thoughts on immutability in D

[Rainer Schuetze]

On 25.09.2011 04:25, Peter Alexander wrote:
> On 24/09/11 4:47 PM, Rainer Schuetze wrote:
>> Sorry for the bad example, here's one that is not thread-safe, and where
>> the invariant can fail in a multi-threaded environment:
>>
>> shared(int) globId;
>>
>> class C
>> {
>> invariant() { assert((globId & 1) == 0); }
>> @property int id() immutable
>> {
>> globId = globId + 1;
>> globId = globId + 1;
>> return globId;
>> }
>> }
>>
>> But the details whether the operation happens to be thread safe or not
>> is irrelevant, thread-safety is not guaranteed by the type system with
>> immutable.
>
> It's still thread safe! I think you are misunderstanding what thread
> safety is.
>
> http://en.wikipedia.org/wiki/Thread_safety
>
> You are right in saying that the invariant may break, but that doesn't
> prove that the code isn't thread safe, it just proves that you have a
> bad invariant. You may as well have written:
>
> invariant() { assert(false); }
>
> and then claimed that the code wasn't thread safe because immutable
> didn't save you.

Grrr, another bad example, where some bad side-effects did not get 
unnoticed ;-)
>
> Thread safety in your example simply means that calling id() 10 times
> from different threads will always result in globId == 20 at the end.
> This isn't guaranteed by many languages.
>

This is what I wanted to show: it is also not guaranteed in D. Checking 
the disassembly there is nothing atomic about the operations. Even 
++globId does not use a locked increment.

> Thread safety does *not* guarantee that your invariant will hold,
> because your invariant not only requires thread safety but mutually
> exclusive access to globId around calls to id().
>
> I should point out that immutable has absolutely no bearing in your
> example. Having immutable methods in a class with no members means
> nothing! The thread-safety comes from the shared(int) here.
>

Which finally brings us back to the original question: What does 
immutable guarantee in the face of non-pure property getter functions?