[R-pkg-devel] Convention or standards for using header library (e.g. Eigen)

[Stephen Wade]

I recently submitted a package to CRAN which downloaded Eigen via Makevars
and Makevars.win. My Makevars.ucrt was empty as I noted that Eigen3 is
installed by default (however, this doesn't ensure that a version of Eigen
compatible/tested with the package is available).

The source is currently on github:
https://github.com/stephematician/literanger

Here is the Makevars

$ more src/Makevars
# downloads eigen3 to extlibs/ and sets include location
PKG_CPPFLAGS = -I../src -I../extlibs/
.PHONY: all clean extlibs
all: extlibs $(SHLIB)
extlibs:
         "${R_HOME}/bin${R_ARCH_BIN}/Rscript" "../tools/extlibs.R"
clean:
         rm -f $(SHLIB) $(OBJECTS)

The details of `extlibs.R` are fairly mundane, it downloads a release from
gitlab and unzips it to `extlibs`.

CRAN gave me this feedback:

> Why do you download eigen here rather than using the system version of
> Eigen if available?
>
> We asked you to do that for Windows as you did in Makevars.ucrt. For
> Unix-like OS you should only fall back (if at all) to some download if
> the system Eigen is unavailable.

The problem is I'm not sure what a minimum standard to 'searching' for a
system version of Eigen looks like. I also note that packages like
RcppEigen simply bundle the Eigen headers within the package (and its
repository) which will certainly ignore any system headers.

I would like a solution that would keep CRAN happy, i.e. i need to meet
some standard for searching for the compiler flags, checking the version of
the system headers, and then falling through to download release if the
system headers fail.

1.  For each platform (Unix, Windows, OS-X) what tool(s) should be invoked
to check for compiler flags for a header-only library like Eigen? e.g.
pkg-config, pkgconf? others?
2.  What is a reasonable approach for the possible package names for Eigen
(e.g. typically libeigen3-dev on Debian, and eigen3 on arch, homebrew,
others)? Is this enough?
3.  If pkg-config/pkgconf (or others) are unavailable, what is a reasonable
standard for checking if the library can be built with some reasonable
guess for the compiler flags (probably empty) - I assume I would need to
try to compile a test program (within Makevars)?
4.  Following on from 3... would a package need to check (again via a test
program) that the _system_ headers have the correct version (e.g. some
static assert on EIGEN_WORLD_VERSION), and if that fails _then_ download
the release from gitlab?

Any and all advice would be appreciated.

Kind regards,
-Stephen Wade

	[[alternative HTML version deleted]]