As a student project, I once prototyped a Soulver-like interface that used Frink for its calculations: https://github.com/roryokane/CalcuDoc/wiki/About-CalcuDoc. You can see screenshots and get the code from that link.

I didn’t continue working on it after the class finished, though – I lost all interest after I realized that the Frink programming language, despite being free, is not open source.