						compile; it requires itself to build itself, and
						bootstrapping is an issue. CMUCL is mostly public domain
						software, though portions are under an X-like or BSD-like
						licence.
					</P
></DD
><DT
><A
HREF="http://www.corman.net/CormanLisp.html"
TARGET="_top"
>Corman
				Lisp</A
></DT
><DD
><P
>						
						Corman Lisp is a compiler for Windows 95/98/2000/ME/NT,
						with integration with the Windows API. Corman Lisp is free
						for personal use. 
					</P
></DD
><DT
><A
HREF="http://ecls.sourceforge.net/"
TARGET="_top"
>ECLS</A
></DT
><DD
><P
>						Standing for `Embeddable Common Lisp "Spain"', ECLS is an
						implementation of Common Lisp running on at least x86
						Linux and FreeBSD. As well as a bytecode compiler, ECLS
						can compile to C. ECLS is licensed under the GNU GPL.
					</P
></DD
><DT
><A
HREF="ftp://rene.ma.utexas.edu/pub/gcl/"
TARGET="_top"
>GCL</A
></DT
><DD
><P
>						Despite its name, Gnu Common Lisp (GCL) is not an actively
						maintained project these days, and is not particularly
						compliant with the ANSI Common Lisp specification as it
						predates that standard. It is not recommended that this be
						used except specifically for running software written for
						it. GCL is licensed under the GNU LGPL.
					</P
></DD
><DT
><A
HREF="http://openmcl.clozure.com/"
TARGET="_top"
>OpenMCL</A
></DT
><DD
><P
>						OpenMCL is an `opensourced' version of a port of a subset
						of MCL 4.2, and runs on powerpc/Linux. Not quite all
						functionality is yet present, but it is improving
						rapidly. OpenMCL is licensed under the GNU LGPL
					</P
></DD
><DT
><A
HREF="http://www.poplog.org/"
TARGET="_top"
>Poplog</A
></DT
><DD
><P
>						Poplog is licensed under an XFree86-style license.
					</P
></DD
><DT
><A
HREF="http://sbcl.sourceforge.net/"
TARGET="_top"
>SBCL</A
></DT
><DD
><P
>						SBCL is a fork from CMUCL aimed at improving the
						maintainability by removing the bootstrapping issues
						involved in recompiling. Currently it runs on x86/Linux
						and alpha/Linux. SBCL is licensed in a similar fashion to
						CMUCL.
					</P
></DD
></DL
></DIV
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN106"
></A
><B
>2.2. </B
>Where can I buy a professional lisp system?</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>There are a fair few commercial vendors of Lisp systems; the
		following are the most active:</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
HREF="http://www.franz.com/"
TARGET="_top"
>Allegro Common Lisp</A
></DT
><DD
><P
>Franz Inc's Allegro Common Lisp is a fine lisp
					development environment. See their website for more
					details</P
></DD
><DT
><A
HREF="http://www.xanalys.com/"
TARGET="_top"
>LCL</A
></DT
><DD
><P
>LCL (formerly Liquid Common Lisp) is an offering from
					Xanalys</P
></DD
><DT
><A
HREF="http://www.xanalys.com/"
TARGET="_top"
>LispWorks</A
></DT
><DD
><P
>Another offering from Xanalys, LispWorks has a
					different set of extensions above the ANSI specification
					from LCL</P
></DD
><DT
><A
HREF="http://www.digitool.com/"
TARGET="_top"
>MCL</A
></DT
><DD
><P
>A commercial implementation of Common Lisp for the
						Macintosh.</P
></DD
><DT
>Symbolics Common Lisp</DT
><DD
><P
></P
></DD
></DL
></DIV
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN136"
></A
><B
>2.3. </B
>What books should I read to learn more about
		lisp?</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN139"
></A
><B
>2.4. </B
>What online resources are there for lisp users?</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
><DT
><A
HREF="http://www.xanalys.com/software_tools/reference/HyperSpec/FrontMatter/index.html"
TARGET="_top"
>The Common Lisp Hyperspec</A
></DT
><DD
><P
>A non-normative transferral of the official ANSI
					standard for Common Lisp to the hypertext medium, by
					Kent Pitman.</P
></DD
><DT
><A
HREF="http://www.lisp.org/"
TARGET="_top"
>Association of
				Lisp Users</A
></DT
><DD
><P
>The website of the association of lisp
					users. Infrequently updated.</P
></DD
><DT
><A
HREF="http://ww.telent.net/cliki/"
TARGET="_top"
>CLiki</A
></DT
><DD
><P
>An on-line Wiki-equivalent with emphasis on Free
					software written in Common Lisp.</P
></DD
></P
></DIV
></DIV
></DIV
><DIV
CLASS="QANDADIV"
><H3
><A
NAME="AEN158"
></A
>3. Terminology</H3
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN161"
></A
><B
>3.1. </B
>I've heard the terms "Lisp-1" and "Lisp-2" &#8211; what do
		they mean?</P
></DIV
></DIV
></DIV
><DIV
CLASS="QANDADIV"
><H3
><A
NAME="AEN163"
></A
>4. Programming questions</H3
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN166"
></A
><B
>4.1. </B
>How do I write a "Hello, World!" programme in Lisp?"</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			This is actually a subtle question; not in the respect of
			computing "Hello, World!", obviously, but because of what being
			a "Hello, World!" programme actually means.
