encoding/json: no way to preserve the order of map keys

[lavalamp]

The encoding/json library apparently offers no way to decode data into a interface{} without scrambling the order of the keys in objects. Generally this isn't a correctness issue (per the spec the order isn't important), but it is super annoying to humans who may look at JSON data before and after it's been processed by a go program.

The most popular yaml library solves the problem like this: https://godoc.org/gopkg.in/yaml.v2#MapSlice

[ALTree]

This was asked in the past. Last week: #27050, and an old issue: #6244, where brad wrote:

JSON objects are unordered, just like Go maps. If you're depending on the order that a specific implementation serializes your JSON objects in, you have a bug.

While it's unlikely that the current behaviour of the json package will change, a proposal to add some new mechanism that preserves order (similar to the one in the yaml package) may be accepted. It would help, though, to have something more concrete to look at, i.e. a complete, full fledged proposal explaining what the new piece of API would look like.

[lavalamp]

My search failed to turn up either of those issues, thanks.

I tried to make it clear that this is not about correctness, it's about being nice to human readers. If that's not a relevant concern then we can close this. If there is some chance of a change being accepted then I can make a proposal.

[ALTree]

If there is some chance of a change being accepted then I can make a proposal.

I don't know if there is but the package owners can probably tell you: cc @rsc @dsnet @bradfitz

[mvdan]

There is a bit of precedent to making things nicer for human readers. For example, fmt sorts maps when printing them, to avoid random ordering in the output.

The fmt and json packages are different, and sorting keys isn't the same as keeping the original order, but I think the purpose is similar. Making the output easy to read or modify by humans.

I imagine there's no way to do this under the hood or by simply adding extra API like Encoder.SetIndent, as native maps simply don't keep the order.

Adding a type like MapSlice sounds fine to me - I'd say file a proposal unless someone else comes up with a better idea in the next few days. There's the question of whether the proposal will get past "publish this as a third-party package", but otherwise it seems like a sound idea.

[mvdan]

Similar proposal in the past - keeping the order of the headers in net/http: #24375

Just like in this case, the big question was where to store the actual order of the map keys.

[dsnet]

Strictly speaking, you can do this today using the raw tokenizer, that json.Decoder provides. That said, I think there will be comprehensive review of all json proposals and issues in the Go1.12 cycle. I can imagine solutions for this issue that also address problems of not being able to detect duplicate keys in objects.

[lavalamp]

I think the proposal would look like a flag (.SetPreserveOrder()) on the decoder that makes a MapSlice instead of a map[string]interface{}, plus making the encoder handle that type.

@dsnet Yeah, that solves the input half of the problem, but it's very inconvenient.

[dsnet]

Alternatively, it could output as [][2]interface{}, in which case you won't need to declare a new type in the json package.

[lavalamp]

I actually like that a lot. It should probably still be declared in the json package just for documentation purposes.

[rsc]

For now it seems like the best answer is a custom type (maybe a slice of pairs) with an UnmarshalJSON method that in turn uses the tokenizer.