memoize.html

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<h1><a>Tie::Memoize - add data to hash when needed</a></h1>
<p><a name="__index__"></a></p>

<!-- INDEX BEGIN -->

<ul>

	<li><a href="#name">NAME</a></li>
	<li><a href="#synopsis">SYNOPSIS</a></li>
	<li><a href="#description">DESCRIPTION</a></li>
	<li><a href="#inheriting_from_tie__memoize">Inheriting from <strong>Tie::Memoize</strong></a></li>
	<li><a href="#example">EXAMPLE</a></li>
	<li><a href="#bugs">BUGS</a></li>
	<li><a href="#author">AUTHOR</a></li>
</ul>
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<p>
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<h1><a name="name">NAME</a></h1>
<p>Tie::Memoize - add data to hash when needed</p>
<p>
</p>
<hr />
<h1><a name="synopsis">SYNOPSIS</a></h1>
<pre>
  <span class="keyword">require</span> <span class="variable">Tie::Memoize</span><span class="operator">;</span>
  <span class="keyword">tie</span> <span class="variable">%hash</span><span class="operator">,</span> <span class="string">'Tie::Memoize'</span><span class="operator">,</span>
      <span class="operator">\&amp;</span><span class="variable">fetch</span><span class="operator">,</span>                  <span class="comment"># The rest is optional</span>
      <span class="variable">$DATA</span><span class="operator">,</span> <span class="operator">\&amp;</span><span class="keyword">exists</span><span class="operator">,</span>
      <span class="operator">{</span><span class="variable">%ini_value</span><span class="operator">}</span><span class="operator">,</span> <span class="operator">{</span><span class="variable">%ini_existence</span><span class="operator">}</span><span class="operator">;</span>
</pre>
<p>
</p>
<hr />
<h1><a name="description">DESCRIPTION</a></h1>
<p>This package allows a tied hash to autoload its values on the first access,
and to use the cached value on the following accesses.</p>
<p>Only read-accesses (via fetching the value or <a href="../../lib/Pod/perlfunc.html#item_exists"><code>exists</code></a>) result in calls to
the functions; the modify-accesses are performed as on a normal hash.</p>
<p>The required arguments during <a href="../../lib/Pod/perlfunc.html#item_tie"><code>tie</code></a> are the hash, the package, and
the reference to the <code>FETCH</code>ing function.  The optional arguments are
an arbitrary scalar $data, the reference to the <code>EXISTS</code> function,
and initial values of the hash and of the existence cache.</p>
<p>Both the <code>FETCH</code>ing function and the <code>EXISTS</code> functions have the
same signature: the arguments are <code>$key, $data</code>; $data is the same
value as given as argument during tie()ing.  Both functions should
return an empty list if the value does not exist.  If <code>EXISTS</code>
function is different from the <code>FETCH</code>ing function, it should return
a TRUE value on success.  The <code>FETCH</code>ing function should return the
intended value if the key is valid.</p>
<p>
</p>
<hr />
<h1><a name="inheriting_from_tie__memoize">Inheriting from <strong>Tie::Memoize</strong></a></h1>
<p>The structure of the <a href="../../lib/Pod/perlfunc.html#item_tied"><code>tied()</code></a> data is an array reference with elements</p>
<pre>
  0:  cache of known values
  1:  cache of known existence of keys
  2:  FETCH  function
  3:  EXISTS function
  4:  $data</pre>
<p>The rest is for internal usage of this package.  In particular, if
TIEHASH is overwritten, it should call SUPER::TIEHASH.</p>
<p>
</p>
<hr />
<h1><a name="example">EXAMPLE</a></h1>
<pre>
  <span class="keyword">sub</span><span class="variable"> slurp </span><span class="operator">{</span>
    <span class="keyword">my</span> <span class="operator">(</span><span class="variable">$key</span><span class="operator">,</span> <span class="variable">$dir</span><span class="operator">)</span> <span class="operator">=</span> <span class="keyword">shift</span><span class="operator">;</span>
    <span class="keyword">open</span> <span class="keyword">my</span> <span class="variable">$h</span><span class="operator">,</span> <span class="string">'&lt;'</span><span class="operator">,</span> <span class="string">"$dir/$key"</span> <span class="keyword">or</span> <span class="keyword">return</span><span class="operator">;</span>
    <span class="keyword">local</span> <span class="variable">$/</span><span class="operator">;</span> <span class="operator">&lt;</span><span class="variable">$h</span><span class="operator">&gt;</span>                      <span class="comment"># slurp it all</span>
  <span class="operator">}</span>
  <span class="keyword">sub</span><span class="variable"> exists </span><span class="operator">{</span> <span class="keyword">my</span> <span class="operator">(</span><span class="variable">$key</span><span class="operator">,</span> <span class="variable">$dir</span><span class="operator">)</span> <span class="operator">=</span> <span class="keyword">shift</span><span class="operator">;</span> <span class="keyword">return</span> <span class="keyword">-f</span> <span class="string">"$dir/$key"</span> <span class="operator">}</span>
</pre>
<pre>
  <span class="keyword">tie</span> <span class="variable">%hash</span><span class="operator">,</span> <span class="string">'Tie::Memoize'</span><span class="operator">,</span> <span class="operator">\&amp;</span><span class="variable">slurp</span><span class="operator">,</span> <span class="variable">$directory</span><span class="operator">,</span> <span class="operator">\&amp;</span><span class="keyword">exists</span><span class="operator">,</span>
      <span class="operator">{</span> <span class="string">fake_file1</span> <span class="operator">=&gt;</span> <span class="variable">$content1</span><span class="operator">,</span> <span class="string">fake_file2</span> <span class="operator">=&gt;</span> <span class="variable">$content2</span> <span class="operator">}</span><span class="operator">,</span>
      <span class="operator">{</span> <span class="string">pretend_does_not_exists</span> <span class="operator">=&gt;</span> <span class="number">0</span><span class="operator">,</span> <span class="string">known_to_exist</span> <span class="operator">=&gt;</span> <span class="number">1</span> <span class="operator">}</span><span class="operator">;</span>
</pre>
<p>This example treats the slightly modified contents of $directory as a
hash.  The modifications are that the keys <em>fake_file1</em> and
<em>fake_file2</em> fetch values $content1 and $content2, and
<em>pretend_does_not_exists</em> will never be accessed.  Additionally, the
existence of <em>known_to_exist</em> is never checked (so if it does not
exists when its content is needed, the user of %hash may be confused).</p>
<p>
</p>
<hr />
<h1><a name="bugs">BUGS</a></h1>
<p>FIRSTKEY and NEXTKEY methods go through the keys which were already read,
not all the possible keys of the hash.</p>
<p>
</p>
<hr />
<h1><a name="author">AUTHOR</a></h1>
<p>Ilya Zakharevich <em><a href="mailto:mailto:perl-module-hash-memoize@ilyaz.org">mailto:perl-module-hash-memoize@ilyaz.org</a></em>.</p>

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