The general idea is to define a surface with a function that goes from a u,v pair, to a 3d vector representing the position of the surface for the given u and v.
The function is written in ISL, and the surface is triangulated at scene load time.
I'm not sure how useful it will be for everyone, but it's certainly fun for me

It can be pretty tricky to get correct shapes.
The shell render uses a parametric surface for the geometry, and an ISL shader for the albedo. So it's basically pure shaders!