Is there a way to evaluate a polar equation?

I’ve figured out how to evaluate more complex equations (e.g. conics) in the Cartesian plane using parseEquation, differenceFunction, and evaluateAt as a way to check whether my students are writing the equation correctly. I think this works because when the variables my students use are “x” and “y” and I can easily identify them when I call the differenceFunction.

Is there a way to do something similar to this when writing polar equations (r = sin \theta")? From my attempts at following the similar steps I can seem to identify the “r” variable but the \theta is not being recognized.