The new, new phobos sneak preview

[Daniel Keep]

Andrei Alexandrescu wrote:
> Daniel Keep wrote:
>> ...
> 
> So essentially we're looking at a symbolic approach - a resumable range
> would need to advertise that. I've used that for isSorted too, and it
> works pretty well.
> 
> The remaining question is one of defaults - are most ranges resumable or
> not? I.e., should a range advertise explicitly when it's resumable or
> when it's *not* resumable? I think the safe approach is to go as you
> say: a range ain't resumable unless it explicitly claims to. So if you
> forget stuff, the compiler will remind you.
> 
> I'd love to hear more opinions on the topic.
> 
> 
> Andrei

Actually, I stuffed up the test; I wanted the default to be resumable.
:P  That'll teach me to not double-check my logic.  A quick truth table
later, and the correct test is this:

> template isResumableRange(R)
> {
>     enum bool isResumableRange = isInputRange!(R)
>         && (!is(typeof(R.isResumableRange))
>             || (is(typeof(R.isResumableRange)) && R.isResumableRange))
>         && is(typeof(
>         {
>             R r1;
>             R r2 = r1;
>         }()));
> }

... I think.  :P

Actually, I've been thinking and I realised that in 95% of cases, you
can assume a range is resumable if it has no references.  If it has no
references, the only way the range can be non-resumable is if the
advance member uses the range's state as an argument to some global
method.  An example might be a Range that statically accesses Stdout
instead of taking it as an argument.

It's a real shame the range interface doesn't support this:

> struct R
> {
>     ...
>     pure R advance();
> }

If it did, we could prove a range was resumable if advance was pure and
R has no mutable or const references.

Honestly, I think the safest option is to *require* resuming to be
explicitly stated by the programmer.  It'd be nice if we could
automatically account for some cases, but I can't think of any you
couldn't escape from.

Maybe we should default to non-resumable for now, then re-visit the
issue when we have an idea of how people are using ranges.

  -- Daniel