D1 -> D2

[Steven Schveighoffer]

On Thu, 18 Nov 2010 05:51:32 -0500, Fawzi Mohamed <fawzi at gmx.ch> wrote:

> Is there any "porting" guide around in a wiki?
> If not a page where to share the best tricks would be nice "D1->D2  
> conversion tricks"?
>
> In the short term I don't think that going D2 only is really an option  
> for me, so how feasible it is to keep the code base compatible to both  
> D1 and D2?
>
> I know that one can define some templates (for example Const(T),....),  
> and maybe use mixins, but how much uglier does the code become as result?
> I choose D to have cleaner code, I am not interested in loosing all that  
> just to be D1 and D2, then I prefer to wait, and convert everything at  
> once.

I have had two experiences moving code from D1 to D2.  First, I tried to  
port Tango to D2, and found that backwards compatibility is not possible.   
So I was simply reorganizing the files.  The hugest problem for going from  
D1 to D2 is string literals.  Every place you use char[] has to be  
switched to const(char)[] or string.  I came up with a set of rules for  
this:

1. if the 99% common case for string arguments will be string literals,  
then use string -- you never have to worry about duping.
2. if you intend to keep a copy of the string, but you aren't sure whether  
the string to be passed in will have mutable aliases, use const(char)[]  
and dup.
3. if you don't intend to keep a copy of the string, use const(char)[] to  
have the most flexibility.
4. if you intend on returning a portion of the string, well, currently  
you're SOL, but eventually inout will come into play here.

For example, Tango's log package fell under rules 1 and 3 (most of the  
time you are creating log objects with string literals, and logging a  
message does not save the message anywhere).
Many many cases fell under rule 4, which is one of the main reasons I gave  
up (inout still isn't ready).

The second experience I had was porting dcollections to D2.  Since  
dcollections has not many places for string literals, it was much easier  
to port.  I don't think I made it backwards compatible, but it's very very  
similar to the original in terms of constancy.  I've added a few features  
that only really make sense in D2 (ranges for one).

Porting dcollections was quite easy, but with my experience trying to port  
Tango, I never tried to make it backwards compatible.

My recommendation -- when you are ready, switch wholly to D2.  Don't  
bother with compatibility, it's just not possible.

-Steve