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</pre>
<p>Other examples include:</p>
<pre>
    <span class="keyword">use</span> <span class="variable">Socket</span> <span class="string">qw(!/^[AP]F_/ !SOMAXCONN !SOL_SOCKET)</span><span class="operator">;</span>
    <span class="keyword">use</span> <span class="variable">POSIX</span>  <span class="string">qw(:errno_h :termios_h !TCSADRAIN !/^EXIT/)</span><span class="operator">;</span>
</pre>
<p>Remember that most patterns (using //) will need to be anchored
with a leading ^, e.g., <code>/^EXIT/</code> rather than <code>/EXIT/</code>.</p>
<p>You can say <code>BEGIN { $Exporter::Verbose=1 }</code> to see how the
specifications are being processed and what is actually being imported
into modules.</p>
<p>
</p>
<h2><a name="exporting_without_using_exporter_s_import_method">Exporting without using Exporter's import method</a></h2>
<p>Exporter has a special method, 'export_to_level' which is used in situations
where you can't directly call Exporter's import method. The export_to_level
method looks like:</p>
<pre>
    <span class="variable">MyPackage</span><span class="operator">-&gt;</span><span class="variable">export_to_level</span><span class="operator">(</span><span class="variable">$where_to_export</span><span class="operator">,</span> <span class="variable">$package</span><span class="operator">,</span> <span class="variable">@what_to_export</span><span class="operator">);</span>
</pre>
<p>where $where_to_export is an integer telling how far up the calling stack
to export your symbols, and @what_to_export is an array telling what
symbols *to* export (usually this is @_).  The $package argument is
currently unused.</p>
<p>For example, suppose that you have a module, A, which already has an
import function:</p>
<pre>
    <span class="keyword">package</span> <span class="variable">A</span><span class="operator">;</span>
</pre>
<pre>
    <span class="variable">@ISA</span> <span class="operator">=</span> <span class="string">qw(Exporter)</span><span class="operator">;</span>
    <span class="variable">@EXPORT_OK</span> <span class="operator">=</span> <span class="string">qw ($b)</span><span class="operator">;</span>
</pre>
<pre>
    <span class="keyword">sub</span><span class="variable"> import
    </span><span class="operator">{</span>
        <span class="variable">$A::b</span> <span class="operator">=</span> <span class="number">1</span><span class="operator">;</span>     <span class="comment"># not a very useful import method</span>
    <span class="operator">}</span>
</pre>
<p>and you want to Export symbol $A::b back to the module that called 
package A. Since Exporter relies on the import method to work, via 
inheritance, as it stands Exporter::import() will never get called. 
Instead, say the following:</p>
<pre>
    <span class="keyword">package</span> <span class="variable">A</span><span class="operator">;</span>
    <span class="variable">@ISA</span> <span class="operator">=</span> <span class="string">qw(Exporter)</span><span class="operator">;</span>
    <span class="variable">@EXPORT_OK</span> <span class="operator">=</span> <span class="string">qw ($b)</span><span class="operator">;</span>
</pre>
<pre>
    <span class="keyword">sub</span><span class="variable"> import
    </span><span class="operator">{</span>
        <span class="variable">$A::b</span> <span class="operator">=</span> <span class="number">1</span><span class="operator">;</span>
        <span class="variable">A</span><span class="operator">-&gt;</span><span class="variable">export_to_level</span><span class="operator">(</span><span class="number">1</span><span class="operator">,</span> <span class="variable">@_</span><span class="operator">);</span>
    <span class="operator">}</span>
</pre>
<p>This will export the symbols one level 'above' the current package - ie: to 
the program or module that used package A.</p>
<p>Note: Be careful not to modify <a href="../lib/Pod/perlvar.html#item___"><code>@_</code></a> at all before you call export_to_level
- or people using your package will get very unexplained results!</p>
<p>
</p>
<h2><a name="exporting_without_inheriting_from_exporter">Exporting without inheriting from Exporter</a></h2>
<p>By including Exporter in your @ISA you inherit an Exporter's <a href="../lib/Pod/perlfunc.html#item_import"><code>import()</code></a> method
but you also inherit several other helper methods which you probably don't
want. To avoid this you can do</p>
<pre>
  <span class="keyword">package</span> <span class="variable">YourModule</span><span class="operator">;</span>
  <span class="keyword">use</span> <span class="variable">Exporter</span> <span class="string">qw( import )</span><span class="operator">;</span>
</pre>
<p>which will export Exporter's own <a href="../lib/Pod/perlfunc.html#item_import"><code>import()</code></a> method into YourModule.
