Here's a quick gist that does what I think you want, modulo any specific error handling. https://gist.github.com/usernamefornow/14f972df9429146b46846...

edit: note that the cleanest way would probably to write your own converter so you could avoid the union type, but I didn't want to put that in and have to write a comment about it.

Makes sense, thanks. For some reason, I was set on parsing the "env" values into instances of the same class, regardless of what type they were in JSON, but it's not really necessary. I'll also look into writing custom converters, because I'll likely need that in the future.

You can pass union types to the macro, so that should handle your "varying types" problem. In terms of nesting, I just create another struct with its own JSON.mapping and use that as the type for the field.