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  <p class="right" id="optional-0"><span id="index-0"></span><strong>object</strong></p>
<section id="optional">
<h1><code class="docutils literal notranslate"><span class="pre">optional</span></code><a class="headerlink" href="#optional" title="Link to this heading"></a></h1>
<p>Constructors for optional terms. An optional term is either empty or holds a value. Optional terms should be regarded as opaque terms and always used with the <code class="docutils literal notranslate"><span class="pre">optional/1</span></code> object by passing the optional term as a parameter.</p>
<div class="line-block">
<div class="line"><strong>Availability:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">logtalk_load(optionals(loader))</span></code></div>
</div>
</div>
<div class="line-block">
<div class="line"><strong>Author:</strong> Paulo Moura</div>
<div class="line"><strong>Version:</strong> 2:1:0</div>
<div class="line"><strong>Date:</strong> 2021-01-03</div>
</div>
<div class="line-block">
<div class="line"><strong>Compilation flags:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">static,</span> <span class="pre">context_switching_calls</span></code></div>
</div>
</div>
<div class="line-block">
<div class="line"><strong>Provides:</strong></div>
<div class="line-block">
<div class="line"><a class="reference internal" href="type_0.html#type-0-type-1"><span class="std std-ref">type::type/1</span></a></div>
<div class="line"><a class="reference internal" href="type_0.html#type-0-check-2"><span class="std std-ref">type::check/2</span></a></div>
</div>
</div>
<div class="line-block">
<div class="line"><strong>Remarks:</strong></div>
</div>
<blockquote>
<div><ul class="simple">
<li><p>Type-checking support: This object also defines a type <code class="docutils literal notranslate"><span class="pre">optional</span></code> for use with the <code class="docutils literal notranslate"><span class="pre">type</span></code> library object.</p></li>
</ul>
</div></blockquote>
<div class="line-block">
<div class="line"><strong>Inherited public predicates:</strong></div>
<div class="line-block">
<div class="line">(none)</div>
</div>
</div>
<nav class="contents local" id="contents">
<ul class="simple">
<li><p><a class="reference internal" href="#public-predicates" id="id1">Public predicates</a></p>
<ul>
<li><p><a class="reference internal" href="#empty-1" id="id2"><code class="docutils literal notranslate"><span class="pre">empty/1</span></code></a></p></li>
<li><p><a class="reference internal" href="#of-2" id="id3"><code class="docutils literal notranslate"><span class="pre">of/2</span></code></a></p></li>
<li><p><a class="reference internal" href="#from-goal-3" id="id4"><code class="docutils literal notranslate"><span class="pre">from_goal/3</span></code></a></p></li>
<li><p><a class="reference internal" href="#from-goal-2" id="id5"><code class="docutils literal notranslate"><span class="pre">from_goal/2</span></code></a></p></li>
<li><p><a class="reference internal" href="#from-generator-3" id="id6"><code class="docutils literal notranslate"><span class="pre">from_generator/3</span></code></a></p></li>
<li><p><a class="reference internal" href="#from-generator-2" id="id7"><code class="docutils literal notranslate"><span class="pre">from_generator/2</span></code></a></p></li>
</ul>
</li>
<li><p><a class="reference internal" href="#protected-predicates" id="id8">Protected predicates</a></p></li>
<li><p><a class="reference internal" href="#private-predicates" id="id9">Private predicates</a></p></li>
<li><p><a class="reference internal" href="#operators" id="id10">Operators</a></p></li>
</ul>
</nav>
<section id="public-predicates">
<h2><a class="toc-backref" href="#contents" role="doc-backlink">Public predicates</a><a class="headerlink" href="#public-predicates" title="Link to this heading"></a></h2>
<section id="empty-1">
<span id="optional-0-empty-1"></span><span id="index-1"></span><h3><a class="toc-backref" href="#contents" role="doc-backlink"><code class="docutils literal notranslate"><span class="pre">empty/1</span></code></a><a class="headerlink" href="#empty-1" title="Link to this heading"></a></h3>
<p>Constructs an empty optional term.</p>
<div class="line-block">
<div class="line"><strong>Compilation flags:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">static</span></code></div>
</div>
</div>
<div class="line-block">
<div class="line"><strong>Template:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">empty(Optional)</span></code></div>
</div>
<div class="line"><strong>Mode and number of proofs:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">empty(--nonvar)</span></code> - <code class="docutils literal notranslate"><span class="pre">one</span></code></div>
</div>
</div>
<hr class="docutils" />
</section>
<section id="of-2">
<span id="optional-0-of-2"></span><span id="index-2"></span><h3><a class="toc-backref" href="#contents" role="doc-backlink"><code class="docutils literal notranslate"><span class="pre">of/2</span></code></a><a class="headerlink" href="#of-2" title="Link to this heading"></a></h3>
<p>Constructs an optional term holding the given value.</p>
<div class="line-block">
<div class="line"><strong>Compilation flags:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">static</span></code></div>
</div>
</div>
<div class="line-block">
<div class="line"><strong>Template:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">of(Value,Optional)</span></code></div>
</div>
<div class="line"><strong>Mode and number of proofs:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">of(&#64;term,--nonvar)</span></code> - <code class="docutils literal notranslate"><span class="pre">one</span></code></div>
</div>
</div>
<hr class="docutils" />
</section>
<section id="from-goal-3">
<span id="optional-0-from-goal-3"></span><span id="index-3"></span><h3><a class="toc-backref" href="#contents" role="doc-backlink"><code class="docutils literal notranslate"><span class="pre">from_goal/3</span></code></a><a class="headerlink" href="#from-goal-3" title="Link to this heading"></a></h3>
