Proposal: Multidimensional opSlice solution

[Norbert Nemec]

Don wrote:
> Norbert Nemec wrote:
>> Hi there,
>>
>> in implementing multi-dimensional arrays, the current way of 
>> overloading the slicing operator does not scale up.
>>
>> Currently, there is opIndex working for an arbitrary number of 
>> indices, but opSlice works only for one dimension. Ultimately, it 
>> should be possible to allow slicing for more than one dimension, and 
>> mixing it with indexing for e.g.:
>>     A[4,7..8,7,2..5]
>> So far, no clean solution for overloading this has been suggested.
> 
> A solution was suggested while you were away.
> You don't need a new opRange operator, a simple tuple struct like:
> struct Slice(T) { T from; T to; }
> in std.object is enough.
> Note that:
> A[4, Slice(7,8), 7, Slice(2,5)]
> 
> will work with the existing compiler. So it's just a tiny syntax sugar 
> issue.

I know this solution. It is exactly the "syntax sugar" issue that I see 
as the problem here:

The compiler would need to be aware of the data type Slice. It would 
therefore have to be something like a "builtin" type. If I am not 
mistaken, the language definition so far never makes use of "builtin" 
struct types.


> And another simple possibility is to turn 7..8 into int[2][7,8].

That solution would unnecessarily get in to way of implementing indexing 
by an array of indices:

	auto A = MyArray(["zero","one","two","three"]);
	assert( A[ [2,3,0] ] == MyArray(["two","three","zero"]) );


> However, Andrei argued that slicing of multidimensional arrays is so 
> rarely used that syntax sugar is not necessary. Thus, it's not in D2.

Outside of numerics, multidimensional arrays are indeed rarely used. In 
numerical programming, however, they are an essential ingredient. My 
ultimate goal is to multidimensional arrays in D as comfortable as in 
Fortran, Matlab or Python/NumPy in order to make D a real competitor in 
that market.