Documentation of FiveArg()

[ruairidhs]

If we set the trait FiveArg(), is it also necessary to implement the three argument mul!? Something like:

LinearMaps.MulStyle(A::MyMap) = FiveArg()

function LinearMaps._unsafe_mul!(y, A::MyMap, x::AbstractVector, α, β)
    ...
end

LinearMaps._unsafe_mul!(y, A::MyMap, x::AbstractVector) = LinearMaps._unsafe_mul!(y, A, x, 1, 0)

If so, it would be useful to note this in the "Custom Maps" section of the documentation. If you don't need to do this then I have some other issue... :)

[dkarrasch]

Sorry for my slow response. That's a good question, I haven't looked things in here for quite a while. It seems that within this package, even the FiveArg() types have the three-arg _unsafe_mul! defined, but that may be "by tradition" or "by necessity". I think we introduced it to indicate whether or not one can do multiply-and-add in-place, without allocating the intermediate multiplication result. I never thought about it in terms of "saving" the three-arg method.

If you don't, which issue comes up?