Are Dual4 and GraDual trying to do the same thing?

@mlubin: I saw the work on Dual4 you started, and that looks VERY exciting to me. I assume you eventually want to use tuples (i.e. fixed size arrays) to store the epsilons, so that this would essentially be a DualN datatype, right?

I am just wondering how that relates to the GraDual type in ForwardDiff.jl? Is that essentially exactly the same thing? Except for the implementation details. Right now GraDual uses an array to store the epsilons, which probably makes it very slow. But once fixed size arrays are part of core julia, one would just have to change the implementation details and probably make this really fast?

I guess this issue is just to clarify whether these two efforts could be unified. In some way it seems to me that this kind of functionality actually fits better into the ForwardDiff.jl package, but not sure.

Also pinging @scidom.

Compare the Dual4 work with ForwardDiff.jl's GraDual implementation in src/typed_fad/GraDual.jl. First determine whether the two efforts represent the same functionality and whether fixed-size epsilon storage changes the comparison. Done means documenting a clear decision on unification and which package should own the functionality.