> I guess it's turtles all the way down, because (from my pov at least) deciding on what "structures a type must implement" is pretty similar to saying "a table is a table only if it's got 4 legs".
The idea is that callers/clients define the structure they’re going to use. If the caller needs something that can hold food, then only things that hold food can be passed in. If the caller depends on having four legs, then only things with four legs can be passed in. If the caller depends on four legs and can hold food, only objects that satisfy both properties can be passed in. The type doesn’t choose which structures it implements (that’s nominal typing) the type just is and it either implements a contract / has the right structure for a given client or it doesn’t. Check out Go’s interfaces for a more concrete example.
> Again, from my pov, a dynamic-like language is a lot more tolerant with the unknown unknowns of which the open world is full of, it doesn't depend on any "structure implementation" being defined or on anyone else counting the legs of said table.
This is inherently untrue because structural typing is simply the formalization of the same logic you use when composing programs in a dynamic language. The same logic that lets Python document an argument as “file-like” is formally expressed in a static language as “anything with methods read(), write(), seek(), close(), etc”. This still gives you the openness you need (you can pass in other types even if they don’t know about your file-like interface) but it guards against runtime TypeErrors and AttributeErrors.
Give static typing an earnest try. And be discerning about the source of friction you encounter: is it because static typing is making it harder for you to write big-prone code? Is it because the language makes heavy use of inheritance (and nothing to do with static typing at all)? Is it because the language is very verbose (and nothing to do with static typing at all)? Or maybe the language has a bizarre syntax and jargon-laden, conflicting documentation, in which case these also aren’t properties of type systems but rather functional languages ;).