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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<TITLE>20.1. Run-Time Evaluation of Forms</TITLE>
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<meta name="description" value=" Run-Time Evaluation of Forms">
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html3775 HREF="node181.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3773 HREF="node179.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3767 HREF="node179.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3777 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3778 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3776 HREF="node181.html"> The Top-Level Loop</A>
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<B>Up:</B> <A NAME=tex2html3774 HREF="node179.html"> The Evaluator</A>
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<B> Previous:</B> <A NAME=tex2html3768 HREF="node179.html"> The Evaluator</A>
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<HR> <P>
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<H1><A NAME=SECTION002410000000000000000>20.1. Run-Time Evaluation of Forms</A></H1>
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<P>
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The function <tt>eval</tt> is the main user interface to the evaluator.
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Hooks are provided for user-supplied debugging routines
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to obtain control during the execution of an interpretive evaluator.
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The functions <tt>evalhook</tt> and <tt>applyhook</tt> provide alternative
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interfaces to the evaluator mechanism for use by these debugging routines.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>eval <i>form</i></tt><P>The <i>form</i> is evaluated in the current dynamic environment and
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a null lexical environment. Whatever results from the evaluation
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is returned from the call to <tt>eval</tt>.
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<P>
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Note that when you write a call to <tt>eval</tt> <i>two</i> levels
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of evaluation occur on the argument form you write.
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First the argument form is evaluated, as for arguments to any function,
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by the usual argument evaluation mechanism
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(which involves an implicit use of <tt>eval</tt>). Then the argument
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is passed to the <tt>eval</tt> function, where another evaluation occurs.
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For example:
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<P><pre>
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(eval (list 'cdr (car '((quote (a . b)) c)))) => b
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</pre><P>
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The argument form <tt>(list 'cdr (car '((quote (a . b)) c)))</tt> is evaluated
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in the usual way to produce the argument <tt>(cdr (quote (a . b)))</tt>;
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this is then given to <tt>eval</tt> because <tt>eval</tt> is being called explicitly,
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and <tt>eval</tt> evaluates its argument <tt>(cdr (quote (a . b)))</tt> to produce <tt>b</tt>.
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<P>
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If all that is required for some application is
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to obtain the current dynamic value of a given symbol, the function
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<tt>symbol-value</tt> may be more efficient than <tt>eval</tt>.
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in January 1989
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(MAPPING-DESTRUCTIVE-INTERACTION) <A NAME=19129> </A>
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to restrict user side effects; see section <A HREF="node92.html#STRUCTURETRAVERSALSECTION">7.9</A>.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR><b>[Variable]</b><BR>
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<tt>*evalhook* <BR></tt><tt>*applyhook*</tt><P>If the value of <tt>*evalhook*</tt> is not <tt>nil</tt>, then <tt>eval</tt> behaves
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in a special way. The non-<tt>nil</tt> value of <tt>*evalhook*</tt> should be a function
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that takes two arguments, a form and an environment;
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this is called the <i>eval hook function</i>.
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When a form is to be evaluated (any form at all, even a number or a symbol),
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whether implicitly or via an explicit call to <tt>eval</tt>, no attempt
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is made to evaluate the form.
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Instead, the hook function is invoked and is passed the form to be evaluated
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as its first argument. The hook function is then responsible for
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evaluating the form; whatever is returned by the hook function is assumed
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to be the result of evaluating the form.
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<P>
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The variable <tt>*applyhook*</tt> is similar to <tt>*evalhook*</tt> but is used
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when a function is about to be applied to arguments.
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If the value of <tt>*applyhook*</tt> is not <tt>nil</tt>, then <tt>eval</tt> behaves
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in a special way.
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<p>
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<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
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The non-<tt>nil</tt> value of <tt>*applyhook*</tt> should be a function
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that takes three arguments: a function, a list of arguments,
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and an environment;
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this is called the <i>apply hook function</i>.
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<br><img align=bottom alt="old_change_end" src="gif/old_change_end.gif">
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in January 1989
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(APPLYHOOK-ENVIROMENT) <A NAME=19152> </A>
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to revise the definition of <tt>*applyhook*</tt>.
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Its value should be a function of <i>two</i> arguments,
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a function and a list of arguments; no environment information is passed
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to an apply hook function.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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This was simply a flaw in the first edition. Sorry about that.
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<P>
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When a function is about to be applied to a list of arguments,
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no attempt is made to apply the function.
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Instead, the hook function is invoked and is passed the function and the list
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of arguments
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as its first and second arguments. The hook function is then responsible for
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evaluating the form; whatever is returned by the hook function is assumed
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to be the result of evaluating the form.
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The apply hook function is used only for application of ordinary functions
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within <tt>eval</tt>. It is not used for applications via <tt>apply</tt> or
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<tt>funcall</tt>, for applications by such functions as <tt>map</tt> or
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<tt>reduce</tt>, or for invocation of macro-expansion functions
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by either <tt>eval</tt> or <tt>macroexpand</tt>.
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in June 1988 (FUNCTION-TYPE) <A NAME=19164> </A> to specify
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that the value of <tt>*macroexpand-hook*</tt> is first coerced to a
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function before being called as the expansion interface hook.
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This vote made no mention of <tt>*evalhook*</tt> or <tt>*applyhook*</tt>,
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but this may have been an oversight.
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<P>
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A proposal was submitted to X3J13 in September 1989 to specify
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that the value of <tt>*evalhook*</tt> or <tt>*applyhook*</tt> is first coerced to a
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function before being called. If this proposal is accepted,
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the value of either variable may be <tt>nil</tt>, any other symbol,
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a lambda-expression, or any object of type <tt>function</tt>.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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The last argument passed to either kind of hook function contains information
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about the lexical environment in an implementation-dependent format.
