complement to $

[Simen kjaeraas]

On Sat, 15 May 2010 23:46:12 +0200, Walter Bright  
<newshound1 at digitalmars.com> wrote:

> bearophile wrote:
>> KennyTM~:
>>> auto a = new OrderedDict!(int, string);
>>> a[-3] = "negative three";
>>> a[-1] = "negative one";
>>> a[0] = "zero";
>>> a[3] = "three";
>>> a[4] = "four";
>>> assert(a[0] == "zero");
>>> return a[0..4]; // which slice should it return?
>>  D slicing syntax and indexing isn't able to represent what you can in  
>> Python, where you can store the last index in a variable:
>>  last_index = -1
>> a = ['a', 'b', 'c', 'd']
>> assert a[last_index] == 'd'
>>  In D you represent the last index as $-1, but you can't store that in  
>> a variable.
>
> Sure you can:
>
>      last_index = a.length - 1;

Ah, but if you then change the length of a, last_index is no longer
correct. Now, if we had a special index type...

enum slice_base {
   START,
   END
}

struct index {
   ptrdiff_t pos;
   slice_base base;

   // Operator overloads here, returns typeof( this ) if +/-
   // integral, ptrdiff_t if subtracted from
   // typeof( this ).
}

immutable $ = index( 0, slice_base.END );
immutable ^ = index( 0, slice_base.START );

auto last_index = $ - 1;
auto third_index = ^ + 2;

These would then stay valid no  matter what you
did to the container.

-- 
Simen