		</P
><P
>			At its simplest, you can simply type "Hello, World!" at the
			REPL, to find that the `P' part of that will print "Hello,
			World!" back to you. However, this won't do what you want if
			this is in a file, as return values aren't printed unless they
			are at the REPL.
		</P
><P
>			Something which is closer to the canonical "Hello, World!"
			attempt is (format t "~&#38;Hello, World!~%").
		</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN173"
></A
><B
>4.2. </B
>Does anyone actually use flet?</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			This question is usually motivated by the existence of labels,
			which is a similar form for binding functions, but also allows
			mutual recursion between the functions being bound. Given
			this, it is perhaps natural to question the utility of flet.
		</P
><P
>			However, there are two reasons for using flet: one idiomatic
			and one programmatic. The idiomatic reason is that flet can be
			useful to signal to the reader of the code that it is not
			expected that the functions will be mutually recursive; in
			other words, it is part of the documentation of the system, so
			that a subsequent programmer can see by inspection the purpose
			of binding the functions.
		</P
><P
>			More usefully, though, flet can be useful to locally modify
			the behaviour of functions, for instance (a contrived example
			due to Kent Pitman):
		</P
><P
>(defun square (x) (* x x))</P
><P
>(flet ((square (x) (make-instance 'square :area (square
			x)))) ...)</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN182"
></A
><B
>4.3. </B
>How do I split a string?</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			There is no `right' answer to this question; many lisp
			programmers have rolled their own solution in the past, and
			others are of the view that it should never be necessary, as
			long as all sequence functions are used with consistent :start
			and :end arguments.
		</P
><P
>			However, a community-based `standard' was developed in June/July
			2001 on <A
HREF="news:comp.lang.lisp"
TARGET="_top"
>comp.lang.lisp</A
>;
			known as SPLIT-SEQUENCE (formerly PARTITION), it works as
			follows in its simplest form:
		</P
><P
>			<TT
CLASS="USERINPUT"
><B
>(split-sequence #\Space "A stitch in time saves
				nine.")</B
></TT
> gives <TT
CLASS="COMPUTEROUTPUT"
>("A" "stitch" "in"
				"time" "saves" "nine."), 28</TT
>
		</P
><P
>			The function synopsis is
				<TT
CLASS="FUNCTION"
>SPLIT-SEQUENCE</TT
> delimiter sequence &#38;KEY
				:count :keep-empty-subseqs :from-end :start :end :test
				:test-not :key =&#62; list of sequences, index
		</P
><P
>			For more details, consult the specification, available from
			<A
HREF="http://ww.telent.net/cliki/SPLIT-SEQUENCE"
TARGET="_top"
>its CLiki
			page</A
>, or download the reference implementation
			(currently available as the cl-split-sequence <A
HREF="http://cclan.sourceforge.net/"
TARGET="_top"
>cCLan</A
> package, or
			for convenience as a single file from <A
HREF="http://www-jcsu.jesus.cam.ac.uk/~csr21/split-sequence.lisp"
TARGET="_top"
>http://www-jcsu.jesus.cam.ac.uk/~csr21/split-sequence.lisp</A
>.
		</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN198"
></A
><B
>4.4. </B
>Why doesn't Common Lisp have continuations?</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			Continuations are a great theoretical tool; if a language has
			first-class, multiply invocable continuations then one can build
			threads, exceptions, coroutines, and the kitchen sink on top.
		</P
><P
>			However, there is an implementation burden with continuations;
			supporting first-class, multiply invocable continuations
			complicates things tremendously for the Lisp implementor. The
			ANSI standardizing committee J13, mindful of this, took the view
			that it would be better to specify the user-level control
			structure (CATCH, UNWIND-PROTECT, and so on) and let
			implementors choose whether to build those on top of
			continuations or not.
		</P
><P
>			If you need to play with continuations, you should use a Scheme
			implementation.
		</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN205"
></A
><B
>4.5. </B
>How does (defun car (x) (car x)) work?</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			This code is probably part of the source to a lisp compiler,
			which knows how to open-code calls to CAR. However, the
			interpreter also needs to know how to call CAR, which is what
			the above defun is doing. This is not recommended in user
			code...
