Why is complex being deprecated again?

[Stewart Gordon]

On 16/04/2012 07:06, Lars T. Kyllingstad wrote:
<snip>
> For any standard type (built-in or library) to be useful, it has to actually be used for
> something.

You mean someone has to use it in order to prove that it's usable and therefore useful? 
Well, if a feature isn't usable, it's probably due to something wrong with its design. 
And in many cases it's a reason to rethink the design, rather than throw the feature out 
of the window.

> And in all my years of using D, I have never seen a *single* real-world use of
> the pure imaginary types.

How many of those years you spent "using" D did you spend looking at other people's 
real-world applications written in it?

Of these, how many have a focus on number crunching?

And of these, how many use complex numbers?

It's bound to be a rarely used feature.  But "nobody's using it" is a prime example of a 
self-fulfilling prophecy if it leads to the feature's removal.

Do you feel the SETI Institute should have given up years ago?

> The reason the imaginaries are so seldomly used is precisely because there are so few
> things you can do with them. Basically, if you do anything beyond addition and
> subtraction, and multiplication with a real number, you are back in the complex plane. And
> if those operations are all you need, the real line is just as good as the imaginary line,
> and you might as well fake it with a real floating-point type.

This doesn't cover the case of multiplying a complex number by an imaginary number.  In 
the absence of imaginary types, one would have to use complex(-z.im * k, z.re * k), just 
because z * complex(0, k) isn't guaranteed to produce the correct result.  Seems a bit 
silly.  Or have you another suggestion for dealing with this?

<snip>
> It is true that the real line can be extended with elements called plus and minus infinity
> (affinely extended real line, see https://en.wikipedia.org/wiki/Extended_real_number), and
> the IEEE floats can be said to approximate this system, but this does not generalise
> directly to complex numbers. The extended complex plane (see
> https://en.wikipedia.org/wiki/Riemann_sphere) only has one "infinity".
<snip>

But the Riemann sphere is only one of the various possible extended complex planes.  You 
could just as well use the polar circle of infinities, the cartesian square of infinities, 
or the real projective plane model (by which -∞ = ∞, but ∞i, ∞(2+i), etc. are distinct).

Stewart.