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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">&quot;as_json_string&quot;</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>&lt;T, S&gt;(value: &amp;T, serializer: S) -&gt; <span class="hljs-built_in">Result</span>&lt;S::<span class="hljs-literal">Ok</span>, S::Error&gt;
    <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>&lt;<span class="hljs-symbol">&apos;de</span>, T, D&gt;(deserializer: D) -&gt; <span class="hljs-built_in">Result</span>&lt;T, D::Error&gt;
    <span class="hljs-keyword">where</span>
        T: DeserializeOwned,
        D: Deserializer&lt;<span class="hljs-symbol">&apos;de</span>&gt;,
    {
        <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(&amp;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">&quot;test_policy&quot;</span>.to_owned(),
        policy: Policy {
            effect: <span class="hljs-string">&quot;Allow&quot;</span>.to_owned(),
            action: <span class="hljs-string">&quot;s3:ListBucket&quot;</span>.to_owned(),
            resource: <span class="hljs-string">&quot;arn:aws:s3:::example_bucket&quot;</span>.to_owned(),
        },
    };

    <span class="hljs-keyword">let</span> y = serde_yaml::to_string(&amp;resource).unwrap();
    <span class="hljs-built_in">println!</span>(<span class="hljs-string">&quot;{}&quot;</span>, y);
}
</code></pre>

                                
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