		</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN210"
></A
><B
>4.6. </B
>			I want an array of foos, but (make-array 10 :initial-element
			(make-foo)) causes strange bugs. Why?
		</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			Well, the array created above contains 10 pointers to the same
			foo, which will indeed cause strange bugs. The correct way to
			initialize your array is probably (map-into (make-array 10) #'make-foo)
		</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN215"
></A
><B
>4.7. </B
>How do I make an executable from my programme?</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN218"
></A
><B
>4.8. </B
>How do I run my programme as a script, then?</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN221"
></A
><B
>4.9. </B
>			What is the Lisp equivalent of the __FILE__ and __LINE__ ANSI C
			preprocessor macros? How do I find out where an error occurred?
		</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			There is no direct equivalent of __FILE__ and __LINE__ in ANSI
			Common Lisp; this is perhaps most simply explained by the fact
			that CL is not particularly a file-oriented and definitely not
			a line-oriented language. That said, your particular
			implementation may carry around some information about where
			functions were compiled, and COMPILE-FILE binds the special
			variables *COMPILE-FILE-TRUENAME* and *COMPILE-FILE-PATHNAME*.
		</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			(defun foo () (break "Stopped inside ~S" (the-function-i-am-in)))
		</P
><P
>			(setf (symbol-function 'bar) (symbol-function 'foo))
		</P
><P
>			(bar)
		</P
><P
>			Are you in a breakpoint in FOO or in BAR?
	</P
></DIV
></DIV
></DIV
><DIV
CLASS="QANDADIV"
><H3
><A
NAME="AEN230"
></A
>5. Common programming pitfalls</H3
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN233"
></A
><B
>5.1. </B
>			The read-eval-print loop is just sitting there after I've
			typed in my form. What's happening?
		</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			One possible explanation for this behaviour is that you have
			typed in a program that causes an infinite loop; for instance
			<TT
CLASS="USERINPUT"
><B
>(loop)</B
></TT
>.
		</P
><P
>			However, the fact that you are surprised by this behaviour
			suggests that this isn't the case; in which case a far more
			likely explanation is that your form is not quite complete. You
			may have typed a doublequote, vertical bar, "#|" comment
			beginning, or left parenthesis that you never matched with
			another doublequote, vertical bar, "|#", or right parenthesis,
			respectively.  Try typing a few right parentheses followed by
			Return.
		</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN240"
></A
><B
>5.2. </B
>Why does (read-from-string "foobar" :start 3) return
   FOOBAR instead of BAR?</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			<TT
CLASS="FUNCTION"
>READ-FROM-STRING</TT
> is one of the rare functions that takes both
			&#38;OPTIONAL and &#38;KEY arguments:
			<P
CLASS="LITERALLAYOUT"
>				<TT
CLASS="FUNCTION"
>READ-FROM-STRING</TT
>&nbsp;string&nbsp;&#38;OPTIONAL&nbsp;eof-error-p&nbsp;eof-value&nbsp;&#38;KEY&nbsp;:start&nbsp;:end&nbsp;:preserve-whitespace<br>
			</P
>
		</P
><P
>			When a function takes both types of arguments, all the
			optional arguments must be specified explicitly before any
			of the keyword arguments may be specified.  In the example
			above, :START becomes the value of the optional
			EOF-ERROR-P parameter and 3 is the value of the optional
			EOF-VALUE parameter.
		</P
><P
>			To get the desired result, you should use
			<B
CLASS="COMMAND"
>(read-from-string "foobar" t nil :start
			3)</B
>.  If you need to understand and use the
			optional arguments, please refer to CLTL2
			under READ-FROM-STRING, otherwise, this will behave as
			desired for most purposes.
		</P
><P
>			The other functions with this peculiarity in the
			COMMON-LISP package are
			<TT
CLASS="FUNCTION"
>PARSE-NAMESTRING</TT
>,
			<TT
CLASS="FUNCTION"
>WRITE-LINE</TT
> and
			<TT
CLASS="FUNCTION"
>WRITE-STRING</TT
>.
		</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN255"
></A
><B
>5.3. </B
>Why can't I apply <TT
CLASS="FUNCTION"
>#'AND</TT
> or
		<TT
CLASS="FUNCTION"
>#'OR</TT
>?</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			Here's the simple, but not necessarily satisfying, answer:
			<TT
CLASS="FUNCTION"
>AND</TT
> and <TT
CLASS="FUNCTION"
>OR</TT
> are
			macros, not functions; <TT
CLASS="FUNCTION"
>APPLY</TT
> and
			<TT
CLASS="FUNCTION"
>FUNCALL</TT
> can only be used to invoke
			functions, not macros and special operators.
