Some missing things in the current threading implementation

[Robert Jacques]

On Sun, 12 Sep 2010 09:35:42 -0400, Sönke Ludwig  
<ludwig at informatik.uni-luebeck.de> wrote:
> 9. unique
>
> 	Unique objects or chunks of data are really important not only to be  
> able to check that a cast to 'immutable' is correct, but also to allow  
> for passing objects to another thread for computations without making a  
> superfluous copy or doing superfluous computation.

Unique (or for those with an Occam background 'mobile') has several  
proponents in the D community (myself included). It was seriously  
considered for inclusion in the type system, but Walter found several  
issues with it on a practical level. If I recall correctly, Walter's exact  
issues weren't made public, but probably stem from the fact that  
unique/mobile types in other languages are generally library defined and  
are 'shallow'. They exist as a 'please use responsibly' / 'here be  
dragons' feature. For unique to be safe, it needs to be transitive, but  
this severely limits the objects that can be represented. For example, a  
doubly-linked-list can not be unique. Unique has been integrated into the  
Clean and Mercury functional languages (or so says Wikipedia), so there  
might be reasonable solutions to these problems.