Hi Guix,

attached is a patch to add a new importer for R packages published on
CRAN.  (I still have to add tests for this.)

The importer fetches the HTML description of an R package from
http://cran.r-project.org, converts it to SXML, and then tries to
extract the relevant information from the SXML expression.

When I finished I thought that maybe this could be done with less effort
by downloading the tarball from CRAN, extracting the plain text
DESCRIPTION file containing the same information and parse that instead.
But in order to get the tarball I’d have to parse the HTML anyway, so
maybe that’s not so much better after all.

This generates package expressions using the r-build-system — this build
system does not yet exist, but I’ll prepare one soon, not least to test
the importer.

Here’s an example:

$ ./pre-inst-env guix import cran ggplot2
following redirection to `http://cran.r-project.org/web/packages/ggplot2/'...
starting download of `/tmp/guix-file.QGL35J' from `http://cran.rstudio.com/src/contrib/ggplot2_1.0.1.tar.gz'...
http://cran.rstudio.com/.../ggplot2_1.0.1.tar.gz	100.0% of 2296.1 KiB (1203. KiB/s)
(package
  (name "r-ggplot2")
  (version "1.0.1")
  (source
    (origin
      (method url-fetch)
      (uri (string-append
             "mirror://cran/src/contrib/ggplot2_"
             version
             ".tar.gz"))
      (sha256
        (base32
          "0794kjqi3lrxb33lr1mykd58959hlgkhdn259vj8fxrh65mqw920"))))
  (build-system r-build-system)
  (inputs
    `(("r-digest" ,r-digest)
      ("r-gtable" ,r-gtable)
      ("r-mass" ,r-mass)
      ("r-plyr" ,r-plyr)
      ("r-proto" ,r-proto)
      ("r-reshape2" ,r-reshape2)
      ("r-scales" ,r-scales)))
  (home-page "http://ggplot2.org")
  (synopsis
    "An Implementation of the Grammar of Graphics")
  (description
    "An implementation of the grammar of graphics
in R.  It combines the advantages of both base and
lattice graphics: conditioning and shared axes are
handled automatically, and you can still build up a
plot step by step from multiple data sources.  It also
implements a sophisticated multidimensional
conditioning system and a consistent interface to map
data to aesthetic attributes.  See http://ggplot2.org
for more information, documentation and examples.")
  (license gpl2))

Without further ado: the patch.