Structural typing doesn’t save you unless you add other stuff to it. Example: var o = ...; // Value comes from somewhere, doesn’t matter where so long as

Structural typing doesn’t save you unless you add other stuff to it. Example: var o = ...; // Value comes from somewhere, doesn’t matter where so long as it’s opaque if (p) ... = o.f else ... = o.g In dynamic typing, we only assert that o has f on the then, and only assert that o has g on the else. Structural typing or abstract base classes or whatever would only save you here if you were very careful about giving o some top type and then casting. But that’s more work - so the tendency is that the static types version of this program will assert that o has both f and g at the point of assignment. And that represents a tighter coupling to whatever produces o.