[smallpaul]

I don't want that in my json/yaml.

If I have Javascript then I already have mobile code.

If I'm using json in a situation where I don't have a Javascript interpreter, then I probably don't have a Lisp interpreter around either.

[failrate]

A decent JSON parser is also only a day's worth of work or less in just about any language.

My experience is that it is as little as and possibly less initial development and maintenance as a custom flat-file format or a really robust csv parser.