		</P
><P
>			OK, so what's the <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>real</I
></SPAN
> reason?  The
			reason that <TT
CLASS="FUNCTION"
>AND</TT
> and
			<TT
CLASS="FUNCTION"
>OR</TT
> are macros rather than functions
			is because they implement control structure in addition to
			computing a boolean value.  They evaluate their subforms
			sequentially from left/top to right/bottom, and stop
			evaluating subforms as soon as the result can be
			determined (in the case of <TT
CLASS="FUNCTION"
>AND</TT
>, as
			soon as a subform returns NIL; in the case of
			<TT
CLASS="FUNCTION"
>OR</TT
>, as soon as one returns non-NIL);
			this is referred to as "short circuiting" in computer
			language parlance.  <TT
CLASS="FUNCTION"
>APPLY</TT
> and
			<TT
CLASS="FUNCTION"
>FUNCALL</TT
>, however, are ordinary
			functions; therefore, their arguments are evaluated
			automatically, before they are called.  Thus, were
			<TT
CLASS="FUNCTION"
>APPLY</TT
> able to be used with
			<TT
CLASS="FUNCTION"
>#'AND</TT
>, the short-circuiting would be
			defeated.
		</P
><P
>			Perhaps you don't really care about the
			short-circuiting, and simply want the functional, boolean
			interpretation.  While this may be a reasonable
			interpretation of trying to apply AND or OR, it doesn't
			generalize to other macros well, so there's no obvious way
			to have the Lisp system "do the right thing" when trying
			to apply macros.  The only function associated with a
			macro is its expander function; this function accepts and
			returns and form, so it cannot be used to compute the
			value.
		</P
><P
>			The Common Lisp functions <TT
CLASS="FUNCTION"
>EVERY</TT
> and
			<TT
CLASS="FUNCTION"
>SOME</TT
> can be used to get the
			functionality you intend when trying to apply
			<TT
CLASS="FUNCTION"
>#'AND</TT
> and <TT
CLASS="FUNCTION"
>#'OR</TT
>.
			For instance, the erroneous form:
			<TT
CLASS="USERINPUT"
><B
>(apply #'and *list*)</B
></TT
>
			can be translated to the correct form:
			<TT
CLASS="USERINPUT"
><B
>(every #'identity *list*)</B
></TT
>.
		</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN284"
></A
><B
>5.4. </B
>Why are my structure contents wrong?</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			Most probably, one of your structure slots is called `p'. The
			accessor for this slot 
			will clash with the default predicate that defstruct defines for
			your structure; so if you have (defstruct bar (p 1) (q 2)), you
			could easily see (make-bar) return #S(bar :p t :q 2).
		</P
><P
>			To avoid this, use the :predicate defstruct option to eliminate
			or rename the predicate function, or else use a different slot
			name.
		</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN290"
></A
><B
>5.5. </B
>			Why isn't there a DEEP-COPY function in the language?
		</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			Copying an arbitrary structure or object needs context to
			determine what is the correct copy.
		</P
><P
>			Example: queue
		</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>		See Kent Pitman's <A
HREF="http://world.std.com/~pitman/PS/EQUAL.html"
TARGET="_top"
>article</A
>
		on the nature of equality.
		</P
></DIV
></DIV
></DIV
><DIV
CLASS="QANDADIV"
><H3
><A
NAME="AEN298"
></A
>6. Debunking myths</H3
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN301"
></A
><B
>6.1. </B
>When did LISP die?</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN304"
></A
><B
>6.2. </B
>Why is LISP slow?</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN307"
></A
><B
>6.3. </B
>How do I manipulate symbols with IMPLODE/EXPLODE?</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>Generally, you don't.</P
><P
>			IMPLODE and EXPLODE were functions in old lisps where there
			was no string data type, so that symbols were the only way of
			manipulating text. Then (explode 'foo) would give you (F O O);
			you could then do (implode (cdr (explode 'crash))) to give you
			back the symbol RASH
		</P
><P
>			If you are taught today about implode/explode in a lisp
			programming class for anything other than historical context,
			complain loudly to your lecturer.
		</P
></DIV
></DIV
><DIV
CLASS="QANDAENTRY"
><DIV
CLASS="QUESTION"
><P
><A
NAME="AEN314"
></A
><B
>6.4. </B
>Why doesn't lisp have structures or arrays?</P
></DIV
><DIV
CLASS="ANSWER"
><P
><B
> </B
>			If arrays and structures don't exist, then obviously MAKE-ARRAY and
			DEFSTRUCT must be figments of the imagination.  Similarly, since
			Lisp only uses association lists to organize "database-like"
			information, MAKE-HASH-TABLE must also be a figment of the
			imagination.
		</P
><P
>			Based on the nonexistence of the above figments of the imagination,
			Perl and Python, with arrays, associative tables, and dictionaries,
			must obviously be manifestly superior.
		</P
></DIV
></DIV
></DIV
></DIV
></DIV
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>