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>eval | Common Lisp Nova Spec</TITLE
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>Common Lisp Nova Spec</A
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> <SPAN CLASS="breadcrumb-item"
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>→ <A HREF="3_Evaluation_and_Compilation.html"
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>3. Evaluation and Compilation</A
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> <SPAN CLASS="breadcrumb-item"
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>→ <A HREF="3_8_Evaluation_and_Compilation_Dictionary.html"
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>3.8 Evaluation and Compilation Dictionary</A
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> <SPAN CLASS="breadcrumb-item"
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>→ <A HREF="f_eval.html"
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>eval</A
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><DIV CLASS="apropos-io"
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><A HREF="f_compile.html" CLASS="prev"
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>←</A
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><SPAN ID="apropos-label"
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>Apropos </SPAN
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><INPUT ID="apropos" AUTOFOCUS="AUTOFOCUS" PLACEHOLDER="Type here to search" ONINPUT="aproposInput(this);" ONKEYUP="aproposKeyup(event);" ONCHANGE="aproposChange(this);" ONFOCUS="aproposFocus(this);" ONFOCUSOUT="aproposFocusout(this);"
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></DIV
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><DIV CLASS="matter"
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><DIV CLASS="com"
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><DIV CLASS="begincom"
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><HR
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><TABLE WIDTH="100%" CELLSPACING="0" CELLPADDING="0"
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><TR
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><TD ALIGN="LEFT" VALIGN="BASELINE" WIDTH="100%" CLASS="name"
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><SPAN CLASS="idx" DATA-KIND="idxref" DATA-TERM="eval"
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></SPAN
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><B
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>eval</B
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></TD
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><TD ALIGN="RIGHT" VALIGN="BASELINE" WIDTH="0" NOWRAP="NOWRAP" CLASS="ftype"
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><I
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>Function</I
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></TD
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></TR
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></TABLE
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><HR
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></DIV
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><UL CLASS="subtoc"
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></UL
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><DL
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><DT
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><B
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>Syntax</B
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></DT
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><DD
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><P CLASS="j"
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><B
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>eval</B
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> <SPAN CLASS="cmssi"
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>form</SPAN
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> <SPAN CLASS="arrow"
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>→</SPAN
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> {<VAR CLASS="param"
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>result</VAR
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>}*</P
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></DD
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><DT
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><B
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>Arguments and Values</B
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></DT
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><DD
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><P CLASS="j"
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><VAR CLASS="param"
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>form</VAR
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> — a <A HREF="26_1_Glossary.html#form"
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><EM CLASS="term"
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>form</EM
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></A
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>. </P
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><P CLASS="j"
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><VAR CLASS="param"
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>results</VAR
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> — the <A HREF="26_1_Glossary.html#value"
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><EM CLASS="term"
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>values</EM
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></A
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> <A HREF="26_1_Glossary.html#yield"
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><EM CLASS="term"
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>yielded</EM
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></A
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> by the <A HREF="26_1_Glossary.html#evaluation"
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><EM CLASS="term"
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>evaluation</EM
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></A
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> of <VAR CLASS="param"
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>form</VAR
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>.</P
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></DD
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><DT
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><B
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>Description</B
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></DT
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><DD
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><P CLASS="j"
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>Evaluates <VAR CLASS="param"
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>form</VAR
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> in the current <A HREF="26_1_Glossary.html#dynamic_environment"
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><EM CLASS="term"
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>dynamic environment</EM
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></A
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> and the <A HREF="26_1_Glossary.html#null_lexical_environment"
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><EM CLASS="term"
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>null lexical environment</EM
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></A
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>. </P
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><P CLASS="j"
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><A HREF="f_eval.html" CLASS="funref"
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><B
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>eval</B
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></A
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> is a user interface to the evaluator. </P
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><P CLASS="j"
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>The evaluator expands macro calls as if through the use of <A HREF="f_macroexpand.html" CLASS="funref"
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><B
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>macroexpand-1</B
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></A
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>. </P
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><P CLASS="j"
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>Constants appearing in code processed by <A HREF="f_eval.html" CLASS="funref"
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><B
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>eval</B
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></A
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> are not copied nor coalesced. The code resulting from the execution of <A HREF="f_eval.html" CLASS="funref"
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><B
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>eval</B
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></A
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> references <A HREF="26_1_Glossary.html#object"
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><EM CLASS="term"
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>objects</EM
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></A
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> that are <A HREF="f_eql.html" CLASS="funref"
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><B
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>eql</B
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></A
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> to the corresponding <A HREF="26_1_Glossary.html#object"
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><EM CLASS="term"
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>objects</EM
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></A
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> in the source code.</P
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></DD
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><DT
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><B
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>Examples</B
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></DT
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><DD
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><PRE CLASS="screen"
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> (setq form '(1+ a) a 999) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> 999
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(eval form) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> 1000
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(eval 'form) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> (1+ A)
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(let ((a '(this would break if eval used local value))) (eval form))
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<SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> 1000</PRE
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></DD
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><DT
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><B
