getNext

[Shin Fujishiro]

Andrei Alexandrescu <SeeWebsiteForEmail at erdani.org> wrote:
> I think I figured out a comfortable and all-encompassing means to define 
> a simplified interface for an input range.
> 
> Currently input ranges need to define empty, front, and popFront. That 
> works but it's a bit heavy for simple input ranges. We've been 
> discussing simplified interfaces in this group but couldn't find one 
> that satisfied all use cases.
> 
> Consider this:
> 
> T* getNext(R, T)(ref R range, ref T item);
> 
> Semantics: if the range wants to expose addresses of its elements, it 
> returns a pointer to the current element and also advances to the next 
> element. Otherwise (i.e. the range does not have or does not want to 
> expose addresses of its elements), the range fills "item" with the 
> current value, again moves on to the next value, and returns &item.
> 
> In all cases, when there are no more elements in the range, getNext 
> returns null.
> 
> getNext is easy to define for e.g. arrays and files. How does it sound? 
> Does it bring significant simplification?

I gave it a try, and it fairly simplified range implementation.
    http://gist.github.com/474562
I think it's good.

(1) But how does it represent an infinite range?

(2) Should user code use getNext for input ranges (like put for output
ranges)?  For example:

void doSomething(R)(R input)
{
    // read first character if any
    dchar* p = getNext!dchar(input);
    if (p == null)
        return;
    dchar a = *p;

    // read subsequent characters...
    dchar b = *enforce( getNext!dchar(input) );
    ...
}


Shin