<p>Hi all,
I'd like to en/de/code JSON with Golang which may have multiple types in a given field, something like:</p>
<pre><code>[{"type": "number", "value": 15}, {"type": "string", "value": "Hello!"}]
</code></pre>
<p>The "value" field can be string, number, boolean, etc: how to handle this?</p>
<p><em>edit: I'm grateful for suggestions to use <code>interface{}</code>, but that's a despicable antipattern I'd really rather avoid. Is there any way to achieve this using multiple type-safe fields and JSON annotations to let more than one field map to the same field-name when encoding and decoding?</em></p>
<p><em>edit 2: To be more specific, is there some way to do this (except that this doesn't work)?:</em></p>
<pre><code>type FooStruct struct{
Type string
StringValue `json:"value"`
IntValue `json:"value"`
FloatValue `json:"value"`
BooleanValue `json:"value"`
}
</code></pre>
<hr/>**评论：**<br/><br/>ilikeorangutans: <pre><p>Take a look at <a href="http://golang.org/pkg/encoding/json/#RawMessage" rel="nofollow">RawMessage</a> which lets you delay parsing of parts of the JSON. The example in the docs does pretty much what you are looking for. </p></pre>cathalgarvey: <pre><p>This might be my answer, thank you! Needs some experimentation to see how to implement the encoding end robustly but this is a clean-seeming way to handle decoding.</p></pre>djherbis: <pre><p>This is a little verbose, but it does what you want.
<a href="https://play.golang.org/p/ZXudlXBdty" rel="nofollow">https://play.golang.org/p/ZXudlXBdty</a></p></pre>cathalgarvey: <pre><p>Thank you! This answers my question as to whether the JSON tagging system would suffice, and goes further to provide a working solution. Really cool, thanks.</p></pre>anhlh2: <pre><p>I have similar problem but the types and data of possible records are a bit more complex. I found <a href="https://github.com/mitchellh/mapstructure" rel="nofollow">mapstructure</a> and I like it, here is the solution to your question using mapstructure: <a href="http://pastebin.com/5Wbzk4ZB" rel="nofollow">http://pastebin.com/5Wbzk4ZB</a> </p></pre>IntellectualReserve: <pre><p>I haven't found a library I like for this yet. I'm thinking about writing one. </p></pre>jerf: <pre><p>Responding to your subsequent edits, <code>interface{}</code> in this case isn't the despicable anti-pattern... it's just a request for <code>encoding/json</code> to use what it uses for interface{}. It isn't truly "unrestricted".</p>
<p>And for edit 2: No. <code>encoding/json</code> is a simple library. It has many weaknesses if you stress it, and you'll have to go find a more complicated library. Among its weaknesses is that it shares something that is true of a lot of static languages with simple JSON libraries, which is that if your JSON is <em>not</em> implicitly a statically-typed document, it has a hard time dealing with that. You'll end up having to do some manual work.</p></pre>drwiggly: <pre><p>Made a small wrapper that decodes with raw message and allows path access to the structure of the doc.</p>
<p><a href="https://github.com/chrhlnd/dynjson" rel="nofollow">https://github.com/chrhlnd/dynjson</a></p>
<p>That being said depending on what you're doing just decoding with 2 struct types may just be easier.</p>
<p><a href="http://play.golang.org/p/9bPhTY66Gr" rel="nofollow">http://play.golang.org/p/9bPhTY66Gr</a></p></pre>cathalgarvey: <pre><p>Using different types for each value and partially decoding the type hint is actually the cleanest way I've yet seen, thanks. It's a pity things like this either impose convoluted method like this and a runtime cost, or simple, dangerous and <em>still</em> runtime costly methods like <code>interface{}</code>…</p></pre>JHunz: <pre><p>The struct you decode into should define the value field as an interface{}. Then once you determine the type via the other field, do a runtime cast into a new variable of the appropriate type. </p>
<p>switch decoded.Type {<br/>
case "number":<br/>
intValue, ok := decoded.Value.(int)<br/>
if !ok {<br/>
// Error handling...<br/>
}<br/>
...</p></pre>
