Input Range addition to associative arrays

[Phil Lavoie]

Hi,

I am aware that you can iterate over associative arrays in 
multiple ways using the structure's opApply and different 
delegates returned by properties. I think adding a "range" 
property returning an input range to the associative array would 
benefit the users of the D language.

I'd like to justify this addition by pointing out that some 
standard library functional features, like filter, do not work on 
associative arrays as of right now. The reason for this is that 
filter returns a range that moves to the next appropriate value 
ON DEMAND rather than constructing the result. I think it is a 
great design choice. However, since no incremental (read on 
demand) iteration can be done on associative arrays, one cannot 
use features such as filter, nor make his own.

I think that the existing code can easily support such an 
addition, which makes it all the more attractive.

Imagine that "front()" would return an entry such that entry.key 
returns the key and entry.value returns the value. Here is an 
example of how one could use it:

float[ item ] itemsCost;
//Items whose price are lower than 5 dollars.
auto myFavoriteItems = filter!"a.value < 5.0"( itemsCost );
foreach( cheapItem; myFavoriteItems ) { 
theMoreComplicatedAlgorithmOnEarth( cheapItem); }

Now don't get me wrong, I am aware that you can do this multiple 
ways. It's just that ranges are so attractive in the way they 
"lazily" fetch values that it would make it easier to wrap 
associative arrays in struct that return relevant ranges based on 
the aa's content of constructing the result, than returning the 
range/or result. I also think it would merge well with D's way of 
doing things.

What are your thoughts?