protocol for using InputRanges

[Steven Schveighoffer]

On Sun, 23 Mar 2014 09:00:17 -0400, Tommi <tommitissari at hotmail.com> wrote:

> On Sunday, 23 March 2014 at 00:50:34 UTC, Walter Bright wrote:
>> It's become clear to me that we've underspecified what an InputRange  
>> is. The normal way to use it is:
>>
>>     while (!r.empty) {
>>         auto e = r.front;
>>         ... do something with e ...
>>         r.popFront();
>>     }
>>
>> no argument there. But there are two issues:
>>
>> 1. If you know the range is not empty, is it allowed to call r.front  
>> without calling r.empty first?
>>
>> If this is true, extra logic will need to be added to r.front in many  
>> cases.
>>
>> 2. Can r.front be called n times in a row? I.e. is calling front()  
>> destructive?
>>
>> If true, this means that r.front will have to cache a copy in many  
>> cases.
>
> Is InputRange supposed to be a one-pass range and am I supposed to able  
> to use InputRange as a lightweight wrapper for a stream? If the answer  
> is yes, then I think the fundamental issue is that the  
> empty-front-popFront interface is not optimal for something like  
> InputRange. But given that's the interface we have, I think that  
> InputRange's front must be allowed to be destructive because the stream  
> it could potentially be wrapping is destructive.

A range interface only works for a buffered stream, which naturally allows  
caching.

-Steve