Repost: make foreach(i, a; range) "just work"

[Regan Heath]

On Fri, 21 Feb 2014 14:29:37 -0000, Steven Schveighoffer  
<schveiguy at yahoo.com> wrote:

> On Fri, 21 Feb 2014 06:21:39 -0500, Regan Heath <regan at netmail.co.nz>  
> wrote:
>
>> On Thu, 20 Feb 2014 17:09:31 -0000, Steven Schveighoffer  
>> <schveiguy at yahoo.com> wrote:
>>
>>> On Thu, 20 Feb 2014 11:07:32 -0500, Regan Heath <regan at netmail.co.nz>  
>>> wrote:
>>>
>>>> Only if the compiler prefers opApply to range methods, does it?
>>>
>>> It should. If it doesn't, that is a bug.
>>>
>>> The sole purpose of opApply is to interact with foreach. If it is  
>>> masked out, then there is no point for having opApply.
>>
>> Thanks.
>>
>> So, if we had this support which I am asking for:
>>
>> 	foreach(index, value; range) { }
>>
>> And, if someone adds opApply to that range, with a different type for  
>> the first variable then an existing foreach (using index, value) is  
>> likely to stop compiling due to type problems.
>>
>> This seems acceptable to me.
>
> I think any type that does both opApply and range iteration is asking  
> for problems :) D has a nasty way of choosing "all or nothing" for  
> overloads, meaning it may decide "this is a range" or "this is opApply",  
> but if you have both, it picks one or the other.
>
> I'd rather see it do:
>
> 1. can I satisfy this foreach using opApply? If yes, do it.
> 2. If not, can I satisfy this foreach using range iteration?
>
> This may be how it works, I honestly don't know.
>
>> There is an outside chance it might keep on compiling, like if 'i' is  
>> not used in a strongly typed way, i.e. passed to a writefln or  
>> similar.  In this case we have silently changed behaviour.
>>
>> Is this acceptable?
>
> Adding opApply is changing the API of the range. If the range does  
> something different based on whether you use the range interface or  
> opApply, then this is a logic error IMO.
>
> The easiest thing is to just not use opApply and range primitives  
> together :) One separation I like to use in my code is that you use  
> opApply on a container, but range primitives on a range for that  
> container. And a container is not a range.

Makes sense to me. :)

R

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