Wednesday, October 08, 2008


The back-in-the-day promise of XSL stylesheets was that you could output an XML document from a HTTP request, and have it processed as-is by a machine recipient, or transformed into a pretty HTML for human viewing automatically. 

Seeing how I (and probably many others) do prefer JSON to XML nowadays for my machine-to-machine communication, I'm thinking of an equivalent for JSON. In similar vein, XSL is almost entirely replaced by CSS today, and I keep thinking that we'd need an XSL or CSS equivalent for JSON.

Actually, we probably don't even need a new language for describing the transform. JavaScript would probably do just fine - read the JSON, build the DOM.

The other problem, assuming we already have the language for transforming JSON to pretty formatted HTML: how would you declare a transform in a JSON document?

In XML, you can use a processing instruction. In HTML, you can use the <link> element. Now, unlike XML's processing instructions, or HTML's <link> element, JSON notoriously lacks any support for any out-of-band information that isn't the actual content. Heck, it doesn't even support comments!

I can see three possible ways around it:
  1. Declare the transform in the transport header, i.e. just slap on an "X-JSON-Stylesheet" or similar header into the HTTP response. The obvious problem is that this ties the solution to the transport.
  2. Add it to a conventional location into the JSON data itself. I.e. have the top-level object literal have a "__metadata__" object, and put this and similar other information into it. i.e.:
    { "__metadata__": { "links": [{"url": "myStyleSheet.cssjs", "type": "stylesheet"}]} ... }
    The obvious problems are: a) the top-level value in a JSON document ain't necessarily an object (can be an array, or even a primitive value) b) some JSON based formats won't be happy about having extra elements with magic names inside of them.
  3. Extend JSON to add a syntactic equivalent of XML processing instructions. Honestly, I prefer this approach. Just adding single-line comment support, and then having a special prefix for the "processing instruction" comment (i.e. using a question mark, to keep it similar with XML PE) would do, I think:
    //?stylesheet myStyleSheet.cssjs
    { real payload goes here ... }
In any case, it is up to browser vendors to get the ball rolling on this - both the declaration mechanism, and the transform language.

I think this would be a great technology to have, where I could just output JSON from my web applications, and have it both consumed by software, and presented in nicely human readable form in browsers.

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