I think `mulAdd a b c` should be implemented as `a*b+c` even for Double/Float. It should only be an "optmization" (as in modular arithmetic), not a semantic changing operation. Thus justifying the optimization.

"fma" should be the "more-precise" version available for Float/Double. I don't think it makes sense to have "fma" for other types. That's why I'm advocating "mulAdd" to be part of "Num" for optimization purposes; and "fma" reserved for true IEEE754 types and semantics.

I understand that Edward doesn't like this as this requires a different class; but really, that's the price to pay if we claim Haskell has proper support for IEEE754 semantics. (Which I think it should.) The operation is just different. It also should account for the rounding-modes properly.

I think we can pull this off just fine; and Haskell can really lead the pack here. The situation with floats is even worse in other languages. This is our chance to make a proper implementation, and we have the right tools to do so.

-Levent.

On Mon, May 4, 2015 at 10:58 AM, Artyom <yom@artyom.me> wrote:
On 05/04/2015 08:49 PM, Levent Erkok wrote:
Artyom: That's precisely the point. The true IEEE754 variants where precision does matter should be part of a different class. What Edward and Yitz want is an "optimized" multiply-add where the semantics is the same but one that goes faster.
 No, it looks to me that Edward wants to have a more precise operation in Num:
I'd have to make a second copy of the function to even try to see the precision win.
 Unless I'm wrong, you can't have the following things simultaneously:
  1. the compiler is free to substitute a+b*c with mulAdd a b c
  2. mulAdd a b c is implemented as fma for Doubles (and is more precise)
  3. Num operations for Double (addition and multiplication) always conform to IEEE754

The true IEEE754 variants where precision does matter should be part of a different class.
So, does it mean that you're fine with not having point #3 because people who need it would be able to use a separate class for IEEE754 floats?