FSet v2.2.0, which is the version included in the recent Quicklisp release, has a new Quicklisp-loadable system, FSet/Jzon. It extends the Jzon JSON parser/printer to construct FSet collections when reading, and to be able to print them.
On parsing, JSON arrays produce FSet seqs; JSON objects produce FSet replay maps by default, but the parser can also be configured to produce ordinary maps or FSet tuples. For printing, any of these can be handled, as well as the standard Jzon types. The tuple representation provides a way to control the printing of `nil`, depending on the type of the corresponding key.
For details, see the GitLab MR.
NOTE: unfortunately, the v2.1.0 release had some bugs in the new seq code, and I didn't notice them until after v2.2.0 was in Quicklisp. If you're using seqs, I strongly recommend you pick up v2.2.2 or newer from GitLab or GitHub.
