with_package is detaching the package if it was attached before call

[moodymudskipper]

I believe it's not expected :

reprex :

library(glue)
x <- "hello"
glue("{x} world")
#> hello world
withr::with_package("glue", glue("{x} world"))
#> hello world
glue("{x} world")
#> Error in glue("{x} world"): could not find function "glue"

^{Created on 2019-06-26 by the reprex package (v0.3.0)}

[jimhester]

If the package is already attached there isn't much point in using with_package() though...

[moodymudskipper]

Sure, but it could used in a function, and the user doesn't call it directly and doesn't know that it has been called, and then the function would have a mysterious side effect.

[jonathan-g]

I ran into this issue with a package I'm writing that will allow the user to specify packages to be temporarily loaded when calling knitr::knit_expand() to expand R macros in a template.

Code like that in #138 is a simple workaround, so there's no reason to change the code in withr:

if (package %in% .packages()) {
  eval(code)
} else {
  withr::with_package(package, code)
}

But it might be nice to mention the side effect in the documentation for withr::withr, which currently says

Upon completion or error, the global environment is restored to the previous state.

[mllg]

+1 for only detaching the package if it has been attached by withr. The package description says "Methods For Temporarily Modifying Global State", but with_package() is performing a permanent global state change.