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