[Haskell-cafe] Why no Floating instance for Data.Fixed / Data.Fixed.Binary

[Scott Turner]

On 2015-04-13 22:08, Douglas McClean wrote:
> I'd certainly be happy to do it, I'm just concerned that it would be
> actively unwanted for a reason that I can't see.
>
> I will look in to what the procedures are for contributing to base.
>
> It wasn't my intention to beg the internet to do it for me.
>
> On Mon, Apr 13, 2015 at 5:49 PM, Albert Y. C. Lai <trebla at vex.net
> <mailto:trebla at vex.net>> wrote:
>
>     On 2015-04-13 11:31 AM, Douglas McClean wrote:
>
>         I'm wondering why the decision was made not to have a Floating
>         instance for Data.Fixed.
>
>
>     I have always found economics to be a powerful answer to this kind
>     of questions. That is, perhaps simply, there has not been
>     sufficient incentive for anyone to do the work. For example, do
>     you want to do it?
>

It looks hairy to me. The big-number cases would need approaches quite
different from floating point.
    sin(31415926.535897932384::Pico)
    sin(31415926535897932384.626433832795::Pico)
    sin(314159265358979323846264338327950288419.716939937510::Pico)
To return the correct value (0) from each of these examples, and their
successors, requires an implementation able to calculate pi to an
unbounded precision.

The value of exp(50::Uni) requires more precision than a double can
provide. How far do you go? exp(100::Uni)?

I hope this doesn't scare you off. Perhaps there's literature on
acceptable limitations when implementing/using such fixed point
transcendental functions. In any case an implementation would be
interesting even if it doesn't provide correct results in the extreme cases.
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