I don't think one can do proper, robust Event Sourcing without having first a few years experience in hardcore functional programming (particularly of the varia

I don't think one can do proper, robust Event Sourcing without having first a few years experience in hardcore functional programming (particularly of the variant that strictly segregates side-effects), something most of us lack. OO and loose FP are fine for a (huge) variety of problems, but the hardest problems need the next level of correctness and elegance. Else you end up authoring yet another Rube Goldberg machine.