Hello Will,

> about the typesetting; should/can we provide an easy way to typeset negative
> zero just as zero?

I suppose there are two cases where one needs to display numbers: for
typesetting, and when writing to a file (terminal, log,...).  For the
first case, I'd advocate using siunitx, which has all the settings you
may want, and more.  For the second case, I feel that in all contexts
where 0 is better than -0 (e.g. pdf specials) there is also a need to
limit the number of significant digits to less than 16.  This requires
rounding.  We should provide formatting functions which produce a
string of characters from floating points.  Doing this correctly is a
bit subtle, and it is not clear what should be implemented and what
can be left out (see e.g.
http://www.cplusplus.com/reference/cstdio/printf/ for a long list of
things C++ does).

> When I was trying it out, I thought perhaps that \fp_eval:n would return
> negative zero but \fp_to_decimal:n would “normalise” it; is that a crazy
> idea?

I'm not keen on that, but I don't have a very strong argument against
it.  Mainly, -0 is the correct result, so I don't see what's wrong
with it.  Actually... thinking about it, there is an easy way to get 0
for -0: add +0 to it.  It turns out that 0+(-0) = +0, so for all x,
0+x is exactly x, except when x is -0, then 0+x is +0.  If I remember
correctly, the signs of (±0)±(±0) are not mandated by the standard, so
we could presumably document that 0+x does the right thing, and make
sure that this is mentioned in the code and tested.

Bruno