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digitalmars.D - Re: Smooth transition to D2 for Tango users?

Sergey Gromov Wrote:

 Sean Kelly <sean invisibleduck.org> wrote:
 Sergey Gromov wrote:
 Steven Schveighoffer <schveiguy 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;

This style of solution won't work for function declarations... There's no way to mixin const/invariant/pure function declarations without taking the function body along for the ride. If only we could find a way to make the D1 compiler ignore those extra function attributes.
 // 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.

Sep 23 2008