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<h1>splain</h1>
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Perl 5 version 22.0 documentation
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<h1>splain</h1>
<!-- -->
<ul><li><a href="#NAME">NAME</a><li><a href="#SYNOPSIS">SYNOPSIS</a><li><a href="#DESCRIPTION">DESCRIPTION</a><ul><li><a href="#The-diagnostics-Pragma">The diagnostics Pragma</a><li><a href="#The-_splain_-Program">The _splain_ Program</a></ul><li><a href="#EXAMPLES">EXAMPLES</a><li><a href="#INTERNALS">INTERNALS</a><li><a href="#BUGS">BUGS</a><li><a href="#AUTHOR">AUTHOR</a></ul><a name="NAME"></a><h1>NAME</h1>
<p>diagnostics, splain - produce verbose warning diagnostics</p>
<a name="SYNOPSIS"></a><h1>SYNOPSIS</h1>
<p>Using the <code class="inline"><span class="w">diagnostics</span></code>
pragma:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/use.html">use</a> <span class="w">diagnostics</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/use.html">use</a> <span class="w">diagnostics</span> -<span class="w">verbose</span><span class="sc">;</span></li><li></li><li> <span class="w">enable</span> <span class="w">diagnostics</span><span class="sc">;</span></li><li> <span class="w">disable</span> <span class="w">diagnostics</span><span class="sc">;</span></li></ol></pre><p>Using the <code class="inline"><span class="w">splain</span></code>
standalone filter program:</p>
<pre class="verbatim"><ol><li> <span class="w">perl</span> <span class="w">program</span> <span class="n">2</span>><span class="w">diag</span>.<span class="w">out</span></li><li> <span class="w">splain</span> <span class="s">[</span>-<span class="w">v</span><span class="s">]</span> <span class="s">[</span>-p<span class="s">]</span> <span class="w">diag</span>.<span class="w">out</span></li></ol></pre><p>Using diagnostics to get stack traces from a misbehaving script:</p>
<pre class="verbatim"><ol><li> <span class="w">perl</span> -<span class="w">Mdiagnostics</span>=-<span class="w">traceonly</span> <span class="w">my_script</span>.<span class="w">pl</span></li></ol></pre><a name="DESCRIPTION"></a><h1>DESCRIPTION</h1>
<a name="The-diagnostics-Pragma"></a><h2>The <code class="inline"><span class="w">diagnostics</span></code>
Pragma</h2>
<p>This module extends the terse diagnostics normally emitted by both the
perl compiler and the perl interpreter (from running perl with a -w
switch or <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">warnings</span></code>
), augmenting them with the more
explicative and endearing descriptions found in <a href="perldiag.html">perldiag</a>. Like the
other pragmata, it affects the compilation phase of your program rather
than merely the execution phase.</p>
<p>To use in your program as a pragma, merely invoke</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/use.html">use</a> <span class="w">diagnostics</span><span class="sc">;</span></li></ol></pre><p>at the start (or near the start) of your program. (Note
that this <i>does</i> enable perl's <b>-w</b> flag.) Your whole
compilation will then be subject(ed :-) to the enhanced diagnostics.
These still go out <b>STDERR</b>.</p>
<p>Due to the interaction between runtime and compiletime issues,
and because it's probably not a very good idea anyway,
you may not use <code class="inline"><a class="l_k" href="functions/no.html">no</a> <span class="w">diagnostics</span></code>
to turn them off at compiletime.
However, you may control their behaviour at runtime using the
disable() and enable() methods to turn them off and on respectively.</p>
<p>The <b>-verbose</b> flag first prints out the <a href="perldiag.html">perldiag</a> introduction before
any other diagnostics. The $diagnostics::PRETTY variable can generate nicer
escape sequences for pagers.</p>
<p>Warnings dispatched from perl itself (or more accurately, those that match
descriptions found in <a href="perldiag.html">perldiag</a>) are only displayed once (no duplicate
descriptions). User code generated warnings a la warn() are unaffected,
allowing duplicate user messages to be displayed.</p>
<p>This module also adds a stack trace to the error message when perl dies.
This is useful for pinpointing what
caused the death. The <b>-traceonly</b> (or
just <b>-t</b>) flag turns off the explanations of warning messages leaving just
the stack traces. So if your script is dieing, run it again with</p>
<pre class="verbatim"><ol><li> <span class="w">perl</span> -<span class="w">Mdiagnostics</span>=-<span class="w">traceonly</span> <span class="w">my_bad_script</span></li></ol></pre><p>to see the call stack at the time of death. By supplying the <b>-warntrace</b>
(or just <b>-w</b>) flag, any warnings emitted will also come with a stack
trace.</p>
<a name="The-_splain_-Program"></a><h2>The <i>splain</i> Program</h2>
<p>While apparently a whole nuther program, <i>splain</i> is actually nothing
more than a link to the (executable) <i>diagnostics.pm</i> module, as well as
a link to the <i>diagnostics.pod</i> documentation. The <b>-v</b> flag is like
the <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">diagnostics</span> -<span class="w">verbose</span></code>
directive.
