[R-pkg-devel] cross-ref possible ext.pkg's --not mentioned in DESCRIPTION file-- in non-code sections in .Rd files

[Peter Ruckdeschel]

Hi,

triggered by some new NOTEs uncovering infallacities in my packages on CRAN, I stumbled upon

Result: NOTE     Undeclared packages ‘distrMod’, ‘RobAStBase’ in Rd xrefs

which is issued in R-devel-linux-x86_64-fedora-clang (but not otherwise).

Digging into it, this comes from a text in a \seealso section where through something like

\code{\{link}[<ext.pkg>]{<generic>}

we want to link to a method provided in some extension package <ext.pkg> for a generic
<generic> documented in Rd-file of the current package.

In this case, we intentionally do not have <ext.pkg> in any of the entries "Depends", "Suggests",
"Imports", "Enhances" in the DESCRIPTION file, as <ext.pkg> is neither needed visible for the
user, nor for the code in the current package nor for testing code nor for example code and vignettes
of the current package, and because having it in "Suggests" would produce a circular dependence
as <ext.pkg> has the current package in its "Imports" section.

Of course, in "Writing R extesions" we find that

"Packages referred to by these ‘other forms’ should be declared in the DESCRIPTION file, in
   the ‘Depends’, ‘Imports’, ‘Suggests’ or ‘Enhances’ fields. "

so I am aware that this is my fault, as I use these 'other forms'  without declaration in the DESCRIPTION file.
Now I guess I could wrap my conditional cross-ref into some R-code, testing whether <ext.pkg> is available,
i.e., something like

\Sexpr[eval=TRUE,stage=render,results=text]{if("<ext.pkg>" %in% installed.packages()[,1] )
  cat("\\code{\\{link}[<ext.pkg>]{<generic>}") else cat("<generic> from package \\pkg{<ext.pkg>})

and so produce the link only if <ext.pkg> is installed at rendering time, but I would guess such a
conditional linkage would be something which could be of interest to a broader scope of R-package
developers and hence could and should be implemented in a less cumbersome, and more
efficient way.

Any suggestions welcome,
best, Peter

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