Comment by jameshart

This feels like a pattern where the naming choices obscure the intent. The ‘Shape’ type doesn’t capture the essence of a ‘shape’ - it captures the essence of being ‘visitable by a visitor that knows about shapes’ or ‘able to handle shapes’. The things which are instances of Shape are functions that accept circles or rectangles, not actual circles or rectangles. So maybe call it ‘VisitableShape’, or ‘ShapeHandler’; and instead of calling its functions ‘circle’ and ‘rectangle’, call them ‘visitCircle’ or ‘handleCircle’...

Also, you seem to have created functions and types with the same names (Circle and Rectangle) which seems dangerous.