What is a type error?

[Andreas Rossberg]

Darren New wrote:
> Andreas Rossberg wrote:
> 
>> Yes, technically you are right. But this makes a pretty weak notion of 
>> mutability. All stateful data structures had to stay within their 
>> lexical scope, and could never be passed to a function. 
> 
> Not really. The way Hermes handles this is with destructive assignment. 
> Each variable holds a value, and you (almost) cannot have multiple 
> variables referring to the same value.

OK, this is interesting. I don't know Hermes, is this sort of like a 
dynamically checked equivalent of linear or uniqueness typing?

> If you want to assign Y to X, you use
>    X := Y
> after which Y is considered to be uninitialized. If you want X and Y to 
> have the same value, you use
>    X := copy of Y
> after which X and Y have the same value but are otherwise unrelated, and 
> changes to one don't affect the other.

Mh, but if I understand correctly, this seems to require performing a 
deep copy - which is well-known to be problematic, and particularly 
breaks all kinds of abstractions. Not to mention the issue with 
uninitialized variables that I would expect occuring all over the place. 
So unless I'm misunderstanding something, this feels like trading one 
evil for an even greater one.

- Andreas