D versionning

[Tobias Pankrath]

> The system adopted in PHP works with a 3 number version. The 
> first number is used for major languages changes (for instance 
> 4 > 5 imply passing object by reference when it was by copy 
> before, 5 > 6 switched the whole thing to unicode).
>
> The second number imply language changes, but either non 
> breaking or very specific, rarely used stuff. For instance 5.2
> > 5.3 added GC, closures and namespace which does not break
> code.


We can also learn from the python community, whose from 
__future__ import <feature> facility is a great success and we 
should adopt a similar scheme.

Consider the -property switch or a future introduction of tuple 
syntax.
If you start a new project, it's no problem. Just use the switch 
and don't
introduce things that will not work without. But if you have an 
old codebase
but want to use a new feature you can either

a) fix the hole codebase at one
b) fix and configure your build systems to build some files with 
and some files without the switch.

Both options are PITA. An alternative is to introduce 
#pragma(future, tuplesyntax);

Now you can insert the pragma in those sourcefiles that are new 
or already fixed and you can immediately benefit from the feature 
and you can upgrade your code file by file or even scope by scope.

Later versions could even introduce #pragma(past, notuplesyntax) 
before dropping the old syntax completely.