I like Google's Jsonnet [1], which has all of this except for 4. Jsonnet is quite mature, with fairly wide language adoption, and has the benefit of supporting

I like Google's Jsonnet [1], which has all of this except for 4. Jsonnet is quite mature, with fairly wide language adoption, and has the benefit of supporting expressions, including conditionals, arithmetic, as well as being able to define reusable blocks inside function definitions or external files. It's not suitable as a serialization format, but great for config. It's popular in some circles, but I'm sad that it has not reached wider adoption. [1] https://jsonnet.org/ https://jsonnet.org/