opSlice and $

[Kirk McDonald]

Janice Caron wrote:
>>I dont know even what signature it will have, how to distinguish such
>>uses:
>>c[1, 2..3]
>>c[1..2,3]
> 
> 
> Just a suggestion, but the signature could be
> 
> opSlice(int[] ...)
> 
> where the passed array always had an even number of parameters, and
> non-slice parameters get passed as the tuple n, n+1. In that case,
> your two cases would be distinguished as [ 1, 2, 2, 3 ] and [ 1, 2, 3,
> 4 ] (since 1 is equivalent to 1..2, and 3 is equivalent to 3..4).
> 
> Only works for ints, but hey! Better than nothing.

If I may wax Pythonic, there is a better way: Slice objects. Take the 
following Python code:

class C(object):
     def __getitem__(self, i):
         return i

__getitem__ is roughly the equivalent of opIndex:

 >>> c = C()
 >>> c[10]
10

However, it handles slicing, as well:

 >>> c[2:10]
slice(2, 10, None)
 >>> c[1:5:2]
slice(1, 5, 2)

Slices in Python have two or three items in them. The first two are the 
start and stop indices, which work just like in D. The third is an 
optional "step". If you set it to (e.g.) 2, you would get every other 
item. As the above code shows, it defaults to None.

The slice object is very simple, and has three attributes of note, 
start, stop, and step, which are the values passed to the slice.

If I may make up some syntax, we could invent a new "slice" type in D. I 
am admittedly not very good at making up syntax, but maybe something 
like this:

alias void[int..int] slice; // a slice from an int to an int

(A better keyword than "void" might be thought of.) You could then 
declare opIndex like this:

class C {
     int[] arr;
     int opIndex(slice s) {
        	return arr[s.start..s.stop];
         // or just:
         return arr[s];
     }
}

The examples mentioned previously become easy as pie. These:

c[1, 2..3]
c[1..2, 3]

Would require the following opIndex methods:

int opIndex(int, slice);
int opIndex(slice, int);

opSlice would, therefore, become entirely obsolete, unless I am missing 
some edge-case or other.

This is possibly a little wild and crazy. I feel the basic idea is 
sound, however. What do other people think?

-- 
Kirk McDonald
http://kirkmcdonald.blogspot.com
Pyd: Connecting D and Python
http://pyd.dsource.org