default random object?

[dsimcha]

== Quote from Bill Baxter (wbaxter at gmail.com)'s article
> On Mon, Feb 16, 2009 at 2:33 AM, Don Clugston <nospam at nospam.com> wrote:
> > Andrei Alexandrescu wrote:
> >>
> >> Don wrote:
> >>>
> >>> Andrei Alexandrescu wrote:
> >>>>
> >>>> auto rng = Random(unpredictableSeed);
> >>>> auto a = 0.0, b = 1.0;
> >>>> auto x1 = uniform!("[]")(rng, a, b);
> >>>> auto x2 = uniform!("[)")(rng, a, b);
> >>>> auto x3 = uniform!("(]")(rng, a, b);
> >>>> auto x4 = uniform!("()")(rng, a, b);
> >>>
> >>> This is a general issue applying to any numeric range. I've been giving
> >>> the issue of numeric ranges some thought, and I have begun an implementation
> >>> of a general abstraction.
> >>> Any open range can be converted into a closed range, but the converse
> >>> does not apply. So any implementation will be using "[]" internally.
> >>>
> >>> -range("[)", a, b) == range("(]", -b, -a)
> >>> range("[)", a, b) == range("[]", a, predecessor(b))
> >>> range("()", a, b) == range("[]", successor(a), predecessor(b))
> >>>
> >>>
> >>> There's a couple of difficult situations involving floating-point
> >>> numbers.
> >>> *  "[)" has the uncomfortable property that (-2,-1, rng) includes -2 but
> >>> not -1, whereas (1, 2, rng) includes 1 but not 2.
> >>>
> >>> * any floating point range which includes 0 is difficult, because there
> >>> are so many numbers which are almost zero. The probability of getting a zero
> >>> for an 80-bit real is so small that you probably wouldn't encounter it in
> >>> your lifetime. I think this weakens arguments based on analogy with the
> >>> integer case.
> >>>
> >>> However, it is much easier to make an unbiased rng for [1,2) than for
> >>> [1,2] or (1,2) (since the number of members in the range is even).
> >>
> >> So what would you recommend? [a, b) for floats and [a, b] for ints, or [a,
> >> b) for everything?
> >>
> >> Andrei
> >
> > I'm leaning towards [a,b) for everything (consistency with arrays), but I'd
> > want to know what the reasoning of the boost/c++0x guys was.
> How do you create a random uint that can take on any of uint's values
> with [a,b)?  That's the main reason I can think of to go with [a,b]
> for integral types.  With floats it's never useful to use the entire
> value range.
> --bb

Keep in mind that we're talking only about defaults here.  At least in the current
implementation, "[)" vs. "[]" can be specified by template parameters.  Wanting
the entire range of an integer type is an edge case, so it's not important that it
be well-supported by the defaults, as long as it can be done without resorting to
serious kludges.