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>See Also</B
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></DT
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><DD
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><P CLASS="j"
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><A HREF="f_macroexpand.html" CLASS="funref"
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><B
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>macroexpand-1</B
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></A
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>, <A HREF="3_1_Evaluation.html#sec_3_1_2" CLASS="secref"
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><SPAN CLASS="cmr"
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>Section</SPAN
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> <SPAN CLASS="cmr"
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>3.1.2</SPAN
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> <SPAN CLASS="cmr"
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>(The</SPAN
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> <SPAN CLASS="cmr"
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>Evaluation</SPAN
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> <SPAN CLASS="cmr"
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>Model)</SPAN
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></A
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></P
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></DD
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><DT
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><B
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>Notes</B
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></DT
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><DD
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><P CLASS="j"
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>To obtain the current dynamic value of a <A HREF="26_1_Glossary.html#symbol"
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><EM CLASS="term"
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>symbol</EM
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></A
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>, use of <A HREF="f_symbol-value.html" CLASS="funref"
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><B
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>symbol-value</B
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></A
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> is equivalent (and usually preferable) to use of <A HREF="f_eval.html" CLASS="funref"
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><B
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>eval</B
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></A
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>. </P
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><P CLASS="j"
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>Note that an <A HREF="f_eval.html" CLASS="funref"
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><B
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>eval</B
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></A
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> <A HREF="26_1_Glossary.html#form"
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><EM CLASS="term"
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>form</EM
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></A
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> involves two levels of <A HREF="26_1_Glossary.html#evaluation"
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><EM CLASS="term"
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>evaluation</EM
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></A
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> for its <A HREF="26_1_Glossary.html#argument"
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><EM CLASS="term"
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>argument</EM
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></A
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>. First, <VAR CLASS="param"
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>form</VAR
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> is <A HREF="26_1_Glossary.html#evaluate"
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><EM CLASS="term"
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>evaluated</EM
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></A
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> by the normal argument evaluation mechanism as would occur with any <A HREF="26_1_Glossary.html#call"
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><EM CLASS="term"
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>call</EM
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></A
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>. The <A HREF="26_1_Glossary.html#object"
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><EM CLASS="term"
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>object</EM
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></A
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> that results from this normal <A HREF="26_1_Glossary.html#argument"
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><EM CLASS="term"
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>argument</EM
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></A
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> <A HREF="26_1_Glossary.html#evaluation"
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><EM CLASS="term"
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>evaluation</EM
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></A
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> becomes the <A HREF="26_1_Glossary.html#value"
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><EM CLASS="term"
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>value</EM
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></A
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> of the <VAR CLASS="param"
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>form</VAR
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> <A HREF="26_1_Glossary.html#parameter"
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><EM CLASS="term"
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>parameter</EM
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></A
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>, and is then <A HREF="26_1_Glossary.html#evaluate"
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><EM CLASS="term"
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>evaluated</EM
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></A
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> as part of the <A HREF="f_eval.html" CLASS="funref"
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><B
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>eval</B
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></A
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> <A HREF="26_1_Glossary.html#form"
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><EM CLASS="term"
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>form</EM
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></A
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>. For example:</P
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><PRE CLASS="screen"
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>(eval (list 'cdr (car '((quote (a . b)) c)))) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> b</PRE
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><P CLASS="j"
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>The <A HREF="26_1_Glossary.html#argument"
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><EM CLASS="term"
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>argument</EM
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></A
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> <A HREF="26_1_Glossary.html#form"
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><EM CLASS="term"
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>form</EM
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></A
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> <CODE CLASS="f"
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>(list 'cdr (car '((quote (a . b)) c)))</CODE
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> is evaluated in the usual way to produce the <A HREF="26_1_Glossary.html#argument"
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><EM CLASS="term"
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>argument</EM
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></A
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> <CODE CLASS="f"
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>(cdr (quote (a . b)))</CODE
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>; <A HREF="f_eval.html" CLASS="funref"
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><B
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>eval</B
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></A
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> then evaluates its <A HREF="26_1_Glossary.html#argument"
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><EM CLASS="term"
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>argument</EM
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></A
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>, <CODE CLASS="f"
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>(cdr (quote (a . b)))</CODE
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>, to produce <CODE CLASS="f"
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>b</CODE
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>. Since a single <A HREF="26_1_Glossary.html#evaluation"
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><EM CLASS="term"
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>evaluation</EM
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></A
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> already occurs for any <A HREF="26_1_Glossary.html#argument"
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><EM CLASS="term"
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>argument</EM
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></A
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> <A HREF="26_1_Glossary.html#form"
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><EM CLASS="term"
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>form</EM
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></A
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> in any <A HREF="26_1_Glossary.html#function_form"
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><EM CLASS="term"
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>function form</EM
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></A
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>, <A HREF="f_eval.html" CLASS="funref"
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><B
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>eval</B
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></A
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> is sometimes said to perform “an extra level of evaluation.”</P
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></DD
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></DL
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