"fma" (a.k.a. fused-multiply-add) is one of those functions; which is the workhorse in many HPC applications. The idea is to multiply two floats and add a third with just one rounding, and thus preserving more precision. There are a multitude of applications for this operation in engineering data-analysis, and modern processors come with custom implementations and a lot of hardware to support it natively.

I created a ticket along these lines already: https://ghc.haskell.org/trac/ghc/ticket/10364

Edward suggested that the matter should further be discussed here.

I think the proposal is rather straightforward, and should be noncontroversial. To wit, we shall add a new method to the RealFloat class:

  class (RealFrac a, Floating a) => RealFloat a where

      ...

      fma :: a -> a -> a -> a

The intention is that

      fma x y z = x * y + z

except the multiplication and addition are done infinitely-precisely, and then rounded only once; as opposed to two roundings as one would get with the above implementation. Most modern architectures directly support this operation so we can map it easily; and in case the architecture does not have it available, we can get it via the C-math libraries, where it appears under the names fma (the double version), and fmaf (the float version.)

There should be no default definitions; as an incorrect (two-rounding version) would essentially beat the purpose of having fma in the first place.

While the name "fma" is well-established in the arithmetic/hardware community and in the C-library, we can also go with "fusedMultiplyAdd," if that is deemed more clear.

Discussion period: 2 weeks.