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<P>
<P>
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<HEAD>
<TITLE>Simply Scheme Appendix B: Common Lisp</TITLE>
</HEAD>
<BODY>
<CITE>Simply Scheme</CITE>:
<CITE>Introducing Computer Science</CITE> 2/e Copyright (C) 1999 MIT
<H2>Appendix B</H2>
<H1>Common Lisp</H1>
<TABLE width="100%"><TR><TD>
<IMG SRC="../simply.jpg" ALT="cover photo">
<TD><TABLE>
<TR><TD align="right"><CITE><A HREF="http://www.cs.berkeley.edu/~bh/">Brian
Harvey</A><BR>University of California, Berkeley</CITE>
<TR><TD align="right"><CITE><A HREF="http://ccrma.stanford.edu/~matt">Matthew
Wright</A><BR>University of California, Santa Barbara</CITE>
<TR><TD align="right"><BR>
<TR><TD align="right"><A HREF="../pdf/ssch27.pdf">Download PDF version</A>
<TR><TD align="right"><A HREF="../ss-toc2.html">Back to Table of Contents</A>
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chapter thread <A HREF="appendix-simply.html"><STRONG>NEXT</STRONG></A>
<TR><TD align="right"><A HREF="http://mitpress.mit.edu/0262082810">MIT
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<HR>
<P>The two most popular dialects of Lisp are Scheme and Common Lisp. This
appendix, which assumes that you have finished the rest of this book,
describes the most important differences between Scheme and Common Lisp so
that you will be able to use Common Lisp if you need to. Common Lisp is the
most popular language among Artificial Intelligence researchers, so AI
courses often use Common Lisp.
<P><H2>Why Common Lisp Exists</H2>
<P>Since the beginning of Lisp, many versions of the language were developed.
Each dialect reflected different ideas about the most important capabilities
to include in the language. This diversity made Lisp an exciting arena for
research, but it also meant that a Lisp program written for one dialect
couldn't be used elsewhere.
<P>In 1984, a group of Lisp developers decided to define a version of Lisp that
would combine the capabilities of all their favorite dialects, so that in
the future they would all use the same language; thus the name "Common"
Lisp. Common Lisp was not the first attempt at a universal Lisp dialect,
but it was more successful than earlier efforts. In 1985 a revision of the
language was begun under the aegis of ANSI, the American National Standards
Institute. This ANSI sponsorship gave Common Lisp an official status that
has contributed to its growing acceptance.
<P>Since Common Lisp was designed by combining the capabilities of many earlier
dialects, it's an enormous language with nearly 1000 primitives, including
versions of several programs in this book. There is a primitive <CODE>sort</CODE>
procedure, a procedure like <CODE>number-name</CODE> that spells numbers in
English, and a <CODE>substitute</CODE> procedure identical to the one you wrote in
an exercise, to name a few.
<P>If you're writing your own programs in Common Lisp, you can ignore all the
extra features and just use the capabilities you already know from Scheme.
If you're trying to read someone else's Common Lisp program, we expect that
you will have to look up many primitive procedures in a reference manual.
<P><H2>Defining Procedures and Variables</H2>
<P>One minor difference between Scheme and Common Lisp is in the way procedures
are defined. In Common Lisp,
<P><PRE>(defun square (x)
(* x x))
</PRE>
<P>means the same as Scheme's
<P><PRE>(define (square x)
(* x x))
</PRE>
<P>In Scheme, <CODE>define</CODE> is used both for procedures and for variables whose
values aren't procedures. In Common Lisp, procedures are given names by a
mechanism separate from the general variable mechanism; <CODE>defun</CODE> is only
for procedures. To define a variable, use <CODE>defvar</CODE>:
<P><PRE>common-lisp> (defvar x 6)
6
common-lisp> x
6
</PRE>
<P>In Common Lisp, <CODE>defvar</CODE> returns the name of the variable you
define. If a variable has already been defined, <CODE>defvar</CODE> will not
change its value; for that you must use <CODE>setq</CODE>.
<P><H2>The Naming Convention for Predicates</H2>
<P>In Common Lisp, names of predicate procedures end in a "<CODE>p</CODE>" (for
"predicate") instead of a question mark. Unfortunately, this convention
isn't followed strictly. For example, Common Lisp's version of the <CODE>null?</CODE> predicate is just "<CODE>null</CODE>," not "<CODE>nullp</CODE>."
