outline of complex 2nd order accumulator

acc.jl

type var{T,S}
    sum::T
    sumsqr::S
    nobjs::Int64
    var() = new(0,0,0)
end

var_cmplx(T=Float64) = var{Complex{T},T}()
var_scalar(T=Float64) = var{T,T}()

function fit!{T,S}(v::var{T,S}, z::T)
    v.sum += z
    v.sumsqr += abs2(z)
    v.nobjs += 1
end

function fit!{T,S}(v::var{T,S}, z::AbstractArray{T})
    for i in eachindex(z)
        fit!(v, z[i])
    end
end

#overload + to call fit!
import Base.+
function +{T,S}(v::var{T,S}, z)
    fit!(v, z)
    return v
end

function variance{T,S}(v::var{T,S})
    return v.sumsqr/v.nobjs - abs2(v.sum/v.nobjs)
end

function mean{T,S}(v::var{T,S})
    return v.sum/v.nobjs
end

This version unified for both scalar and complex