Associative Array .byKey / .byValue: Counter and Tuples

[Q. Schroll via Digitalmars-d]

On Sunday, 3 April 2016 at 11:17:17 UTC, Mike Parker wrote:
> On Sunday, 3 April 2016 at 10:59:47 UTC, Q. Schroll wrote:
>> Simple as that, suppose
>>     uint[uint] aa;
>> Any range supports carrying an index. Not so does the Range 
>> returned by byKey and byValue.
>>     foreach (i, k; aa.byKey) { }
>> and
>>     foreach (i, v; aa.byValue) { }
>> both don't compile.
>
> That's incorrect. Only Random Access Ranges are indexable. The 
> ranges returned by aa.byKey and aa.byValue are simply Input 
> Ranges. Moreover, ranges do not by default allow for an index 
> value in a foreach loop. That only works out of the box with 
> arrays.

It looks like I've used Random Access Ranges so far (without 
realizing that matters that much).

> To get the same for a range, you can use std.range.enumerate:
>
> import std.range : enumerate;
> foreach(i, k; aa.byKey.enumerate) {}

Thanks. That solves the problem.

>> Reason (I found out by chance):
>>
>> If the key or value type is a std.typecons.Tuple, iteration 
>> over aa.by* decomposes the Tuple if there is the right number 
>> of arguments. For 2-Tuples, there cannot be both possible.
>>
>>     alias Tup = Tuple!(int, int);
>>     int[Tup] it;
>>     Tup[int] ti;
>>
>>     foreach (x, y; it.byKey) { }
>>     foreach (x, y; ti.byValue) { }
>>
>> Why is this undocumented? http://dlang.org/spec/hash-map.html 
>> doesn't mention Tuples at all!
>
> D's associative arrays don't know anything about Tuples, so 
> there's no reason for the aa docs to talk about them. This 
> behavior comes from how std.typecons.Tuple is implemented.
>
>> Why is this useful? Anyone can decompose the Tuple with 
>> .expand if they like. I would prefer allowing an index.
>
> If you look at the source of Tuple, alias this is used on 
> .expand, which is likely why they are automatically decomposed 
> in an aa.

I know about the alias expand this in Tuple, but simply didn't 
expect it can go that far. On the other hand, it sounds 
plausible. I never expected a special preference of AAs for 
Tuples.
It's very interesting that alias this can work like this.