Everything will work as before but you won't need to include Exporter in
@YourModule::ISA.</p>
<p>
</p>
<h2><a name="module_version_checking">Module Version Checking</a></h2>
<p>The Exporter module will convert an attempt to import a number from a
module into a call to $module_name-&gt;require_version($value). This can
be used to validate that the version of the module being used is
greater than or equal to the required version.</p>
<p>The Exporter module supplies a default require_version method which
checks the value of $VERSION in the exporting module.</p>
<p>Since the default require_version method treats the $VERSION number as
a simple numeric value it will regard version 1.10 as lower than
1.9. For this reason it is strongly recommended that you use numbers
with at least two decimal places, e.g., 1.09.</p>
<p>
</p>
<h2><a name="managing_unknown_symbols">Managing Unknown Symbols</a></h2>
<p>In some situations you may want to prevent certain symbols from being
exported. Typically this applies to extensions which have functions
or constants that may not exist on some systems.</p>
<p>The names of any symbols that cannot be exported should be listed
in the <code>@EXPORT_FAIL</code> array.</p>
<p>If a module attempts to import any of these symbols the Exporter
will give the module an opportunity to handle the situation before
generating an error. The Exporter will call an export_fail method
with a list of the failed symbols:</p>
<pre>
  <span class="variable">@failed_symbols</span> <span class="operator">=</span> <span class="variable">$module_name</span><span class="operator">-&gt;</span><span class="variable">export_fail</span><span class="operator">(</span><span class="variable">@failed_symbols</span><span class="operator">);</span>
</pre>
<p>If the export_fail method returns an empty list then no error is
recorded and all the requested symbols are exported. If the returned
list is not empty then an error is generated for each symbol and the
export fails. The Exporter provides a default export_fail method which
simply returns the list unchanged.</p>
<p>Uses for the export_fail method include giving better error messages
for some symbols and performing lazy architectural checks (put more
symbols into @EXPORT_FAIL by default and then take them out if someone
actually tries to use them and an expensive check shows that they are
usable on that platform).</p>
<p>
</p>
<h2><a name="tag_handling_utility_functions">Tag Handling Utility Functions</a></h2>
<p>Since the symbols listed within %EXPORT_TAGS must also appear in either
@EXPORT or @EXPORT_OK, two utility functions are provided which allow
you to easily add tagged sets of symbols to @EXPORT or @EXPORT_OK:</p>
<pre>
  <span class="variable">%EXPORT_TAGS</span> <span class="operator">=</span> <span class="operator">(</span><span class="string">foo</span> <span class="operator">=&gt;</span> <span class="operator">[</span><span class="string">qw(aa bb cc)</span><span class="operator">]</span><span class="operator">,</span> <span class="string">bar</span> <span class="operator">=&gt;</span> <span class="operator">[</span><span class="string">qw(aa cc dd)</span><span class="operator">]</span><span class="operator">);</span>
</pre>
<pre>
  <span class="variable">Exporter::export_tags</span><span class="operator">(</span><span class="string">'foo'</span><span class="operator">);</span>     <span class="comment"># add aa, bb and cc to @EXPORT</span>
  <span class="variable">Exporter::export_ok_tags</span><span class="operator">(</span><span class="string">'bar'</span><span class="operator">);</span>  <span class="comment"># add aa, cc and dd to @EXPORT_OK</span>
</pre>
<p>Any names which are not tags are added to @EXPORT or @EXPORT_OK
unchanged but will trigger a warning (with <a href="../lib/Pod/perlrun.html#item__2dw"><code>-w</code></a>) to avoid misspelt tags
names being silently added to @EXPORT or @EXPORT_OK. Future versions
may make this a fatal error.</p>
<p>
</p>
<h2><a name="generating_combined_tags">Generating combined tags</a></h2>
<p>If several symbol categories exist in %EXPORT_TAGS, it's usually
useful to create the utility &quot;:all&quot; to simplify &quot;use&quot; statements.</p>
<p>The simplest way to do this is:</p>
<pre>
  <span class="variable">%EXPORT_TAGS</span> <span class="operator">=</span> <span class="operator">(</span><span class="string">foo</span> <span class="operator">=&gt;</span> <span class="operator">[</span><span class="string">qw(aa bb cc)</span><span class="operator">]</span><span class="operator">,</span> <span class="string">bar</span> <span class="operator">=&gt;</span> <span class="operator">[</span><span class="string">qw(aa cc dd)</span><span class="operator">]</span><span class="operator">);</span>
</pre>
<pre>
