What does the r function requirenamespace() mean?

Question

Every package installation instruction starts with:

if (!requireNamespace("BiocManager", quietly = TRUE))
  install.packages("BiocManager")

I was wondering what the requireNamespace() function means?

The definition given in the documentation doesn't make much sense to me. How is requireNamespace() different to loadNamespace?

a wrapper for loadNamespace analogous to require that returns a logical value"

And are the following two lines always necessary to precede each new package installation?

if (!requireNamespace("BiocManager", quietly = TRUE))
  install.packages("BiocManager")

Answer 1

requireNamespace() is a wrapper around loadNamespace() which returns a logical value. loadNamespace() loads the specified name space and registers it in an internal data base. (See function documentation accessible by running ?requireNamespace.)

It does not attach the namespace to the search path. So, assuming I have the package "vegan" installed and I run:

requireNamespace("vegan")
as.mlm()

It will return:

Error in as.mlm() : could not find function "as.mlm"

The first line, i.e., requireNamespace() itself returns TRUE, if the package is installed. E.g., you could use that output to install a package if it is missing:

if (!requireNamespace("vegan") {
  install.packages("vegan")
}

If you do not check whether a package is already available, R will try to install it each time you run the script. Depending on the package, this may take quite some time (or cause errors, if you are not connected to the internet or for other reasons). This can be quite annoying, so, while checking the availability of a package prior to installing it is not strictly neccessary, it is generally good practice I would say.

If, instead, I run something like

require("vegan")
as.mlm()

I will (as of the current vegan version) get a message that the function I want to use is deprecated, because the function is actually available and R tries to do something with it.

Thus, you can use requireNamespace() to verify some package exists while preventing potential namespace conflicts.