119 lines
4.6 KiB
HTML
119 lines
4.6 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
|
|
<!Originally converted to HTML using LaTeX2HTML 95 (Thu Jan 19 1995) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
|
|
<HEAD>
|
|
<TITLE> Quote</TITLE>
|
|
</HEAD>
|
|
<BODY>
|
|
<meta name="description" value=" Quote">
|
|
<meta name="keywords" value="lp">
|
|
<meta name="resource-type" value="document">
|
|
<meta name="distribution" value="global">
|
|
<P>
|
|
<BR> <HR>
|
|
<A HREF="node10.html"><IMG ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A>
|
|
<A HREF="node7.html"><IMG ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A>
|
|
<A HREF="node8.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif"></A> <BR>
|
|
<A HREF="lp.html"><B>Contents</B></A>
|
|
<B> Next:</B>
|
|
<A HREF="node10.html"> Selectors: First and </A>
|
|
<B>Up:</B>
|
|
<A HREF="node7.html"> Some Primitive Functions</A>
|
|
<B> Previous:</B>
|
|
<A HREF="node8.html"> Constructors: ConsList, </A>
|
|
<BR> <HR> <P>
|
|
<H2> Quote</H2>
|
|
<P>
|
|
The answer to the previous question is to use the single quote mark, ', in front of an item that you do not wish the interpreter to evaluate. It works
|
|
as follows:
|
|
<P>
|
|
<BLOCKQUOTE>
|
|
<PRE>> 'a
|
|
a
|
|
</PRE>
|
|
</BLOCKQUOTE>
|
|
Without the quote you would have received an error message. So now
|
|
you might try (cons 'a (b c d)), but this still won't work since the
|
|
interpreter tries to evaluate the second argument (b c d), treating b
|
|
as the name of a function. But (we are assuming) you have not
|
|
defined b to be a function, so it is an error. Once again you can
|
|
use the ' to block evaluation of the list (b c d). Thus:
|
|
<P>
|
|
<BLOCKQUOTE>
|
|
<PRE>> (cons 'a '(b c d))
|
|
(a b c d)
|
|
</PRE>
|
|
</BLOCKQUOTE>
|
|
Notice that you need only one quote on the second argument. The
|
|
quote blocks evaluation of the whole thing, so you don't need quotes
|
|
on the internal elements of the list.
|
|
<P>
|
|
Notice, also, that in using setq, the first argument -- the variable to
|
|
be set -- does not need a quote; the interpreter does not evaluate it
|
|
if it is an atom. Originally, Lisp had only the function set, whose
|
|
proper use included things like (set 'a 4) or (set 'a 'b). Lisp
|
|
programmers got tired of putting (or, more likely, forgetting to put)
|
|
the quote on the first argument, so they wrote setq to take care of
|
|
it automatically (now you understand the name, right?!) So, (setq a
|
|
4) is equivalent to (set 'a 4).
|
|
<P>
|
|
In fact, the ' itself is shorthand for another function. The interpreter expands 'a to the expression (quote a). So (setq a 4) is really
|
|
short for (set (quote a) 4). Shorthand commands like setq and ' are
|
|
called ``macros'' by programmers.
|
|
<P>
|
|
Cons always takes two arguments. (Try it with one or three and see
|
|
that you get an error message.) Usually, the second argument is a
|
|
list, but the first can be either an atom or a list. (Cons can be
|
|
used with an atom as the second argument. What gets returned is a
|
|
slightly esoteric data type called a ``dotted pair.'' The use of
|
|
dotted pairs is relatively rare and will be ignored here.)
|
|
<P>
|
|
The list building functions list and append both allow arbitrary
|
|
numbers of arguments. List takes all its arguments and makes a list
|
|
with them as elements. Arguments to list will typically be either
|
|
atoms or lists. For example:
|
|
<P>
|
|
<BLOCKQUOTE>
|
|
<PRE>> (list 2 3 4)
|
|
(2 3 4)
|
|
> (list 'a '(a s d f))
|
|
(a (a s d f))
|
|
</PRE>
|
|
</BLOCKQUOTE>
|
|
Make sure you understand why list works as shown in these examples.
|
|
<P>
|
|
Append is normally used with lists as arguments. (Only the last
|
|
argument may be an atom, resulting in a dotted pair.) When used with
|
|
lists, append has the effect of taking all the members of the lists
|
|
that are its arguments and creating a new list from those elements.
|
|
For example:
|
|
<P>
|
|
<BLOCKQUOTE>
|
|
<PRE>> (append '(a b) '(c d))
|
|
(a b c d)
|
|
</PRE>
|
|
</BLOCKQUOTE>
|
|
Beginning Lisp programmers (and even some experienced ones)
|
|
frequently have trouble deciding whether they should use cons or list
|
|
or append to achieve a particular effect. The best way to get around
|
|
the difficulty is to play with the different functions and see what
|
|
works.
|
|
<P>
|
|
<BR> <HR>
|
|
<A HREF="node10.html"><IMG ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A>
|
|
<A HREF="node7.html"><IMG ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A>
|
|
<A HREF="node8.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif"></A> <BR>
|
|
<A HREF="lp.html"><B>Contents</B></A>
|
|
<B> Next:</B>
|
|
<A HREF="node10.html"> Selectors: First and </A>
|
|
<B>Up:</B>
|
|
<A HREF="node7.html"> Some Primitive Functions</A>
|
|
<B> Previous:</B>
|
|
<A HREF="node8.html"> Constructors: ConsList, </A>
|
|
<BR> <HR> <P>
|
|
<BR> <HR>
|
|
<P>
|
|
<ADDRESS>
|
|
<I>© Colin Allen & Maneesh Dhagat <BR>
|
|
March 2007 </I>
|
|
</ADDRESS>
|
|
</BODY>
|