JSON is a common data format, especially when interfacing with non-Clojure systems. All keys in JSON objects are strings (not keywords, as is common in Clojure).

It is desirable to be able to validate incoming JSON data and provide helpful error messages when data is poorly formed. Spec is an excellent tool for both, but `s/keys` only works with keyword keys.

It would be useful to be able to specify string keys, for instance, given some JSON data like

I would like to write a spec like:

where `:req-str` is like `:req` and `:req-un-str` would be like `:req-un`. The specs would still be fully-qualified keywords.

The current workaround:

1. Convert string keys to keyword keys using `clojure.walk/keywordize-keys`
2. Validate with spec
3. If there are problems, map over the problems and use `stringify-keys` on each val
4. Format the problems appropriately (basically, reproduce the formatting of `explain`).

This workaround is not particularly difficult, but since I suspect working with JSON is a common case, it may be useful to support this use case more directly.