[Rd] R CMD check and missing imports from base packages

[Gábor Csárdi]

When a symbol in a package is resolved, R looks into the package's
environment, and then into the package's imports environment. Then, if the
symbol is still not resolved, it looks into the base package. So far so
good.

If still not found, it follows the 'search()' path, starting with the
global environment and then all attached packages, finishing with base and
recommended packages.

This can be a problem if a package uses a function from a base package, but
it does not formally import it via the NAMESPACE file. If another package
on the search path also defines a function with the same name, then this
second function will be called.

E.g. if package 'ggplot2' uses 'stats::density()', and package 'igraph'
also defines 'density()', and 'igraph' is on the search path, then
'ggplot2' will call 'igraph::density()' instead of 'stats::density()'.

I think that for a better solution, either
1) the search path should not be used at all to resolve symbols in
packages, or
2) only base packages should be searched.

I realize that this is something that is not easy to change, especially 1)
would break a lot of packages. But maybe at least 'R CMD check' could
report these cases. Currently it reports missing imports for non-base
packages only. Is it reasonable to have a NOTE for missing imports from
base packages as well?

[As usual, please fix me if I am missing or misunderstood something.]

Thank you, Best,
Gabor

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