Excellent point, I honestly hadn't considered that. I've spent more time dealing with functional programming than in actual languages with immutable structures. Thanks for pointing that out.

I think my other point still stands, things like optional keys, or missing or required keys still must be shared throughout the app. Everything that touches that hash needs to know its structure.

Having to know the structure of a hash/map/dict is strictly superior to having to know the methods and fields of an object. For any given task the two are equivalent, except that one is easily modified/reused/passed to other callers without creating an extremely brittle class hierarchy you're only going to have to rearchitect when your requirements change. As an IBM researcher looking at Watson waaaay before Watson actually existed: "OOP requires you to treat all extrinsic information as if it were intrinsic to the object itself." This is tantamount to saying you must have a complete and consistent programme design before you write a single line. Which is crazy (and probably the reason UML exists.) But don't take my word for it, think it through, it's the only conclusion you can reach, I think.

Yeah, but that structure is also easy to explore and understand.

And in case you want some security you can use libraries like prismatic/schema, that allow you to declaratively describe your expected data-structures akin to a type system.

The web is build on json and not corba because plain maps and vectors are far easier to work with than domain specific objects.

There is a discussion of deep vs shallow hashes. Take Datomic or Datascript for instance. Very shallow datastructures, with a defined schema. Indexes to make this fast.

There’s tradeoffs. Shallow datastructures are under-used.

But OOP objects share fundamental flaws with deep hashes. (And add some more.)

Exactly; the whole idea of "deep dup" doesn't even exist in Clojure. It's a non-issue. Some languages go so far as to encourage actually serializing and then reading back in a data structure as a form of doing a deep duplication! (I've done this in Objective-C, based on advice in popular books.) This is an extreme example of the pitfalls of mutable data in many circumstances.