Ranges

[Robert Fraser]

Steve Teale wrote:
> template isInputRange(R)
> {
>     enum bool isInputRange = is(typeof(
>     {
>         R r;             // can define a range object
>         if (r.empty) {}  // can test for empty
>         r.popFront;          // can invoke next
>         auto h = r.front; // can get the front of the range
>     }()));
> }
> 
> I can not possibly be the only D enthusiast who finds this completely incomprehensible. 

Yeah, that one is a bit tricky, and what makes it worse is that it seems 
officially sanctioned by Walter/Andrei as the "right way" to check if a 
type supports some operations. Basically, if you have:

is(typeof({ @@@ }()));

this means "if I made a function containing @@@, would that function 
compile?". It's a hack which stems from the way the is expression works.

> What is a range?

As others have mentioned, it's just a struct (or other type) that 
happens to support certain operations.