isInfinite isInadequate

[Steven Schveighoffer]

On Tue, 12 Mar 2013 12:16:10 -0400, Andrei Alexandrescu  
<SeeWebsiteForEmail at erdani.org> wrote:

> On 3/12/13 11:47 AM, Steven Schveighoffer wrote:
>> On Tue, 12 Mar 2013 11:25:54 -0400, deadalnix <deadalnix at gmail.com>  
>> wrote:
>>
>>> On Tuesday, 12 March 2013 at 10:49:57 UTC, monarch_dodra wrote:
>>
>>>> Having "isInfinite" can also have the advantage of protecting users
>>>> from stupid calls. For example, calling "count" on an infinite range
>>>> is forbidden => shifting problems from runtime to compile time is a
>>>> HUGE gain.
>>>>
>>>
>>> Clearly this is a good point. I however think that a static assert
>>> within count is much better because it allow to give nicer feedback.
>>> The problem with InfiniteRange is that it does gives you cryptic error
>>> message like the count function do not exists.
>>>
>>
>> Hm... is there a way to test for inifinitness without requiring to be an
>> enum?
>>
>> Wouldn't this also be valid?
>>
>> if(!R.init.empty)
>>
>> Essentially, you can evaluate R.init.empty at compile time AND it's
>> false on an uninitialized range. How can a correctly written
>> non-infinite range pass that?
>>
>> That would make forwarding much easier, as the 'dumb' implementation
>> still would result in an infinite range.
>
> Crossed my mind a few times that fresh non-infinite ranges should be  
> empty. But there's no explicit requirement stating that, and I think  
> e.g. one may define a k-elements buffer backed by in-situ storage.

So something like this:

struct R
{
   int[4] elems;
   int idx;
   int front() { return elems[idx];}
   void popFront() {++idx;}
   bool empty() {idx < elems.length;}
}

which would make this an infinite range by my measurement.  Crap.

How can we make wrapping ranges easier?  I get the point that it's a pain  
in the ass in order to implement a wrapper for another range for things  
like isInfinite, and save.  We need something in std.range that allows you  
to wrap such functions, like a nice mixin.

-Steve