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<a href="." >Convert error types</a>
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<h1 id="converting-error-types"><a name="converting-error-types" class="plugin-anchor" href="#converting-error-types"><i class="fa fa-link" aria-hidden="true"></i></a>Converting error types</h1>
<p>In some situations, values in some format must be contained inside of data in
some other format. For example an <a href="https://www.terraform.io/docs/providers/aws/r/iam_policy.html" target="_blank">IAM policy in Terraform</a> is represented as a
JSON string contained inside of an HCL config.</p>
<p>It would be simple to treat the inner value as just a String, but if we are
going to be manipulating both the inner and outer value it can often be
convenient to serialize and deserialize them all at once.</p>
<p>One occasional stumbling block in such situations is correctly handling errors.
The two formats may (probably do) have different Error types, so some conversion
is necessary.</p>
<p>This example shows a simplified HCL resource containing a simplified IAM policy.
The policy document is represented as a JSON string when serialized.</p>
<pre><code class="lang-rust"><span class="hljs-keyword">use</span> serde::{Serialize, Deserialize};
<span class="hljs-meta">#[derive(Serialize, Deserialize)]</span>
<span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">Resource</span></span> {
name: <span class="hljs-built_in">String</span>,
<span class="hljs-meta">#[serde(with = <span class="hljs-meta-string">"as_json_string"</span>)]</span>
policy: Policy,
}
<span class="hljs-meta">#[derive(Serialize, Deserialize)]</span>
<span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">Policy</span></span> {
effect: <span class="hljs-built_in">String</span>,
action: <span class="hljs-built_in">String</span>,
resource: <span class="hljs-built_in">String</span>,
}
<span class="hljs-comment">// Serialize and deserialize logic for dealing with nested values represented as</span>
<span class="hljs-comment">// JSON strings.</span>
<span class="hljs-keyword">mod</span> as_json_string {
<span class="hljs-keyword">use</span> serde_json;
<span class="hljs-keyword">use</span> serde::ser::{Serialize, Serializer};
<span class="hljs-keyword">use</span> serde::de::{Deserialize, DeserializeOwned, Deserializer};
<span class="hljs-comment">// Serialize to a JSON string, then serialize the string to the output</span>
<span class="hljs-comment">// format.</span>
<span class="hljs-keyword">pub</span> <span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">serialize</span></span><T, S>(value: &T, serializer: S) -> <span class="hljs-built_in">Result</span><S::<span class="hljs-literal">Ok</span>, S::Error>
<span class="hljs-keyword">where</span>
T: Serialize,
S: Serializer,
{
<span class="hljs-keyword">use</span> serde::ser::Error;
<span class="hljs-keyword">let</span> j = serde_json::to_string(value).map_err(Error::custom)?;
j.serialize(serializer)
}
<span class="hljs-comment">// Deserialize a string from the input format, then deserialize the content</span>
<span class="hljs-comment">// of that string as JSON.</span>
<span class="hljs-keyword">pub</span> <span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize</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: DeserializeOwned,
D: Deserializer<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-keyword">use</span> serde::de::Error;
<span class="hljs-keyword">let</span> j = <span class="hljs-built_in">String</span>::deserialize(deserializer)?;
serde_json::from_str(&j).map_err(Error::custom)
}
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">main</span></span>() {
<span class="hljs-keyword">let</span> resource = Resource {
name: <span class="hljs-string">"test_policy"</span>.to_owned(),
policy: Policy {
effect: <span class="hljs-string">"Allow"</span>.to_owned(),
action: <span class="hljs-string">"s3:ListBucket"</span>.to_owned(),
resource: <span class="hljs-string">"arn:aws:s3:::example_bucket"</span>.to_owned(),
},
};
<span class="hljs-keyword">let</span> y = serde_yaml::to_string(&resource).unwrap();
<span class="hljs-built_in">println!</span>(<span class="hljs-string">"{}"</span>, y);
}
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