Mustache: Render Map as JSON

[tlrx]

This commit changes Mustache script engine so that every map is rendered as a JSON string (instead of default Map.toString())

This pull request adds two util functions to the Mustache templating engine:

• {{#toJson}}my_map{{/toJson}} to render a Map parameter as a JSON string
• {{#join}}my_iterable{{/join}} to render any iterable (including arrays) as a comma separated list of values like 1, 2, 3. It's also possible de change the default delimiter (comma) to something else.

closes #18970

[rjernst]

I don't think this can ever result in a ClassCastException because of type erasure.

[tlrx]

I updated the code to make it work with the {{#toJson}}ctx.payload.someMap{{/toJson}} syntax. I also added a {{#join}}ctx.payload.someIterable{{/join}} to render things like comma separated values.

It would be great if you can review and test it.

@rjernst Thanks for your review! The pull request was marked as work in progress and I changed the whole implementation after discussing with @spinscale. I took your comment into consideration, I'd be glad if you find the time to review again.

[rjernst]

I have no opinion on the technical side of this change. I'm happy this allows rendering json, but I also think the syntax looks funky based on other templating languages I have used in the past. I would have expected something simpler like {{ctx.payload.someMap.asJson}} or {{asJson(ctx.payload.someMap)}}

[spinscale]

does this make sense? is that valid JSON?

[tlrx]

Not really. Let's change that so that it renders "John Smith".

[spinscale]

I had a quick look but did not find any possibility for having arguments to configure the string joiner char instead of hardcoding it as a comma, that would be nice.

Apart from that there needs to be some documentation in the search template docs for those functions

[tlrx]

Thanks @spinscale for the review. I updated the code, changing few things around and added the possibility to set a custom delimiter in the join function. Please let me know what you think.

I have no opinion on the technical side of this change. I'm happy this allows rendering json, but I also think the syntax looks funky based on other templating languages I have used in the past. I would have expected something simpler like {{ctx.payload.someMap.asJson}} or {{asJson(ctx.payload.someMap)}}

@rjernst Yes, I don't have a strong feeling here. I agreed with you when I discussed with @spinscale about implementing a toJSON() (you called it asJson) method on arrays and maps.

But the Mustache specs also propose wrapped functions section with {{#wrapped}}...{{/wrapped}} so I think having {{#toJson}}...{{/toJson}} or {{#join}}...{{/join}} syntax is OK. It also has the side effect to allow things like {{#join}}{{#toJson}}terms{{/toJson}}{{/join}} which would be more difficult to express using the other syntax.

[spinscale]

return JSON_CONTENT_TYPE.equals(params.getOrDefault(CONTENT_TYPE_PARAM, JSON_CONTENT_TYPE))?

[tlrx]

Sure, this switch statement is really ugly. I suppose my brain was in auto-pilot mode when writing this.

[spinscale]

left two minor comments. I like it, very helpful for mustache users!

LGTM

[tlrx]

Thanks @spinscale