foreach range with index

[Steven Schveighoffer via Digitalmars-d]

On 6/14/17 6:46 PM, Luís Marques wrote:
> On Wednesday, 14 June 2017 at 19:22:24 UTC, Steven Schveighoffer wrote:
>> For example:
>>
>> foreach(i, v; hashmap) => i is counter, v is value
>>
>> Later hashmap adds support for iterating key and value. Now i is key,
>> v is value. Code means something completely different.
>
> If we take a step back, I think we are discussing the generalization of
> foreach. Let's assume we want to preserve the current foreach behavior
> where associative types iterate with key and value pairs. If the
> "hashmap" in your example was supposed to be an associative type then
> `i` wasn't a counter, it was the key already. If "hashmap" didn't define
> the iteration key (using a new kind of opApply or whatever) then it
> wasn't an associative type, and therefore your change wouldn't be valid.
>
> So, we just have to be careful that when foreach(i, v; foo) is
> introduced the relevant types in phobos also implement the new behavior,
> so that this situation doesn't arise afterwards..

The point I'm making is that we have mechanisms for a type to define 
what to do when you foreach with 2 parameters per loop iteration.

What you are proposing is to allow the compiler to define what to do in 
the case where the type doesn't. But what happens if it then defines the 
behavior later? code that was written expecting the first item to be an 
arbitrary index is now hijacked by the type to mean something else. This 
can't happen with arrays, because the language says so. The language 
doesn't have control over an arbitrary struct.

It could be any type, I just used a hypothetical hashmap that didn't 
already have key/value iteration. Probably an unlikely scenario, but it 
was what popped into my head as a demonstration.

And what if you DID want to iterate just an incrementing index with the 
values when the type defines 2-parameter iteration otherwise? That 
mechanism isn't available to you without some effort. So again, we still 
need an enumerate to do what you want.

-Steve