Thank you for your interest!

I have implemented everything from scratch using bare C++, it was a lots of fun and very educational experience.

The challenges related to creation of a new programming language really depend on what kind of language you want to implement, i.e.: compiled vs interpreted, imperative vs purely functional, static vs dynamic typing, so on and so forth. All those choices bring its separate set of challenges.

In case of Huginn the most difficult part was to understand responsibilities of consecutive interpretation stages, i.e.: preprocessing, parsing, "compiling", execution. Other challenge was the design of internal data structures that represent/handle Huginn programs run time state.

I can go into more details if you are interested.

-A.