Transience of .front in input vs. forward ranges

[H. S. Teoh]

On Thu, Nov 01, 2012 at 12:05:18AM +0100, deadalnix wrote:
> Le 31/10/2012 18:19, H. S. Teoh a écrit :
[...]
> >Actually, there is another problem: many algorithms' output will be
> >transient or not depending on the input range. For example, we could
> >write map to use the transient version of byLine, say, but then the
> >range that map returns will also be transient (no longer safe).
> >
> >IOW, transience of .front is transitive in some cases. This again
> >makes things complicated: as soon as you use a single transient
> >range, it makes downstream ranges transient as well. So we're back to
> >the problem of how to mark the range as transient in a transparent
> >way. :-(
> >
> 
> No it isn't always transitive. But that is true that it is in some
> cases.
> 
> This is why the code should default to the safe behavior. It is up to
> the programmer to ensure correctness when opting for an alternate
> behavior.

But then can the alternate behaviour be used at all? For example, joiner
will return a range which will be transient if the original range is
transient. But since this is "unsafe" behaviour, joiner's implementation
can't take advantage of the faster behaviour at all, since otherwise the
user, not knowing that it is switching to, say, byLine.fast instead of
the safe version of byLine, may assume that the resulting range is
non-transient, but it is actually transient.

So this nullifies the usefulness of having .fast, since many Phobos
algorithms that would have been able to take advantage of .fast can't,
because their result will be unsafe. In which case, is it even worth the
bother to implement such a thing?

I think we still haven't addressed the fundamental issue, that is, we
need a way to differentiate between transient and non-transient .front
values. Being able to work with transient values is the best, because it
automatically also works for non-transient values, but some algos need
to be able to assume non-transience. This core problem is still
unsolved.


T

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