isInfinite isInadequate

[Steven Schveighoffer]

On Tue, 12 Mar 2013 11:02:43 -0400, monarch_dodra <monarchdodra at gmail.com>  
wrote:

> On Tuesday, 12 March 2013 at 14:30:00 UTC, Steven Schveighoffer wrote:
>> On Tue, 12 Mar 2013 06:49:56 -0400, monarch_dodra  
>> <monarchdodra at gmail.com> wrote:
>>
>>> On Tuesday, 12 March 2013 at 10:01:38 UTC, deadalnix wrote:
>>>> I want to resurrect that thread. Can someone explains the benefices  
>>>> of isInfinite ? I fail to see how it really benefit the code.
>>>
>>> The advantage of "enum empty = false" is that algorithms gain a great  
>>> performance boost by optimizing out any "if (r.empty)". This can be  
>>> exploited for things like take, or anything that iterates as a matter  
>>> of fact. I don't think anybody will argue that this is a bad approach.
>>
>> Wouldn't it automatically be optimized out?  I mean if r.empty is an  
>> enum, it's like saying if(false)  I would think even with optimizations  
>> off, this might be done.
>>
>> Not that I'm questioning the value of isInfinite (I'm neutral on it),  
>> but this is not a benefit.
>>
>> -Steve
>
> Right, that's what I said. This first paragraph was just about enum  
> empty = false.

Oh, ok.  When you said "optmizing out" I thought you meant using  
isInfinite to optionally remove calls to empty by hand.

> Another point: isInfinite is useful, if only to propagate infiniteness.  
> For example: "1.repeat().map"a * 2"()". If "map" didn't know that repeat  
> is infinite, it would simply provide the "dumb" empty implementation,  
> and the final range will have lost it's infinite trait.

With inlining, this should propagate properly:

struct WrapperRange(R)
{
    R src;
    @property auto front() { return src.front;}
    @property bool empty() { return src.empty;} // should be optimized to  
'false' with inlining
    void popFront() { src.popFront();}
}

If R's empty is an enum, then WrapperRange's empty should inline to the  
enum also.

-Steve