<p>Constructs an optional term holding a value bound by calling the given goal. Returns an empty optional term if the goal fails or throws an error.</p>
<div class="line-block">
<div class="line"><strong>Compilation flags:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">static</span></code></div>
</div>
</div>
<div class="line-block">
<div class="line"><strong>Template:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">from_goal(Goal,Value,Optional)</span></code></div>
</div>
<div class="line"><strong>Meta-predicate template:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">from_goal(0,*,*)</span></code></div>
</div>
<div class="line"><strong>Mode and number of proofs:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">from_goal(+callable,--term,--nonvar)</span></code> - <code class="docutils literal notranslate"><span class="pre">one</span></code></div>
</div>
</div>
<hr class="docutils" />
</section>
<section id="from-goal-2">
<span id="optional-0-from-goal-2"></span><span id="index-4"></span><h3><a class="toc-backref" href="#contents" role="doc-backlink"><code class="docutils literal notranslate"><span class="pre">from_goal/2</span></code></a><a class="headerlink" href="#from-goal-2" title="Link to this heading"></a></h3>
<p>Constructs an optional term holding a value bound by calling the given closure. Returns an empty optional term if the closure fails or throws an error.</p>
<div class="line-block">
<div class="line"><strong>Compilation flags:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">static</span></code></div>
</div>
</div>
<div class="line-block">
<div class="line"><strong>Template:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">from_goal(Closure,Optional)</span></code></div>
</div>
<div class="line"><strong>Meta-predicate template:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">from_goal(1,*)</span></code></div>
</div>
<div class="line"><strong>Mode and number of proofs:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">from_goal(+callable,--nonvar)</span></code> - <code class="docutils literal notranslate"><span class="pre">one</span></code></div>
</div>
</div>
<hr class="docutils" />
</section>
<section id="from-generator-3">
<span id="optional-0-from-generator-3"></span><span id="index-5"></span><h3><a class="toc-backref" href="#contents" role="doc-backlink"><code class="docutils literal notranslate"><span class="pre">from_generator/3</span></code></a><a class="headerlink" href="#from-generator-3" title="Link to this heading"></a></h3>
<p>Constructs optional terms with the values generated by calling the given goal. On goal error or failure, returns an empty optional.</p>
<div class="line-block">
<div class="line"><strong>Compilation flags:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">static</span></code></div>
</div>
</div>
<div class="line-block">
<div class="line"><strong>Template:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">from_generator(Goal,Value,Optional)</span></code></div>
</div>
<div class="line"><strong>Meta-predicate template:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">from_generator(0,*,*)</span></code></div>
</div>
<div class="line"><strong>Mode and number of proofs:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">from_generator(+callable,--term,--nonvar)</span></code> - <code class="docutils literal notranslate"><span class="pre">one_or_more</span></code></div>
</div>
</div>
<hr class="docutils" />
</section>
<section id="from-generator-2">
<span id="optional-0-from-generator-2"></span><span id="index-6"></span><h3><a class="toc-backref" href="#contents" role="doc-backlink"><code class="docutils literal notranslate"><span class="pre">from_generator/2</span></code></a><a class="headerlink" href="#from-generator-2" title="Link to this heading"></a></h3>
<p>Constructs optional terms with the values generated by calling the given closure. On closure error or failure, returns an empty optional.</p>
<div class="line-block">
<div class="line"><strong>Compilation flags:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">static</span></code></div>
</div>
</div>
<div class="line-block">
<div class="line"><strong>Template:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">from_generator(Closure,Optional)</span></code></div>
</div>
<div class="line"><strong>Meta-predicate template:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">from_generator(1,*)</span></code></div>
</div>
<div class="line"><strong>Mode and number of proofs:</strong></div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">from_generator(+from_generator,--nonvar)</span></code> - <code class="docutils literal notranslate"><span class="pre">one_or_more</span></code></div>
</div>
</div>
</section>
</section>
<hr class="docutils" />
<section id="protected-predicates">
<h2><a class="toc-backref" href="#contents" role="doc-backlink">Protected predicates</a><a class="headerlink" href="#protected-predicates" title="Link to this heading"></a></h2>
<p>(no local declarations; see entity ancestors if any)</p>
</section>
<section id="private-predicates">
<h2><a class="toc-backref" href="#contents" role="doc-backlink">Private predicates</a><a class="headerlink" href="#private-predicates" title="Link to this heading"></a></h2>
<p>(no local declarations; see entity ancestors if any)</p>
</section>
<section id="operators">
<h2><a class="toc-backref" href="#contents" role="doc-backlink">Operators</a><a class="headerlink" href="#operators" title="Link to this heading"></a></h2>
<p>(none)</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><a class="reference internal" href="optional_1.html#optional-1"><span class="std std-ref">optional(Optional)</span></a>, <a class="reference internal" href="type_0.html#type-0"><span class="std std-ref">type</span></a></p>
</div>
</section>
</section>


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