D2 Short list -- opSlice

[Don]

Andrei Alexandrescu wrote:
> We're entering the finale of D2 and I want to keep a short list of 
> things that must be done and integrated in the release. It is clearly 
> understood by all of us that there are many things that could and 
> probably should be done.
> 
> 1. Currently Walter and Don are diligently fixing the problems marked on 
> the current manuscript.
> 
> 2. User-defined operators must be revamped.

Should opIndex and opSlice be merged?
This would be simpler, and would allow multi-dimensional slicing.
Probably the simplest way to do this would be to use fixed length arrays 
  of length 2 for slices.
So, for example, if the indices are integers, then
opIndex(int x) { } is the 1-D index, and
opIndex(int[2] x) {} is a slice from x[0] to x[1],
which exactly corresponding to the current opSlice(x[0]..x[1]).

Since fixed-length arrays are now passed by value (yay!) this would be 
just as efficient as the current method. It'd be simple to implement.

Downsides: (1) Arguably not terribly intuitive.
(2) This would not let you have a slicing from A..A, while 
simultaneously allowing indexing by A[2]. But I don't think that makes 
sense anyway -- what would $ return? (Note that it doesn't stop you from 
having indexes of type A[2], and slice from A[2]..A[2], which might be 
important for containers of 2D vectors. It's only the case where 
indexing and slicing are different types, which is prohibited).

This is the only solution I have which doesn't introducing more coupling 
between the language and standard library.
Another possible solution is to define a special Slice struct in 
std.object; downside is that it creates yet another pseudo-keyword.

A third solution would be to use tuples;  currently I think it's 
impossible, but if tuple auto-flattening is removed there may be a 
chance. Even so, it might be a little clumsy to use.

A fourth solution is to retain the status quo, and not provide the 
multi-dimensional slicing feature <g>. This is actually about the same 
amount of work as option (1), since opDollar would need to work for 
opSlice (I only implemented it for opIndex).