Smooth transition to D2 for Tango users?

[Sergey Gromov]

Sean Kelly <sean at invisibleduck.org> wrote:
> Sergey Gromov wrote:
> > Steven Schveighoffer <schveiguy at yahoo.com> wrote:
> >> oops, D1 doesn't support that type of enum ;)
> > 
> > I think you've got the idea. The code
> > 
> > version (D_Version2)
> > {
> >   mixin("some very specific code");
> > }
> > 
> > works in any version of D, current or future.
> 
> String mixins don't work directly for everything though.  If you want to 
> change a return value from non-const to const, for example, you need to 
> make an alias for it using a versioned string mixin and then use the 
> alias in the function declaration.  And then there are things like this:

template Const(T) {
  version (D_Version2) {
    alias const(T) Const;
  } else {
    alias T Const;
  }
}

then

Const!(int) bar;

is just one char more than

const(int) bar;

> // D1
> const x = "hello";
> 
> // D2
> enum x = "hello";
> 
> With string mixins you end up having to duplicate the entire line, and 
> this can be a disaster if you're trying to maintain large header files 
> with tons of declarations.

const keyword can be used to declare manifest constants in D2:

const x = "foo";
pragma(msg, typeof(x).stringof);	// invariant(char[3u])

> Finally, I think the version(D_Version2) idea is backwards.  It should 
> be version(D_Version1).  The current method isn't forwards-compatible, 
> so all the code with these version statements in it will break when we 
> get D version 3.

I agree here, there's not enough flexibility.  The version identifiers 
should also have a numeric value, so that you can write:

version (D_Version, 2)
{
  ...
}

and the versioned code compiles only if D_Version has value of 2 or 
greater.