For those interested, I ended up in rolling my own parser since there wasn’t a grammer file to be found. It seems the original parser is handed coded.
I read up on leex and yecc and came up with these two jsonata_leex and jsonata_yecc which sort of do the job, at least they cover most of my use cases.
What I decided on was to convert JSONata code to Erlang code and the Erlang code is evaluated in the presence of a msg object/map. In the long term all JSONata stanzas would be precompiled upon deployment and then directly executed on the message objects - hence the conversion to Erlang.
Of course I am nowhere near covering all possibilities of JSONata (my parser can’t even add 1 plus 1 - forgot I needed that use-case) but at least there is a start!