<P><H2>No Words or Sentences</H2>
<P>We've mentioned that Scheme doesn't really have words and
sentences built in; neither does Common Lisp. So none of the following
procedures have Common Lisp equivalents: <CODE>accumulate</CODE>, <CODE>appearances</CODE>, <CODE>before?</CODE>, <CODE>bf</CODE>, <CODE>bl</CODE>, <CODE>butfirst</CODE>, <CODE>butlast</CODE>, <CODE>count</CODE>, <CODE>empty?</CODE>, <CODE>every</CODE>, <CODE>first</CODE>, <CODE>item</CODE>,
<CODE>keep</CODE>, <CODE>last</CODE>, <CODE>member?</CODE>, <CODE>se</CODE>, <CODE>sentence</CODE>, <CODE>word</CODE>,
and <CODE>word?</CODE>. (Common Lisp does have lists, though, and list-related
procedures such as <CODE>map</CODE>, <CODE>reduce</CODE>, <CODE>append</CODE>, and so on <EM>do</EM>
have equivalents.)
<P><H2>True and False</H2>
<P>Common Lisp doesn't have the Boolean values <CODE>#t</CODE> and <CODE>#f</CODE>.
Instead, it has a single false value, <CODE>nil</CODE>, which is also the empty
list.
<P><PRE>common-lisp> (= 2 3)
NIL
common-lisp> (cdr '(one-word-list))
NIL
common-lisp> '()
NIL
</PRE>
<P><CODE>Nil</CODE> is a strange beast in Common Lisp. It isn't a variable with the
empty list as its value; it's a special self-evaluating symbol. There is
also <CODE>t</CODE>, a self-evaluating symbol with a true value.
<P><PRE>common-lisp> 'nil
NIL
common-lisp> nil
NIL
common-lisp> t
T
</PRE>
<P>Like Scheme, Common Lisp treats every non-false (i.e., non-<CODE>nil</CODE>) value
as true. But be careful; in Common Lisp
<P><PRE>common-lisp> (if (cdr '(one-word-list)) 'yes 'no)
</PRE>
<P>has the value <CODE>NO</CODE>, because the empty list is <CODE>nil</CODE>.
<P>In Common Lisp's <CODE>cond</CODE>, there is no equivalent to <CODE>else</CODE>; Common
Lisp programmers instead use <CODE>t</CODE> as the condition for their last clause,
like this:
<P><PRE>(defun sign (n)
(cond ((> n 0) 'positive)
((= n 0) 'zero)
(t 'negative)))
</PRE>
<P><H2>Files</H2>
<P>Common Lisp's mechanism for dealing with files is trivially different from
Scheme's. What Scheme calls "ports," Common Lisp calls "streams." Also,
there is only one procedure for opening streams; the direction is specified
this way:
<P><PRE>common-lisp> (defvar out-stream (open "outfile" :direction :output))
#<OUTPUT STREAM "outfile">
common-lisp> (close out-stream)
T
common-lisp> (defvar in-stream (open "infile" :direction :input))
#<INPUT STREAM "infile">
common-lisp> (close in-stream)
T
</PRE>
<P>Note that the <CODE>close</CODE> procedure closes both input streams and
output streams.
<P>To <CODE>read</CODE> from an input stream, you must invoke <CODE>read</CODE> with three
arguments:
<P><PRE>common-lisp> (read stream nil <EM>anything</EM>)
</PRE>
<P>The <CODE>nil</CODE> indicates that reaching the end of the file should
not be an error. If <CODE>read</CODE> does reach the end of the file, instead of
returning a special end-of-file object it returns its third argument.
It's possible to choose any value as the indicator for reaching the end
of the file:
<P><PRE>(let ((next (read stream nil 'xyzzy)))
(if (equalp next 'xyzzy)
'done
(do-something next)))
</PRE>
<P>It's important to choose an end-of-file indicator that couldn't
otherwise appear as a value in the file.
<P><H2>Arrays</H2>
<P>In Common Lisp, vectors are just a special case of the multidimensional <EM>array</EM> data type that you invented in Exercise <A HREF="../ssch23/vectors.html#arrays">23.15</A>. There are quite a
few differences between Common Lisp arrays and Scheme vectors, none very
difficult, but too numerous to describe here. If you need to use arrays,
read about them in a Common Lisp book.