  <span class="comment"># add all the other ":class" tags to the ":all" class,</span>
  <span class="comment"># deleting duplicates</span>
  <span class="operator">{</span>
    <span class="keyword">my</span> <span class="variable">%seen</span><span class="operator">;</span>
</pre>
<pre>
    <span class="keyword">push</span> <span class="variable">@</span><span class="operator">{</span><span class="variable">$EXPORT_TAGS</span><span class="operator">{</span><span class="string">all</span><span class="operator">}}</span><span class="operator">,</span>
      <span class="keyword">grep</span> <span class="operator">{!</span><span class="variable">$seen</span><span class="operator">{</span><span class="variable">$_</span><span class="operator">}</span><span class="operator">++}</span> <span class="variable">@</span><span class="operator">{</span><span class="variable">$EXPORT_TAGS</span><span class="operator">{</span><span class="variable">$_</span><span class="operator">}}</span> <span class="keyword">foreach</span> <span class="keyword">keys</span> <span class="variable">%EXPORT_TAGS</span><span class="operator">;</span>
      <span class="operator">}</span>
</pre>
<p>CGI.pm creates an &quot;:all&quot; tag which contains some (but not really
all) of its categories.  That could be done with one small
change:</p>
<pre>
  <span class="comment"># add some of the other ":class" tags to the ":all" class,</span>
  <span class="comment"># deleting duplicates</span>
  <span class="operator">{</span>
    <span class="keyword">my</span> <span class="variable">%seen</span><span class="operator">;</span>
</pre>
<pre>
    <span class="keyword">push</span> <span class="variable">@</span><span class="operator">{</span><span class="variable">$EXPORT_TAGS</span><span class="operator">{</span><span class="string">all</span><span class="operator">}}</span><span class="operator">,</span>
      <span class="keyword">grep</span> <span class="operator">{!</span><span class="variable">$seen</span><span class="operator">{</span><span class="variable">$_</span><span class="operator">}</span><span class="operator">++}</span> <span class="variable">@</span><span class="operator">{</span><span class="variable">$EXPORT_TAGS</span><span class="operator">{</span><span class="variable">$_</span><span class="operator">}}</span>
        <span class="keyword">foreach</span> <span class="string">qw/html2 html3 netscape form cgi internal/</span><span class="operator">;</span>
      <span class="operator">}</span>
</pre>
<p>Note that the tag names in %EXPORT_TAGS don't have the leading ':'.</p>
<p>
</p>
<h2><a name="autoloaded_constants"><code>AUTOLOAD</code>ed Constants</a></h2>
<p>Many modules make use of <code>AUTOLOAD</code>ing for constant subroutines to
avoid having to compile and waste memory on rarely used values (see
<a href="../lib/Pod/perlsub.html">the perlsub manpage</a> for details on constant subroutines).  Calls to such
constant subroutines are not optimized away at compile time because
they can't be checked at compile time for constancy.</p>
<p>Even if a prototype is available at compile time, the body of the
subroutine is not (it hasn't been <code>AUTOLOAD</code>ed yet). perl needs to
examine both the <code>()</code> prototype and the body of a subroutine at
compile time to detect that it can safely replace calls to that
subroutine with the constant value.</p>
<p>A workaround for this is to call the constants once in a <code>BEGIN</code> block:</p>
<pre>
   <span class="keyword">package</span> <span class="variable">My</span> <span class="operator">;</span>
</pre>
<pre>
   <span class="keyword">use</span> <span class="variable">Socket</span> <span class="operator">;</span>
</pre>
<pre>
   <span class="variable">foo</span><span class="operator">(</span> <span class="variable">SO_LINGER</span> <span class="operator">);</span>     <span class="comment">## SO_LINGER NOT optimized away; called at runtime</span>
   <span class="keyword">BEGIN</span> <span class="operator">{</span> <span class="variable">SO_LINGER</span> <span class="operator">}</span>
   <span class="variable">foo</span><span class="operator">(</span> <span class="variable">SO_LINGER</span> <span class="operator">);</span>     <span class="comment">## SO_LINGER optimized away at compile time.</span>
</pre>
<p>This forces the <code>AUTOLOAD</code> for <code>SO_LINGER</code> to take place before
SO_LINGER is encountered later in <code>My</code> package.</p>
<p>If you are writing a package that <code>AUTOLOAD</code>s, consider forcing
an <code>AUTOLOAD</code> for any constants explicitly imported by other packages
or which are usually used when your package is <a href="../lib/Pod/perlfunc.html#item_use"><code>use</code></a>d.</p>

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