That's one side of the expression problem; the other is adding a new operation. With dynamic dispatch, you can add new shapes without altering the others, but i

That's one side of the expression problem; the other is adding a new operation. With dynamic dispatch, you can add new shapes without altering the others, but if you want to add a new operation (e.g., perimeter()) then you have to modify the base class and all the children. With discriminated unions, adding a new shape requires modifying all the operations, but adding a new operation only requires the creation of a new function.