"un-register" a doParallel cluster
The only official way to "unregister" a foreach backend is to register the sequential backend:
registerDoSEQ()This makes sense to me because you're supposed to declare which backend to use, so I didn't see any point in providing a way to "undeclare" which backend to use. Instead, you declare that you want to use the sequential backend, which is the default.
I originally considered including an "unregister" function, but since I couldn't convince myself that it was useful, I decided to leave it out since it's much easier to add a function than to remove one.
That being said, I think all you need to do is to remove all of the variables from foreach:::.foreachGlobals which is where foreach keeps all of its state:
unregister <- function() { env <- foreach:::.foreachGlobals rm(list=ls(name=env), pos=env)}After calling this function, any parallel backend will be deregistered and the warning will be issued again if %dopar% is called.
cl <- makeCluster(2) registerDoParallel(cl) on.exit(stopCluster(cl))This worked fine for me.