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<a href="." >Discarding data</a>
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<h1 id="discarding-data"><a name="discarding-data" class="plugin-anchor" href="#discarding-data"><i class="fa fa-link" aria-hidden="true"></i></a>Discarding data</h1>
<p>The <a href="https://docs.rs/serde/1/serde/de/struct.IgnoredAny.html" target="_blank"><code>IgnoredAny</code></a> type gives an efficient way of discarding data from a
deserializer.</p>
<p>Think of this like <code>serde_json::Value</code> in that it can be deserialized from
any type, except that it does not store any information about the data that
gets deserialized.</p>
<pre><code class="lang-rust"><span class="hljs-keyword">use</span> std::fmt;
<span class="hljs-keyword">use</span> std::marker::PhantomData;
<span class="hljs-keyword">use</span> serde::de::{
<span class="hljs-keyword">self</span>, Deserialize, DeserializeSeed, Deserializer, Visitor, SeqAccess,
IgnoredAny,
};
<span class="hljs-keyword">use</span> serde_json::json;
<span class="hljs-comment">// A seed that can be used to deserialize only the `n`th element of a sequence</span>
<span class="hljs-comment">// while efficiently discarding elements of any type before or after index `n`.</span>
<span class="hljs-comment">//</span>
<span class="hljs-comment">// For example to deserialize only the element at index 3:</span>
<span class="hljs-comment">//</span>
<span class="hljs-comment">// NthElement::new(3).deserialize(deserializer)</span>
<span class="hljs-keyword">pub</span> <span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">NthElement</span></span><T> {
n: usize,
marker: PhantomData<<span class="hljs-function"><span class="hljs-keyword">fn</span></span>() -> T>,
}
<span class="hljs-keyword">impl</span><T> NthElement<T> {
<span class="hljs-keyword">pub</span> <span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">new</span></span>(n: usize) -> <span class="hljs-keyword">Self</span> {
NthElement {
n: n,
marker: PhantomData,
}
}
}
<span class="hljs-keyword">impl</span><<span class="hljs-symbol">'de</span>, T> Visitor<<span class="hljs-symbol">'de</span>> <span class="hljs-keyword">for</span> NthElement<T>
<span class="hljs-keyword">where</span>
T: Deserialize<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-class"><span class="hljs-keyword">type</span> <span class="hljs-title">Value</span></span> = T;
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">expecting</span></span>(&<span class="hljs-keyword">self</span>, formatter: &<span class="hljs-keyword">mut</span> fmt::Formatter) -> fmt::<span class="hljs-built_in">Result</span> {
<span class="hljs-built_in">write!</span>(formatter, <span class="hljs-string">"a sequence in which we care about element {}"</span>, <span class="hljs-keyword">self</span>.n)
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">visit_seq</span></span><V>(<span class="hljs-keyword">self</span>, <span class="hljs-keyword">mut</span> seq: V) -> <span class="hljs-built_in">Result</span><Self::Value, V::Error>
<span class="hljs-keyword">where</span>
V: SeqAccess<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-comment">// Skip over the first `n` elements.</span>
<span class="hljs-keyword">for</span> i <span class="hljs-keyword">in</span> <span class="hljs-number">0</span>..<span class="hljs-keyword">self</span>.n {
<span class="hljs-comment">// It is an error if the sequence ends before we get to element `n`.</span>
<span class="hljs-keyword">if</span> seq.next_element::<IgnoredAny>()?.is_none() {
<span class="hljs-keyword">return</span> <span class="hljs-literal">Err</span>(de::Error::invalid_length(i, &<span class="hljs-keyword">self</span>));
}
}
<span class="hljs-comment">// Deserialize the one we care about.</span>
<span class="hljs-keyword">let</span> nth = seq.next_element()?
.ok_or_else(|| de::Error::invalid_length(<span class="hljs-keyword">self</span>.n, &<span class="hljs-keyword">self</span>))?;
<span class="hljs-comment">// Skip over any remaining elements in the sequence after `n`.</span>
<span class="hljs-keyword">while</span> <span class="hljs-keyword">let</span> <span class="hljs-literal">Some</span>(IgnoredAny) = seq.next_element()? {
<span class="hljs-comment">// ignore</span>
}
<span class="hljs-literal">Ok</span>(nth)
}
}
<span class="hljs-keyword">impl</span><<span class="hljs-symbol">'de</span>, T> DeserializeSeed<<span class="hljs-symbol">'de</span>> <span class="hljs-keyword">for</span> NthElement<T>
<span class="hljs-keyword">where</span>
T: Deserialize<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-class"><span class="hljs-keyword">type</span> <span class="hljs-title">Value</span></span> = T;
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize</span></span><D>(<span class="hljs-keyword">self</span>, deserializer: D) -> <span class="hljs-built_in">Result</span><Self::Value, D::Error>
<span class="hljs-keyword">where</span>
D: Deserializer<<span class="hljs-symbol">'de</span>>,
{
deserializer.deserialize_seq(<span class="hljs-keyword">self</span>)
}
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">main</span></span>() {
<span class="hljs-keyword">let</span> array = json!([<span class="hljs-string">"a"</span>, <span class="hljs-string">"b"</span>, <span class="hljs-string">"c"</span>, <span class="hljs-string">"d"</span>, <span class="hljs-string">"e"</span>]);
<span class="hljs-keyword">let</span> nth: <span class="hljs-built_in">String</span> = NthElement::new(<span class="hljs-number">3</span>).deserialize(&array).unwrap();
<span class="hljs-built_in">println!</span>(<span class="hljs-string">"{}"</span>, nth);
<span class="hljs-built_in">assert_eq!</span>(nth, array[<span class="hljs-number">3</span>]);
}
</code></pre>
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