protocol for using InputRanges

[Steven Schveighoffer]

On Thu, 27 Mar 2014 16:58:12 -0400, Walter Bright  
<newshound2 at digitalmars.com> wrote:

> On 3/27/2014 12:50 PM, Andrei Alexandrescu wrote:
>> Yah, agreed. -- Andrei
>
> Completely unworkable. To determine if a range is empty or not, it may  
> have to actually read from its input. TTYs are an example. Although  
> empty may then cache the result, and not read the second time it is  
> called, an observer of TTY could see that an item was read from the TTY.

In the land of ranges, it's construction and popFront that generally  
advances the range. Empty does not. This is why assert(!empty) is all over  
the place, it's considered to be non-destructive.

Note that a stream makes a terrible range, simply because of what you say  
-- reading is destructive, and determining emptiness is dependent on  
reading. You need a buffered stream to make a good range, and then the  
logic becomes much more straightforward.

The awkwardness for shoehorning streams into ranges I see is that the  
advancement (popFront) and the check for termination (empty) are  
decoupled, when in reality they are synced for a stream. This REQUIRES a  
sticky bit for popFront to communicate with empty on whether it is EOF or  
not.

Even a single byte buffer is not enough, you need a bool to indicate the  
stream is done.

-Steve