[Haskell-cafe] How odd...

[Andrew Coppin]

Paul Johnson wrote:
> Andrew Coppin wrote:
>> > 0**2
>> 0
>>
>> > (0 :+ 0)**2
>> NaN :+ NaN
>>
>> (Is this a bug?)
> According to the Standard Prelude,
> #   x ** y           =  exp (log x * y)

I had a feeling this would be the cause.

> > log 0
> -Infinity

Oh. So... since when does Haskell know about infinity?

BTW, I recently had some code like this:

  foo x
    | x < 0 = ...
    | x == 0 = ...
    | x > 0 = ...

I was most perplexed when I got a "non-exhaustive patterns" exception... 
It turns out there was a NaN in there. I forget about that.

> > exp (log 0 * 2)
> 0.0

Well that's interesting. I did wonder why it *doesn't* break in the real 
case...

> On to the complex number case.  From the standard for Complex:
>
> # log z          =  log (magnitude z) :+ phase z
>
> # phase (0 :+ 0) = 0
> This is a special case for the phase of zero.
>
> # (x:+y) * (x':+y') =  (x*x'-y*y') :+ (x*y'+y*x')
>
> > log (0 :+ 0)
> (-Infinity) :+ 0.0
>
> > log (0 :+ 0) * 2
> (-Infinity) :+ NaN
>
> Which is the source of the problem.  The imaginary part involves 
> multiplying (-Infinity) by the
> imaginary part of 2 (i.e. 0), which is NaN.

Um... why would infinity * 0 be NaN? That doesn't make sense...

> So no, its not a bug, its according to the standard.

So I'm the only person who was expecting zero squared to be zero? (IMHO 
the standard should try to implement mathematical operations in a 
mathematically sensible way...)

> While working through this I also came across the following case which 
> technically is a bug:
>
> > 0 ** 0
> 1.0
>
> > exp (log 0 * 0)
> NaN
>
> I suspect that GHCi is using a built-in exponentiation operator that 
> doesn't quite conform to the standard in this case.

Now that really *is* odd...