Hi Todd,
In CRAM (a Common Lisp robot programming framework) we are using json_prolog to talk from SBCL to SWI Prolog.
Is the json-prolog call somehow using the normal service and messaging infrastructure of ROS or is it something different?
It is based on the ROS service mechanism, for your case of querying rosprolog from Lisp, json-prolog would help you register a rosprolog ROS service that can answer queries over ROS and send the results back to Lisp. The query and the result are encoded in JSON format. Nothing fancy.
Does ROS even support these two running on different machines at all?
Not sure if I understood your question correctly but if you're asking if you can run a ROS service server on one machine and query it from another machine, yes, sure, just make sure you set up your $ROS_MASTER_URI (and eventually $ROS_IP) entries right.
Should I just have a swank client running local to the prolog?
On the Lisp side we have a cram_json_prolog package which contains a number of utility functions to call the service using the persistent service API, take a look here: https://github.com/cram2/cram_json_pr... . We use lazy lists to represent the result of the Prolog query as it can have an infinite length, so this package has a dependency on two small CRAM packages from the cram_core repository, which implement lazy lists for Common Lisp etc..
You would then call the service from Lisp like this:
(json-prolog:prolog
`(and ("some_swi_predicate" "some_known_binding_as_string" ?some_resulting_binding)
("other_swi_prolog_predicate" 123 ?another_resulting_binding)))
and the variables ?some_resulting_binding and ?another_resulting_binding will have the values that you are interested in in your query.
Unfortunately, I cannot point you to any self-contained documentation about `json_prolog, as the packages are themselves not self-contained and depend on other parts of our systems. If you like the approach and would like to know more feel free to edit your question / comment or drop me an email at gaya [at] cs [dot] uni-bremen [dot] de or open an issue on the corresponding Github repo.
