How to unload a package without restarting R

I'd like to unload a package without having to restart R (mostly because restarting R as I try out different, conflicting packages is getting frustrating, but conceivably this could be used in a program to use one function and then another--although namespace referencing is probably a better idea for that use).

?library doesn't show any options that would unload a package.

There is a suggestion that detach can unload package, but the following both fail:

detach(vegan)

Error in detach(vegan) : invalid name argument

detach("vegan")

Error in detach("vegan") : invalid name argument

So how do I unload a package?

Answer

Try this (see ?detach for more details):

detach("package:vegan", unload=TRUE)

---

It is possible to have multiple versions of a package loaded at once (for example, if you have a development version and a stable version in different libraries). To detach guarantee that all copies are detached, use this function.

detach_package <- function(pkg, character.only = FALSE)

{
  if(!character.only)
  {
    pkg <- deparse(substitute(pkg))
  }
  search_item <- paste("package", pkg, sep = ":")
  while(search_item %in% search())
  {
    detach(search_item, unload = TRUE, character.only = TRUE)
  }

}

Usage is, for example

detach_package(vegan)

or

detach_package("vegan", TRUE)