<P><H2>Equivalents to Scheme Primitives</H2>
<P>Other than the word and sentence procedures, here is a table of the
Scheme primitives from the table on page <A HREF="appendix-funlist.html#funlist">funlist</A> that have
different names, slightly different behavior, or do not exist at all
in Common Lisp. Scheme procedures not in this list (other than the
word and sentence ones) can be used identically in Common Lisp.
<P>
<P><TABLE>
<TR><TH>Scheme<TH>Common Lisp
<TR><TD><CODE>align</CODE>
<TD>Common Lisp's <CODE>format</CODE> primitive has a similar purpose
<TR><TD><CODE>begin</CODE>
<TD><CODE>progn</CODE>
<TR><TD><CODE>boolean?</CODE>
<TD>Doesn't exist; see the section in this appendix about true and
false values.
<TR><TD><CODE>c...r</CODE>
<TD>The same, but <CODE>(c...r nil)</CODE> is <CODE>nil</CODE> instead
of an error.
<TR><TD><CODE>children</CODE>
<TD>You can use our version from Chapter 18.
<TR><TD><CODE>close-...-port</CODE>
<TD><CODE>close</CODE>
<TR><TD><CODE>close-all-ports</CODE>
<TD>Doesn't exist.
<TR><TD><CODE>cond</CODE>
<TD>The same, except for <CODE>else</CODE>; use <CODE>t</CODE> instead.
<TR><TD><CODE>datum</CODE>
<TD>You can use our version from Chapter 18.
<TR><TD><CODE>define</CODE>
<TD>Either <CODE>defun</CODE>, for procedure, or
<CODE>defvar</CODE>, otherwise.
<TR><TD><CODE>display</CODE>
<TD><CODE>princ</CODE>
<TR><TD><CODE>eof-object?</CODE>
<TD>See the section on files.
<TR><TD><CODE>equal?</CODE>
<TD><CODE>equalp</CODE>
<TR><TD><CODE>even?</CODE>
<TD><CODE>evenp</CODE>
<TR><TD><CODE>filter</CODE>
<TD><CODE>remove-if-not</CODE>
<TR><TD><CODE>for-each</CODE>
<TD><CODE>mapc</CODE>
<TR><TD><CODE>integer?</CODE>
<TD><CODE>integerp</CODE>
<TR><TD><CODE>lambda</CODE>
<TD>Discussed later in this appendix.
<TR><TD><CODE>list?</CODE>
<TD><CODE>listp</CODE>, except that <CODE>listp</CODE> also returns true
for improper lists.
<TR><TD><CODE>list-ref</CODE>
<TD><CODE>nth</CODE>, except that the arguments come in reverse order.
<TR><TD><CODE>list->vector</CODE>
<TD>See the section about arrays.
<TR><TD><CODE>make-node</CODE>
<TD>You can use our version from Chapter 18.
<TR><TD><CODE>make-vector</CODE>
<TD>See the section about arrays.
<TR><TD><CODE>map</CODE>
<TD><CODE>mapcar</CODE>
<TR><TD><CODE>newline</CODE>
<TD><CODE>terpri</CODE>
<TR><TD><CODE>null?</CODE>
<TD><CODE>null</CODE>
<TR><TD><CODE>number?</CODE>
<TD><CODE>numberp</CODE>
<TR><TD><CODE>odd?</CODE>
<TD><CODE>oddp</CODE>
<TR><TD><CODE>open-...-file</CODE>
<TD>See the section on files.
<TR><TD><CODE>procedure?</CODE>
<TD><CODE>functionp</CODE>
<TR><TD><CODE>quotient</CODE>
<TD><CODE>truncate</CODE>
<TR><TD><CODE>read</CODE>
<TD>Identical except for end of file. See the section on files.
<TR><TD><CODE>read-line</CODE>
<TD>Doesn't exist. (Common Lisp's <CODE>read-line</CODE> is like our
<CODE>read-string</CODE>.)
<TR><TD><CODE>read-string</CODE>
<TD><CODE>read-line</CODE>
<TR><TD><CODE>reduce</CODE>
<TD>The same, but computes <CODE>(f (f a b) c)</CODE> instead of
<CODE>(f a (f b c))</CODE>
<TR><TD><CODE>remainder</CODE>
<TD><CODE>rem</CODE>
<TR><TD><CODE>repeated</CODE>
<TD>Doesn't exist.
