It has taken me about 90 minutes to write an initial FIQL to LDAP converter which outputs the query formatted according to RFC 4515 and document it here.
I have copied most of the boiler-plate code from the FIQL to SQL converter and given how simple the LDAP query is it was very easy to finish it off fast. The point is that other custom converters (example FIQL to CQL) can likely be written even faster by copying and pasting the LDAP converter :-).
More work will likely need to be done to get RFC-4515 supported completely, but the converter should already be capable to support many simple or composite LDAP queries which do not use complex characters, etc.
You may want to ask why would not one support LDAP query language by directly encoding it into request URIs. There are two issues here, first, the LDAP language is not quite URI friendly, the second one is that it will leak the details about the data technology supporting a given REST endpoint.
This is why FIQL or other dedicated language like OData is the way to go for doing medium complexity queries over HTTP. It is easy to read and it lets the users completely encapsulate the data source details.
I think with this latest update CXF Search extension has enough 'material' for users to start experimenting with using FIQL on top of the well-known and traditional data sources.
I can nearly see some of you starting thinking of creating some cool UI search module which will let users do more interesting search queries over the containers such as LDAP stores or OSGI runtimes with this new converter and may be even sharing the tested queries with the other users who may find it useful.
The next phase is to get FIQL a bit closer to all those No SQL data stores and it should be quite interesting.
Watch this space :-)