Hey! jqjq author here. Some time :) but surpassingly not that much, after getting over some major hurdles like how to mix recursive decent parsing and infix operators (precedence climbing is nice) and how to eval jq's call semantic that uses thunk/lambdas args it was mostly to grind through the rest. Actually string interpolation was quite tricky also. Also helped that i've worked on various jq other implementations and also written quite a lot of jq in general :)