[duckerude]

PHP's arrays are a bit like this. You can treat them either like an (ordered) hashmap or like a growable array. The underlying representation will adapt depending on whether the current state can be represented as an array.

And I hate it! I'm always unsure about an array's contents. I write functions that expect to operate on linear arrays and might misbehave if given a map, or vice versa. It's rare for the conflation to be helpful. Arrays and maps have different meanings and are used in different ways.

Maybe a very mild version of this would be helpful, like automatically converting an array to an array-backed deque—though even that case seems better handled by a linter making the suggestion, so the programmer can make an informed decision.

Going all the way would be terrible. Types are helpful. Even in a dynamically typed language they give you a vocabulary.