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<head><title>OK-IF.html -- ACL2 Version 3.1</title></head>
<body text=#000000 bgcolor="#FFFFFF">
<h2>OK-IF</h2>conditional exit from <code>break-rewrite</code>
<pre>Major Section: <a href="BREAK-REWRITE.html">BREAK-REWRITE</a>
</pre><p>
<pre>
Example Form:
:ok-if (null (brr@ :wonp))
<p>
General Form:
:ok-if expr
</pre>
where <code>expr</code> is a term involving no free variables other than <code>state</code>
and returning one non-<code>state</code> result which is treated as Boolean.
This form is intended to be executed from within <code>break-rewrite</code>
(see <a href="BREAK-REWRITE.html">break-rewrite</a>).<p>
Consider first the simple situation that the <code>(ok-if term)</code> is a
command read by <code>break-rewrite</code>. Then, if the term is non-<code>nil</code>,
<code>break-rewrite</code> exits and otherwise it does not.<p>
More generally, <code>ok-if</code> returns an ACL2 error triple
<code>(mv erp val state)</code>. (See <a href="LD.html">ld</a> for more on error triples.) If
any form being evaluated as a command by <code>break-rewrite</code> returns
the triple returned by <code>(ok-if term)</code> then the effect of that form
is to exit <a href="BREAK-REWRITE.html">break-rewrite</a> if term is non-<code>nil</code>. Thus, one
might define a function or macro that returns the value of <code>ok-if</code>
expressions on all outputs and thus create a convenient new way to
exit <code>break-rewrite</code>.<p>
The exit test, <code>term</code>, generally uses <code>brr@</code> to access context sensitive
information about the attempted rule application. See <a href="BRR_at_.html">brr@</a>.
<code>Ok-if</code> is useful inside of command sequences produced by break
conditions. See <a href="MONITOR.html">monitor</a>. <code>:ok-if</code> is most useful after an <code>:eval</code>
command has caused <code>break-rewrite</code> to try to apply the rule because in
the resulting break environment <code>expr</code> can access such things as
whether the rule succeeded, if so, what term it produced, and if
not, why. There is no need to use <code>:ok-if</code> before <code>:eval</code>ing the rule
since the same effects could be achieved with the break condition on
the rule itself. Perhaps we should replace this concept with
<code>:eval-and-break-if</code>? Time will tell.
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