<TR><TD><CODE>show</CODE>
<TD><CODE>Doesn't exist but easy to write.</CODE>
<TR><TD><CODE>show-line</CODE>
<TD><CODE>Doesn't exist.</CODE>
<TR><TD><CODE>vector-</CODE><I>anything</I>
<TD>See the section about arrays.
<TR><TD><CODE>write</CODE>
<TD><CODE>prin1</CODE>
</TABLE><P>
<P><H2>A Separate Name Space for Procedures</H2>
<P>All of the differences noted in this table are fairly minor ones, in the
sense that the translation needed to account for these differences requires
little more than renaming. There is one major conceptual difference between
the two languages, however, in the way they treat names of procedures.
Common Lisp allows a procedure and a variable to have the same name. For
example, the program
<P><PRE>(defun three-copies (list)
(list list list list))
</PRE>
<P>is perfectly legal.
<P><PRE>common-lisp> (three-copies '(drive my car))
((DRIVE MY CAR) (DRIVE MY CAR) (DRIVE MY CAR))
</PRE>
<P>How can Common Lisp tell that one of the <CODE>list</CODE>s means the primitive
procedure, but the other ones mean the formal parameter? Symbols in the
first position in a list (right after an open parenthesis) are taken
to be names of globally defined procedures.
<P>In Chapter 7 we introduced the image of a blackboard with all the
global variables written on it, which all the Scheme little people can see.
In Common Lisp, there are <EM>two</EM> blackboards: one for global
variables, just as in Scheme, and another one for procedures. The procedure
blackboard contains the primitive procedures and the procedures you define
with <CODE>defun</CODE>. Names in the first position of an expression are looked
up on the procedure blackboard.
<P>Therefore, the names of procedures are not variables and cannot be used as
actual argument expressions:
<P><PRE>common-lisp> (sqrt 144)
12
common-lisp> (mapcar sqrt '(9 16 25 36))
ERROR: The variable SQRT is unbound.
</PRE>
<P>(Common Lisp's equivalent of <CODE>map</CODE> is named <CODE>mapcar</CODE>.)
<P>How, then, do you tell Common Lisp that you want to use the procedure named
<CODE>sqrt</CODE> as data? You must use the <CODE>function</CODE> special
form.<A NAME="text1" HREF="appendix-cl.html#ft1">[1]</A>
<P><PRE>common-lisp> (function sqrt)
#<PROCEDURE>
common-lisp> (mapcar (function sqrt) '(9 16 25 36))
(3 4 5 6)
</PRE>
<P><CODE>Function</CODE>'s job is to look up names on the procedure
blackboard. (<CODE>Function</CODE> actually has a more general definition, as
you'll see in a few paragraphs.)
<P><H2><CODE><B>Lambda</B></CODE></H2>
<P>In Common Lisp, as in Scheme, procedures can be named or unnamed. Just as
procedure names in Common Lisp are meaningful only in certain contexts, so
are <CODE>lambda</CODE> expressions. They make sense at the beginning of an
expression:
<P><PRE>common-lisp> ((lambda (x) (* x x)) 4)
16
</PRE>
<P>or as the argument to <CODE>function</CODE>:
<P><PRE>common-lisp> (function (lambda (x) (* x x)))
#<PROCEDURE>
common-lisp> (mapcar (function (lambda (x) (* x x))) '(3 4 5 6))
(9 16 25 36)
</PRE>
<P>but they're meaningless on their own:
<P><PRE>common-lisp> (lambda (x) (* x x))
ERROR: LAMBDA is not a function
common-lisp> (mapcar (lambda (x) (* x x)) '(3 4 5 6))
ERROR: LAMBDA is not a function
</PRE>
<P><H2>More about <CODE><B>Function</B></CODE></H2>
<P>The official rule is that <CODE>function</CODE> returns the "functional
interpretation" of its argument. If the argument is a symbol, that means
looking up the procedure associated with that name. If the argument is a
<CODE>lambda</CODE> expression, it means creating a new procedure. <CODE>Function</CODE>
uses the same rule that's used to interpret the first element of a procedure
invocation.