The <b>-p</b> flag is like the
$diagnostics::PRETTY variable. Since you're post-processing with
<i>splain</i>, there's no sense in being able to enable() or disable() processing.</p>
<p>Output from <i>splain</i> is directed to <b>STDOUT</b>, unlike the pragma.</p>
<a name="EXAMPLES"></a><h1>EXAMPLES</h1>
<p>The following file is certain to trigger a few errors at both
runtime and compiletime:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/use.html">use</a> <span class="w">diagnostics</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="i">NOWHERE</span> <span class="q">"nothing\n"</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="i">STDERR</span> <span class="q">"\n\tThis message should be unadorned.\n"</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/warn.html">warn</a> <span class="q">"\tThis is a user warning"</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">"\nDIAGNOSTIC TESTER: Please enter a <CR> here: "</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">$a</span><span class="cm">,</span> <span class="i">$b</span> = <a class="l_k" href="functions/scalar.html">scalar</a> <span class="q"><STDIN></span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">"\n"</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="i">$x</span>/<span class="i">$y</span><span class="sc">;</span></li></ol></pre><p>If you prefer to run your program first and look at its problem
afterwards, do this:</p>
<pre class="verbatim"><ol><li> perl -w test.pl 2>test.out</li><li> ./splain < test.out</li></ol></pre><p>Note that this is not in general possible in shells of more dubious heritage,
as the theoretical</p>
<pre class="verbatim"><ol><li> (perl -w test.pl >/dev/tty) >& test.out</li><li> ./splain < test.out</li></ol></pre><p>Because you just moved the existing <b>stdout</b> to somewhere else.</p>
<p>If you don't want to modify your source code, but still have on-the-fly
warnings, do this:</p>
<pre class="verbatim"><ol><li> exec 3>&1; perl -w test.pl 2>&1 1>&3 3>&- | splain 1>&2 3>&-</li></ol></pre><p>Nifty, eh?</p>
<p>If you want to control warnings on the fly, do something like this.
Make sure you do the <code class="inline"><a class="l_k" href="functions/use.html">use</a></code> first, or you won't be able to get
at the enable() or disable() methods.</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/use.html">use</a> <span class="w">diagnostics</span><span class="sc">;</span> <span class="c"># checks entire compilation phase </span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">"\ntime for 1st bogus diags: SQUAWKINGS\n"</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="i">BOGUS1</span> <span class="q">'nada'</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">"done with 1st bogus\n"</span><span class="sc">;</span></li><li></li><li> <span class="w">disable</span> <span class="w">diagnostics</span><span class="sc">;</span> <span class="c"># only turns off runtime warnings</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">"\ntime for 2nd bogus: (squelched)\n"</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="i">BOGUS2</span> <span class="q">'nada'</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">"done with 2nd bogus\n"</span><span class="sc">;</span></li><li></li><li> <span class="w">enable</span> <span class="w">diagnostics</span><span class="sc">;</span> <span class="c"># turns back on runtime warnings</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">"\ntime for 3rd bogus: SQUAWKINGS\n"</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="i">BOGUS3</span> <span class="q">'nada'</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">"done with 3rd bogus\n"</span><span class="sc">;</span></li><li></li><li> <span class="w">disable</span> <span class="w">diagnostics</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">"\ntime for 4th bogus: (squelched)\n"</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="i">BOGUS4</span> <span class="q">'nada'</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">"done with 4th bogus\n"</span><span class="sc">;</span></li></ol></pre><a name="INTERNALS"></a><h1>INTERNALS</h1>
<p>Diagnostic messages derive from the <i>perldiag.pod</i> file when available at
runtime. Otherwise, they may be embedded in the file itself when the
splain package is built. See the <i>Makefile</i> for details.</p>
<p>If an extant $SIG{__WARN__} handler is discovered, it will continue
to be honored, but only after the diagnostics::splainthis() function
(the module's $SIG{__WARN__} interceptor) has had its way with your
warnings.</p>
<p>There is a $diagnostics::DEBUG variable you may set if you're desperately
curious what sorts of things are being intercepted.</p>
<pre class="verbatim"><ol><li> BEGIN <span class="s">{</span> <span class="i">$diagnostics::DEBUG</span> = <span class="n">1</span> <span class="s">}</span></li></ol></pre><a name="BUGS"></a><h1>BUGS</h1>
<p>Not being able to say "no diagnostics" is annoying, but may not be
insurmountable.</p>
<p>The <code class="inline">-<span class="w">pretty</span></code>
directive is called too late to affect matters.
You have to do this instead, and <i>before</i> you load the module.</p>
<pre class="verbatim"><ol><li> BEGIN <span class="s">{</span> <span class="i">$diagnostics::PRETTY</span> = <span class="n">1</span> <span class="s">}</span></li></ol></pre><p>I could start up faster by delaying compilation until it should be
needed, but this gets a "panic: top_level" when using the pragma form
in Perl 5.001e.</p>
<p>While it's true that this documentation is somewhat subserious, if you use
a program named <i>splain</i>, you should expect a bit of whimsy.</p>
<a name="AUTHOR"></a><h1>AUTHOR</h1>
<p>Tom Christiansen <<i>tchrist@mox.perl.com</i>>, 25 June 1995.</p>
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