[nly]

To have 'structs that implement various traits' you must first have structs. That is, objects with some known immutable shape. Implementing any kind of trait (interface) to an 'object' having a mutable structure (like the dictionaries used for modelling objects in most dynamic languages) would be tricky without exposing some crap to the programmer at the language level. Off the top of my head you'd need at least something like Perls 'bless'[0] coupled with Rusts strong protections against mutation.

Whether you choose to define your classes in the form of (possibly intersecting) sets of objects having some particular trait, or set of traits, or in some taxonomic tree is fairly irrelevant. The difference is largely a matter of taste. I think the point Bob (munificent) is making here is that 'scripting languages' (whatever you consider those to be) generally don't give you anything.

[0] http://perldoc.perl.org/functions/bless.html