<P>Since <CODE>function</CODE> is a very commonly used special form, it has an
abbreviation:
<P><PRE>common-lisp> (mapcar #'(lambda (x) (* x x)) '(3 4 5 6))
(9 16 25 36)
common-lisp> (mapcar #'cdr '((hey jude) (eleanor rigby) (yes it is)))
((JUDE) (RIGBY) (IT IS))
</PRE>
<P>Don't confuse
<P><PRE>#'(lambda (x) (* x x))
</PRE>
<P>with
<P><PRE>'#(lambda (x) (* x x))
</PRE>
<P>The first of these is a function that squares its argument; the
second is an array containing three elements.
<P>It's unfortunate that the abbreviation for <CODE>function</CODE> contains a single
quote mark, because the job of <CODE>function</CODE> is nothing like the job of
<CODE>quote</CODE>. You'll just have to get used to the "hashquote" notation.
<P><H2>Writing Higher-Order Procedures</H2>
<P>Think about this attempted translation of the <CODE>map</CODE> procedure:
<P><PRE>(defun map (fn lst) ;; wrong!
(if (null lst)
'()
(cons (fn (car lst))
(map fn (cdr lst)))))
</PRE>
<P>(In Common Lisp, <CODE>null</CODE> is one of the predicates whose names
don't end in "p." Otherwise, this is the same program we showed you in
Chapter 19, except for the <CODE>defun</CODE>, of course.)
<P>According to our rule about names in the front of a list, this procedure
doesn't work. Think about what happens when we say
<P><PRE>(map #'square '(1 2 3 4 5))
</PRE>
<P>According to the substitution model, the parameters <CODE>fn</CODE> and
<CODE>lst</CODE> are replaced in the body with <CODE>#'square</CODE> and <CODE>'(1 2 3 4 5)</CODE>. But Common Lisp makes an exception for the first
element of a compound expression. It uses the procedure blackboard instead
of substitution:
<P><PRE>(if (null '(1 2 3 4 5))
'()
(cons (fn (car '(1 2 3 4 5))
(map #'square (cdr '(1 2 3 4 5))))))
</PRE>
<P>Note that one of the appearances of <CODE>fn</CODE> was left unchanged.
Since there is no global procedure named <CODE>fn</CODE>, this program will produce
an error:
<P><PRE>common-lisp> (map #'square '(1 2 3 4 5))
ERROR: FN is not a procedure.
</PRE>
<P>How, then, do you write higher-order procedures in Common Lisp? The answer is
that you must use <CODE>funcall</CODE>:
<P><PRE>(defun map (fn lst)
(if (null lst)
'()
(cons (funcall fn (car lst))
(map fn (cdr lst)))))
</PRE>
<P><CODE>Funcall</CODE> takes one or more arguments. The first is a
procedure and the rest are arguments for that procedure. It applies that
procedure to the given arguments.<A NAME="text2" HREF="appendix-cl.html#ft2">[2]</A> Since <CODE>fn</CODE> is no longer
at the beginning of a compound expression, the corresponding argument,
<CODE>#'square</CODE>, is substituted for it.
<P>
<HR>
<A NAME="ft1" HREF="appendix-cl.html#text1">[1]</A> Common Lisp uses the word "function" to mean "procedure,"
whether or not the procedure implements a function.<P>
<A NAME="ft2" HREF="appendix-cl.html#text2">[2]</A> This is a lot like <CODE>apply</CODE>,
you may have noticed. Look at the difference:
<P><PRE>common-lisp> (funcall #'+ 1 2 3)
6
common-lisp> (apply #'+ '(1 2 3))
6
</PRE>
<P>In the first case, each argument to <CODE>+</CODE> is a separate argument to <CODE>funcall</CODE>. In the second case, a list of the arguments to <CODE>+</CODE> is a
single argument to <CODE>apply</CODE>. <CODE>Apply</CODE> always takes exactly two
arguments, the procedure and the argument list.<P>
<P><A HREF="../ss-toc2.html">(back to Table of Contents)</A><P>
<A HREF="appendix-running.html"><STRONG>BACK</STRONG></A>
chapter thread <A HREF="appendix-simply.html"><STRONG>NEXT</STRONG></A>
<P>
<ADDRESS>
<A HREF="../index.html">Brian Harvey</A>,
<CODE>bh@cs.berkeley.edu</CODE>
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