131 lines
4.5 KiB
HTML
131 lines
4.5 KiB
HTML
<!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>Atoms</TITLE>
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</HEAD>
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<meta name="description" value="Atoms">
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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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<A HREF="lp.html"><B>Contents</B></A>
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<B> Next:</B>
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<A HREF="node6.html"> Lists</A>
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<B>Up:</B>
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<A HREF="node4.html"> Basic Data Types</A>
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<B> Previous:</B>
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<A HREF="node4.html"> Basic Data Types</A>
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<BR> <HR> <P>
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<H2> Atoms</H2>
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<P>
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The first rule of evaluation is that for any atom the evaluator, known as ``eval,'' attempts to find a value for that atom.
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<P>
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For most atoms, eval will return an error unless you have previously
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assigned a value to it. To assign a value to an atom use setq
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(or you can use its more sophisticated cousin, setf; more on setf in
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later chapters). So, for instance, to assign the value 9 to the
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atom ``my-age'' type the following to the interpreter:
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<BLOCKQUOTE>
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<PRE>> (setq my-age 9) ; you assign 9 to the atom my-age
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9 ; interpreter responds with value
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</PRE>
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</BLOCKQUOTE>
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Now, you may test what you have done by giving the atom to the
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interpreter.
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<BLOCKQUOTE>
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<PRE>> my-age ; you tell interpreter to eval my-age
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9 ; it responds with the set value
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</PRE>
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</BLOCKQUOTE>
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If a birthday has just passed, you can change the value of my-age as
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follows:
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<BLOCKQUOTE>
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<PRE>> (setq my-age 10)
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10
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> my-age
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10
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</PRE>
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</BLOCKQUOTE>
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<P>
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9, 10, 1.234 and all the other numbers are special atoms in
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Lisp -- they are pre-defined to evaluate to themselves. So, you may
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give any number to the interpreter, and it will respond by repeating
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the number.
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<P>
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In addition to numbers, two other special atoms are predefined, t and nil (think of them as true and false respectively). The interpreter considers nil to be identical to the empty list. Typing () directly to the interpreter
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will cause it to respond with nil.
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<P>
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Try the following sequence:
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<P>
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<BLOCKQUOTE>
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<PRE>> 9
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9
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> 10
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10
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> my-age
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10
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> t
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T
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> nil
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NIL
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> ()
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NIL
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> your-age
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Error: The variable YOUR-AGE is unbound.
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Error signalled by EVAL.
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</PRE>
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</BLOCKQUOTE>
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The last item illustrates what happens if you try to
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evaluate an atom that has not been set to a value. (The exact error
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message will vary between versions of Lisp, but all will say
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something about an unbound variable).
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<P>
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Most Lisp systems throw you into a debugger mode when an error
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occurs. From the debugger you can find out lots of useful things
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about the state of the interpreter when the problem occurred.
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Unfortunately, Lisp debuggers are not at all standardized so it is
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impossible to give a description here. Even in debugger
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mode, although the prompt usually is different, the Lisp interpreter
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continues to evaluate Lisp expressions normally. So we will ignore
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what is going on when an error occurs and assume that you can just
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carry on giving expressions to the interpreter for evaluation.
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<P>
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Notice that it is an error to attempt to set a value for special
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atoms: numbers, t, or nil.
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<P>
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<BLOCKQUOTE>
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<PRE>> (setq 1 2)
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Error: 1 is not a symbol.
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Error signalled by SETQ.
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> (setq t nil)
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Error: Cannot assign to the constant T.
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Error signalled by SETQ.
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</PRE>
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</BLOCKQUOTE>
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<P>
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From these error messages you can see that the interpreter distinguishes between symbols, numbers, constants. Numbers and symbols are mutually exclusive subcategories of atoms. Constants (such as t and nil) are a subcategory of symbol. Only symbols which are not constants may be assigned a value with setq.
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<P>
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<BR> <HR>
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<A HREF="node6.html"><IMG ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A>
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<A HREF="node4.html"><IMG ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A>
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<A HREF="node4.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="node6.html"> Lists</A>
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<B>Up:</B>
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<A HREF="node4.html"> Basic Data Types</A>
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<B> Previous:</B>
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<A HREF="node4.html"> Basic Data Types</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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