It's frustratingly like that and not like that. For example, you cannot write a routine that says "get me the edges of this object generated by this other rout

It's frustratingly like that and not like that. For example, you cannot write a routine that says "get me the edges of this object generated by this other routine, and apply a 1mm fillet to them all". Because while the code manipulates geometry, the result is not geometry. The result is a 3D approximated picture of the geometry rendered in another form of geometry. To do anything like operate on result vertexes or edges you have to replicate the result geometry, by hand, in data structures in the code. So it is not just fillets as an operation that are missing. It is any notion of being able to access the edges that you would wish to fillet. This is why I think OpenSCAD does not "aid design" in the CAD sense. It does nothing but compile your wishes into a mesh. The most basic notion of "aiding design" ought to be exposing the result geometry of one operation so it can be used as the variable input for another. Is it good for quickly understanding basic geometry or displaying the results of formulae? Yeah. Undeniably so.