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68 lines
6.7 KiB
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<!-- Common Lisp HyperSpec (TM), version 7.0 generated by Kent M. Pitman on Mon, 11-Apr-2005 2:31am EDT -->
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<TITLE>CLHS: Function EVAL</TITLE>
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<A NAME="eval"><I>Function</I> <B>EVAL</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>eval</B> <I>form</I> => <I><I>result</I><B>*</B></I><P>
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<P>
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<P><B>Arguments and Values:</B><P>
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<P>
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<I>form</I>---a <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>. <P>
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<I>results</I>---the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>values</I></A> <I>yielded</I> by the <A REL=DEFINITION HREF="26_glo_e.htm#evaluation"><I>evaluation</I></A> of <I>form</I>. <P>
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<P><B>Description:</B><P>
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<P>
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Evaluates <I>form</I> in the current <A REL=DEFINITION HREF="26_glo_d.htm#dynamic_environment"><I>dynamic environment</I></A> and the <A REL=DEFINITION HREF="26_glo_n.htm#null_lexical_environment"><I>null lexical environment</I></A>. <P>
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<A REL=DEFINITION HREF="#eval"><B>eval</B></A> is a user interface to the evaluator. <P>
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The evaluator expands macro calls as if through the use of <A REL=DEFINITION HREF="f_mexp_.htm#macroexpand-1"><B>macroexpand-1</B></A>. <P>
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Constants appearing in code processed by <A REL=DEFINITION HREF="#eval"><B>eval</B></A> are not copied nor coalesced. The code resulting from the execution of <A REL=DEFINITION HREF="#eval"><B>eval</B></A> references <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>objects</I></A> that are <A REL=DEFINITION HREF="f_eql.htm#eql"><B>eql</B></A> to the corresponding <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>objects</I></A> in the source code. <P>
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<P><B>Examples:</B><P>
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<P>
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<PRE>
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(setq form '(1+ a) a 999) => 999
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(eval form) => 1000
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(eval 'form) => (1+ A)
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(let ((a '(this would break if eval used local value))) (eval form))
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=> 1000
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</PRE>
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</TT> <P>
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<P><B>Affected By:</B> None.
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<P>
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<P><B>Exceptional Situations:</B> None.
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<P>
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<P><B>See Also:</B><P>
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<P>
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<A REL=DEFINITION HREF="f_mexp_.htm#macroexpand-1"><B>macroexpand-1</B></A>, <A REL=CHILD HREF="03_ab.htm">Section 3.1.2 (The Evaluation Model)</A> <P>
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<P><B>Notes:</B><P>
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<P>
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To obtain the current dynamic value of a <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A>, use of <A REL=DEFINITION HREF="f_symb_5.htm#symbol-value"><B>symbol-value</B></A> is equivalent (and usually preferable) to use of <A REL=DEFINITION HREF="#eval"><B>eval</B></A>. <P>
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Note that an <A REL=DEFINITION HREF="#eval"><B>eval</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A> involves two levels of <A REL=DEFINITION HREF="26_glo_e.htm#evaluation"><I>evaluation</I></A> for its <A REL=DEFINITION HREF="26_glo_a.htm#argument"><I>argument</I></A>. First, <I>form</I> is <I>evaluated</I> by the normal argument evaluation mechanism as would occur with any <A REL=DEFINITION HREF="26_glo_c.htm#call"><I>call</I></A>. The <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> that results from this normal <A REL=DEFINITION HREF="26_glo_a.htm#argument"><I>argument</I></A> <A REL=DEFINITION HREF="26_glo_e.htm#evaluation"><I>evaluation</I></A> becomes the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>value</I></A> of the <I>form</I> <A REL=DEFINITION HREF="26_glo_p.htm#parameter"><I>parameter</I></A>, and is then <I>evaluated</I> as part of the <A REL=DEFINITION HREF="#eval"><B>eval</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>. For example: <P>
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<PRE>
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(eval (list 'cdr (car '((quote (a . b)) c)))) => b
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</PRE>
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</TT> The <A REL=DEFINITION HREF="26_glo_a.htm#argument"><I>argument</I></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A> <TT>(list 'cdr (car '((quote (a . b)) c)))</TT> is evaluated in the usual way to produce the <A REL=DEFINITION HREF="26_glo_a.htm#argument"><I>argument</I></A> <TT>(cdr (quote (a . b)))</TT>; <A REL=DEFINITION HREF="#eval"><B>eval</B></A> then evaluates its <A REL=DEFINITION HREF="26_glo_a.htm#argument"><I>argument</I></A>, <TT>(cdr (quote (a . b)))</TT>, to produce <TT>b</TT>. Since a single <A REL=DEFINITION HREF="26_glo_e.htm#evaluation"><I>evaluation</I></A> already occurs for any <A REL=DEFINITION HREF="26_glo_a.htm#argument"><I>argument</I></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A> in any <A REL=DEFINITION HREF="26_glo_f.htm#function_form"><I>function form</I></A>, <A REL=DEFINITION HREF="#eval"><B>eval</B></A> is sometimes said to perform ``an extra level of evaluation.'' <P>
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<P>
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<P><HR>The following <A REL=META HREF="../Front/X3J13Iss.htm">X3J13 cleanup issues</A>, <I>not part of the specification</I>, apply to this section:<P><UL><LI> <A REL=CHILD HREF="../Issues/iss149.htm">EVALHOOK-STEP-CONFUSION:FIX</A><LI> <A REL=CHILD HREF="../Issues/iss150.htm">EVALHOOK-STEP-CONFUSION:X3J13-NOV-89</A><LI> <A REL=CHILD HREF="../Issues/iss282.htm">QUOTE-SEMANTICS:NO-COPYING</A><P></UL><HR>
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