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<html>
<head><title>USER-DEFINED-FUNCTIONS-TABLE.html -- ACL2 Version 3.1</title></head>
<body text=#000000 bgcolor="#FFFFFF">
<h2>USER-DEFINED-FUNCTIONS-TABLE</h2>an advanced <a href="TABLE.html">table</a> used to replace certain system functions
<pre>Major Section: <a href="EVENTS.html">EVENTS</a>
</pre><p>
<pre>
Examples:
(table user-defined-functions-table 'untranslate-preprocess 'my-preprocess)
(table user-defined-functions-table 'untranslate 'my-untranslate)
</pre>
<p>
This feature should perhaps only be used by advanced users who have a
thorough understanding of the system functions being replaced. There are
currently two ways a user can affect the way ACL2 prints terms.<p>
The first example associates the user-defined function symbol
<code>my-preprocess</code> with <code>untranslate-preprocess</code>. As a result, when ACL2
prints a term, say during a proof, using its so-called ``untranslate''
process the first thing it does is to call <code>my-preprocess</code> on two
arguments: that term and the current ACL2 logical <a href="WORLD.html">world</a>. If the call
produces a non-<code>nil</code> result, then that result is passed to the untranslate
process.<p>
The second example associates the user-defined function symbol
<code>my-untranslate</code> with the built-in function symbol <code>untranslate</code>. As a
result, the code for <code>my-untranslate</code> will be run whenever the untranslate
process is run. The formals of the two functions must agree and must not
contain any <a href="STOBJ.html">stobj</a> names. Note that these overrides fail to occur upon
guard violations and some other evaluation errors.
<p>
The <code>untranslate-preprocess</code> approach may suffice for most cases in which a
user wants to modify the way output is produced by the theorem prover. We
present an example immediately below, but see
<code>books/misc/untranslate-patterns.lisp</code> for a more elaborate example. If
the <code>untranslate-preprocess</code> approach does not seem sufficient for your
purposes, you are invited to look at file <code>books/misc/rtl-untranslate.lisp</code>
for an example of user-defined <code>untranslate</code> (i.e., following the second
example displayed above).<p>
Suppose you have a large constant that you would prefer not to see in
proofs. For example, you may have submitted the following definition (but
imagine a much larger constant, say, a list of length 1,000,000).
<pre>
(defconst *a* '(a b c d))
</pre>
If you submit the following (silly) theorem
<pre>
(thm (equal (cons x *a*) (car (cons yyy zzz))))
</pre>
then you will see the following output:
<pre>
(EQUAL (CONS X '(A B C D)) YYY).
</pre>
If <code>*a*</code> had represented a much larger structure, we would wish we could
see the following instead.
<pre>
(EQUAL (CONS X *A*) YYY)
</pre>
That can be accomplished as follows. First we make the following definition.
<pre>
(defun my-preprocess (term wrld)
(declare (ignore wrld))
(if (equal term (list 'quote *a*))
'*a*
term))
</pre>
Now we submit the following <code><a href="TABLE.html">table</a></code> event.
<pre>
(table user-defined-functions-table
'untranslate-preprocess
'my-preprocess)
</pre>
This will install <code>my-preprocess</code> as a preprocessor before the normal
untranslation routine is applied to printing a term. When the untranslation
routine encounters the constant <code>(QUOTE (A B C D))</code>, it will replace it
with <code>*a*</code>, and the usual untranlation routine will print this as
<code>*A*</code>.
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