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These arguments are suitable for the functions
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<tt>evalhook</tt>, <tt>applyhook</tt>, and <tt>macroexpand</tt>.
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<P>
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When either kind of hook function is invoked, both of the variables
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<tt>*evalhook*</tt>
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and <tt>*applyhook*</tt> are rebound to the value <tt>nil</tt> around the invocation
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of the hook function. This is so that the hook function will not be
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invoked recursively on evaluations and applications that occur
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in the course of executing the code of the hook function.
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The functions <tt>evalhook</tt>
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and <tt>applyhook</tt> are useful for performing recursive evaluations
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and applications within the hook function.
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<P>
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The hook feature is provided as an aid to debugging.
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The <tt>step</tt> facility is implemented using this hook.
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<P>
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If a non-local exit causes a throw back to the top level
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of Lisp, perhaps because an error could not
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be corrected, then <tt>*evalhook*</tt> and <tt>*applyhook*</tt> are
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automatically reset to <tt>nil</tt> as a safety feature.
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<P>
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<BR><b>[Function]</b><BR>
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<pre>
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evalhook <i>form</i> <i>evalhookfn</i> <i>applyhookfn</i> &optional <i>env</i>
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applyhook <i>function</i> <i>args</i> <i>evalhookfn</i> <i>applyhookfn</i> &optional <i>env</i>
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</pre>
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<P>The functions <tt>evalhook</tt> and <tt>applyhook</tt> are provided to make it
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easier to exploit the hook feature.
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<P>
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In the case of <tt>evalhook</tt>, the <i>form</i> is evaluated.
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In the case of <tt>applyhook</tt>, the <i>function</i> is applied to the
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list of arguments <i>args</i>. In either case,
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for the duration of the operation
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the variable <tt>*evalhook*</tt> is bound to <i>evalhookfn</i>, and
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<tt>*applyhook*</tt> is bound to <i>applyhookfn</i>.
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Furthermore, the <i>env</i> argument
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is used as the lexical environment for the operation;
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<i>env</i> defaults to the null environment.
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The check for a hook function is <i>bypassed</i> for the evaluation
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of the <i>form</i> itself (for <tt>evalhook</tt>) or for the application
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of the <i>function</i> to the <i>args</i> itself (for <tt>applyhook</tt>),
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but not for subsidiary evaluations and applications
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such as evaluations of subforms. It is this one-shot bypass that makes
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<tt>evalhook</tt> and <tt>applyhook</tt> so useful.
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in January 1989
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(APPLYHOOK-ENVIROMENT) <A NAME=19209> </A>
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to eliminate the optional <i>env</i>
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parameter to <tt>applyhook</tt>, because it is not (and cannot)
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be useful. Any function that can be applied carries its own
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environment and does not need another environment to be specified
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separately.
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This was a flaw in the first edition.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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Here is an example of a very simple tracing routine that uses just the
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<tt>evalhook</tt> feature.
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<P><pre>
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(defvar *hooklevel* 0)
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(defun hook (x)
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(let ((*evalhook* 'eval-hook-function))
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(eval x)))
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(defun eval-hook-function (form <tt>&rest</tt> env)
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(let ((*hooklevel* (+ *hooklevel* 1)))
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(format *trace-output* "~%~V@TForm: ~S"
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(* *hooklevel* 2) form)
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(let ((values (multiple-value-list
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(evalhook form
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#'eval-hook-function
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<tt>nil</tt>
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env))))
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(format *trace-output* "~%~V@TValue:~{ ~S~}"
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(* *hooklevel* 2) values)
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(values-list values))))
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</pre><P>
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Using these routines, one might see the following interaction:
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<P><pre>
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(hook '(cons (floor *print-base* 2) 'b))
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Form: (CONS (FLOOR *PRINT-BASE* 2) (QUOTE B))
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Form: (FLOOR *PRINT-BASE* 3)
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Form: *PRINT-BASE*
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Value: 10
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Form: 3
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Value: 3
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Value: 3 1
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Form: (QUOTE B)
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Value: B
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Value: (3 . B)
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(3 . B)
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</pre><P>
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<P>
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<BR><b>[Function]</b><BR>
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<tt>constantp <i>object</i></tt><P>If the predicate <tt>constantp</tt> is true of an object,
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then that object, when considered as a form to
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be evaluated, always evaluates to the same thing;
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it is a constant.
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This includes self-evaluating objects such as numbers, characters,
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strings, bit-vectors, and keywords, as well as all constant symbols
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declared by <tt>defconstant</tt>,
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such as <tt>nil</tt>, <tt>t</tt>, and <tt>pi</tt>.
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In addition, a list whose <i>car</i> is <tt>quote</tt>,
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such as <tt>(quote foo)</tt>, is considered to be a constant.
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<P>
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If <tt>constantp</tt> is false of an object, then
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that object, considered as a form,
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might or might not always evaluate to the same thing.
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<P>
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<BR> <HR><A NAME=tex2html3775 HREF="node181.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3773 HREF="node179.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3767 HREF="node179.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3777 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3778 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3776 HREF="node181.html"> The Top-Level Loop</A>
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<B>Up:</B> <A NAME=tex2html3774 HREF="node179.html"> The Evaluator</A>
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<B> Previous:</B> <A NAME=tex2html3768 HREF="node179.html"> The Evaluator</A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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