What is a type error?

[Andreas Rossberg]

Marshall wrote:
> 
> However if the mutable types are not first class, then there
> is no way to have the aliasing. Thus, if we do not have pointers
> or objects or identity but retain mutability, there is no aliasing
> problem.

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. For example, 
this essentially precludes object-oriented programming, because you 
could not have objects with state (the alternative, second class 
objects, would be even less "objective").

Generally, second-classness is an ad-hoc restriction that can work 
around all kinds of problems, but rarely with satisfactory results. So I 
would tend to say that this is not an overly interesting point in the 
design space. But YMMV.

>>In other words, pointers are essentially just an *aspect* of mutability
>>in lower-level languages.
> 
> Again, I disagree: it is posible to have mutability without
> pointers/identity/objects.

OK, if you prefer: it is an aspect of first-class mutability - which is 
present in almost all imperative languages I know. :-)

- Andreas

-- 
Andreas Rossberg, rossberg at ps.uni-sb.de