doMC vs doSNOW vs doSMP vs doMPI:为什么“foreach”在功能上的相同之处不同?

I've got a few test pieces of code that I've been running on various machines, always with the same results. I thought the philosophy behind the various do... packages was that they could be used interchangeably as a backend for foreach's %dopar%. Why is this not the case?I've got a few test pieces of code that I've be