It seems the goal is to make it easier to craft JSON by having curl perform escaping, while the proposal would seem to require some sort of in-memory tree representation of the data.

One alternative would be to provide escaping more directly like this:

    curl --json '{
      "map": %s,
      "prime": [
        %i,
        %i
      ],
      "target": {
        "x": %i,
        "y": %i
      }
    }' "$continent" "$p1" "$p2" "$x" "$y" https://example.com

And then curl would do the substitution with the appropriate type-specific escaping for each variable. This has a few nice properties:

1. What's on the command line resembles what's actually going to be sent.

2. Curl doesn't actually need to parse (nor validate) the JSON, or to create a tree representation of the data within itself. %s is invalid JSON anyway, so you can do a string substitution - all you need to keep track of are matching quotes (including escape sequences).

I've used a printf style format string here, which could be expanded for extra convenience. For example the Python-style `%(env_var)s` sequences could be used which could expand environment variables directly. Or something could be added for convenient handling of bash arrays.