69 lines
2.7 KiB
HTML
69 lines
2.7 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> Constructors: Cons, List, and Append</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Constructors: Cons, List, and Append">
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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="node9.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="node7.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="node9.html"> Quote</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="node7.html"> Some Primitive Functions</A>
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<BR> <HR> <P>
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<H2> Constructors: Cons, List, and Append</H2>
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<P>
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The primitive function ``cons'' allows you to add a member to the front of a list. Here are two examples:
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<P>
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<BLOCKQUOTE>
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<PRE>> (cons 1 nil)
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(1)
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> (cons 1 (cons 2 nil))
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(1 2)
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</PRE>
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</BLOCKQUOTE>
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It is worth looking at what is going on in these examples. In the first, cons is given two arguments. The first is 1 and the second is nil, which, you will remember, is the same as (). So, cons takes the first argument and inserts it as the first element of the second.
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<P>
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To understand the second example, it is useful to anthropomorphize the interpreter. It says, ``OK, cons is a function I know and I've
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got two arguments to that function, 1 and (cons 2
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nil). The first argument is an atom, but a special one, and
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evaluates to 1. The second is a list, so its first element names a
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function, i.e. cons, and there are two arguments to that function, 2
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and nil. 2 evaluates to 2, and nil evaluates to nil (the empty
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list). So, putting 2 into the empty list we get (2). Now I know
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what the second argument to the first cons is, we can go ahead and
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insert the first argument, i.e. 1. Hence we get (1 2).''
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<P>
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Now we discuss what happens when we deal with atoms that are not
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special. Suppose you wanted to construct the list (a b c d) and you
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try to do this by saying:
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<P>
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<BLOCKQUOTE>
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<PRE>> (cons a (b c d))
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</PRE>
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</BLOCKQUOTE>
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What would happen? Well, the interpreter would say ``OK, cons I know,
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and I've got two arguments to evaluate. First, eval a.''
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<P>
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Immediately you get an error because (we are assuming) a is an
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unbound variable (you haven't set it to anything). How do you fix
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this problem?
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<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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