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Either string or struct
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<h1>
<i class="fa fa-circle-o-notch fa-spin"></i>
<a href="." >Either string or struct</a>
</h1>
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<section class="normal markdown-section">
<h1 id="deserialize-either-a-string-or-a-struct"><a name="deserialize-either-a-string-or-a-struct" class="plugin-anchor" href="#deserialize-either-a-string-or-a-struct"><i class="fa fa-link" aria-hidden="true"></i></a>Deserialize either a string or a struct</h1>
<p>The <a href="https://docs.docker.com/compose/compose-file/#/build" target="_blank"><code>docker-compose.yml</code></a> configuration file has a "build" key which can be
either a string or a struct.</p>
<pre><code class="lang-yaml"><span class="hljs-attr">build:</span> ./dir
<span class="hljs-comment"># --- or ---</span>
<span class="hljs-attr">build:</span>
<span class="hljs-attr"> context:</span> ./dir
<span class="hljs-attr"> dockerfile:</span> Dockerfile-alternate
<span class="hljs-attr"> args:</span>
<span class="hljs-attr"> buildno:</span> <span class="hljs-number">1</span>
</code></pre>
<p>The configuration file uses the same pattern in other places as well, typically
where a previously existing string field has been expanded to handle more
complex data.</p>
<p>We can use Rust's <a href="https://doc.rust-lang.org/std/str/trait.FromStr.html" target="_blank"><code>FromStr</code></a> trait and Serde's <code>deserialize_with</code> attribute to
handle this pattern in a general way.</p>
<pre><code class="lang-rust"><span class="hljs-keyword">use</span> std::collections::BTreeMap <span class="hljs-keyword">as</span> Map;
<span class="hljs-keyword">use</span> std::fmt;
<span class="hljs-keyword">use</span> std::marker::PhantomData;
<span class="hljs-keyword">use</span> std::str::FromStr;
<span class="hljs-keyword">use</span> serde::{Deserialize, Deserializer};
<span class="hljs-keyword">use</span> serde::de::{<span class="hljs-keyword">self</span>, Visitor, MapAccess};
<span class="hljs-keyword">use</span> void::Void;
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">main</span></span>() {
<span class="hljs-keyword">let</span> build_string = <span class="hljs-string">"
build: ./dir
"</span>;
<span class="hljs-keyword">let</span> service: Service = serde_yaml::from_str(build_string).unwrap();
<span class="hljs-comment">// context="./dir"</span>
<span class="hljs-comment">// dockerfile=None</span>
<span class="hljs-comment">// args={}</span>
<span class="hljs-built_in">println!</span>(<span class="hljs-string">"{:?}"</span>, service);
<span class="hljs-keyword">let</span> build_struct = <span class="hljs-string">"
build:
context: ./dir
dockerfile: Dockerfile-alternate
args:
buildno: '1'
"</span>;
<span class="hljs-keyword">let</span> service: Service = serde_yaml::from_str(build_struct).unwrap();
<span class="hljs-comment">// context="./dir"</span>
<span class="hljs-comment">// dockerfile=Some("Dockerfile-alternate")</span>
<span class="hljs-comment">// args={"buildno": "1"}</span>
<span class="hljs-built_in">println!</span>(<span class="hljs-string">"{:?}"</span>, service);
}
<span class="hljs-meta">#[derive(Debug, Deserialize)]</span>
<span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">Service</span></span> {
<span class="hljs-comment">// The `string_or_struct` function delegates deserialization to a type's</span>
<span class="hljs-comment">// `FromStr` impl if given a string, and to the type's `Deserialize` impl if</span>
<span class="hljs-comment">// given a struct. The function is generic over the field type T (here T is</span>
<span class="hljs-comment">// `Build`) so it can be reused for any field that implements both `FromStr`</span>
<span class="hljs-comment">// and `Deserialize`.</span>
<span class="hljs-meta">#[serde(deserialize_with = <span class="hljs-meta-string">"string_or_struct"</span>)]</span>
build: Build,
}
<span class="hljs-meta">#[derive(Debug, Deserialize)]</span>
<span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">Build</span></span> {
<span class="hljs-comment">// This is the only required field.</span>
context: <span class="hljs-built_in">String</span>,
dockerfile: <span class="hljs-built_in">Option</span><<span class="hljs-built_in">String</span>>,
<span class="hljs-comment">// When `args` is not present in the input, this attribute tells Serde to</span>
