Using iterator interface, we can make the optimizer like a white box,

for info in optimize_iterator(func, x0, solver)
    println("Step = ", info["istep"])
    println("x = ", info["x"])
    println("f = ", info["f"])
   solver.PARAMETER = PARAMETER
    ...
end

This kind of interface is very welcome is debugging, tunning optimizer et. al.
Please consider ~

A python version is of white box optimizer is https://github.com/BRML/climin