pwpwp's example is just the trivial case, the point is the change happens outside the usage callstack. Structural typing can fit a lot of the issues your paren

pwpwp's example is just the trivial case, the point is the change happens outside the usage callstack. Structural typing can fit a lot of the issues your parent post raised as well. Simply type check on what the function using the value cares about, not that it belongs to same name.