<span class="hljs-comment">// use `Default::default()` which in this case is an empty map. See the</span>
<span class="hljs-comment">// "default value for a field" example for more about `#[serde(default)]`.</span>
<span class="hljs-meta">#[serde(default)]</span>
args: Map<<span class="hljs-built_in">String</span>, <span class="hljs-built_in">String</span>>,
}
<span class="hljs-comment">// The `string_or_struct` function uses this impl to instantiate a `Build` if</span>
<span class="hljs-comment">// the input file contains a string and not a struct. According to the</span>
<span class="hljs-comment">// docker-compose.yml documentation, a string by itself represents a `Build`</span>
<span class="hljs-comment">// with just the `context` field set.</span>
<span class="hljs-comment">//</span>
<span class="hljs-comment">// > `build` can be specified either as a string containing a path to the build</span>
<span class="hljs-comment">// > context, or an object with the path specified under context and optionally</span>
<span class="hljs-comment">// > dockerfile and args.</span>
<span class="hljs-keyword">impl</span> FromStr <span class="hljs-keyword">for</span> Build {
<span class="hljs-comment">// This implementation of `from_str` can never fail, so use the impossible</span>
<span class="hljs-comment">// `Void` type as the error type.</span>
<span class="hljs-class"><span class="hljs-keyword">type</span> <span class="hljs-title">Err</span></span> = Void;
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">from_str</span></span>(s: &<span class="hljs-keyword">str</span>) -> <span class="hljs-built_in">Result</span><<span class="hljs-keyword">Self</span>, Self::<span class="hljs-literal">Err</span>> {
<span class="hljs-literal">Ok</span>(Build {
context: s.to_string(),
dockerfile: <span class="hljs-literal">None</span>,
args: Map::new(),
})
}
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">string_or_struct</span></span><<span class="hljs-symbol">'de</span>, T, D>(deserializer: D) -> <span class="hljs-built_in">Result</span><T, D::Error>
<span class="hljs-keyword">where</span>
T: Deserialize<<span class="hljs-symbol">'de</span>> + FromStr<<span class="hljs-literal">Err</span> = Void>,
D: Deserializer<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-comment">// This is a Visitor that forwards string types to T's `FromStr` impl and</span>
<span class="hljs-comment">// forwards map types to T's `Deserialize` impl. The `PhantomData` is to</span>
<span class="hljs-comment">// keep the compiler from complaining about T being an unused generic type</span>
<span class="hljs-comment">// parameter. We need T in order to know the Value type for the Visitor</span>
<span class="hljs-comment">// impl.</span>
<span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">StringOrStruct</span></span><T>(PhantomData<<span class="hljs-function"><span class="hljs-keyword">fn</span></span>() -> T>);
<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> StringOrStruct<T>
<span class="hljs-keyword">where</span>
T: Deserialize<<span class="hljs-symbol">'de</span>> + FromStr<<span class="hljs-literal">Err</span> = Void>,
{
<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> {
formatter.write_str(<span class="hljs-string">"string or map"</span>)
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">visit_str</span></span><E>(<span class="hljs-keyword">self</span>, value: &<span class="hljs-keyword">str</span>) -> <span class="hljs-built_in">Result</span><T, E>
<span class="hljs-keyword">where</span>
E: de::Error,
{
<span class="hljs-literal">Ok</span>(FromStr::from_str(value).unwrap())
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">visit_map</span></span><M>(<span class="hljs-keyword">self</span>, map: M) -> <span class="hljs-built_in">Result</span><T, M::Error>
<span class="hljs-keyword">where</span>
M: MapAccess<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-comment">// `MapAccessDeserializer` is a wrapper that turns a `MapAccess`</span>
<span class="hljs-comment">// into a `Deserializer`, allowing it to be used as the input to T's</span>
<span class="hljs-comment">// `Deserialize` implementation. T then deserializes itself using</span>
<span class="hljs-comment">// the entries from the map visitor.</span>
Deserialize::deserialize(de::value::MapAccessDeserializer::new(map))
}
}
deserializer.deserialize_any(StringOrStruct(PhantomData))
}
</code></pre>
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