Re: Add external data to a package

[Parnikkapore]

A lot of objects can be ungexp-ed into a gexp to substitute in its store path. 
`<origin>`s are one of them:

```scheme
(define WSRT_Measures
  (origin
   (method url-fetch)
   (uri "[...not sure I can disclose...]/WSRT_Measures.ztar")
   (sha256
     (base32 "1d9qlcmkpsnw8wm7sidp17iwd2d771nsgjwwp568886g5f239a8k"))))

(define-public casacore
  (package
   (name "casacore")
   (version "3.5.0")
   (source
     (origin
       (method git-fetch)
       (uri (git-reference
              (url "https://github.com/casacore/casacore.git";)
              (recursive? #t)
              (commit "v3.5.0"))))
    (file-name (git-file-name name version))
    (sha256 (base32 "0zw70slc3681mnadg169avjg7d3jwn1x49xn0022207blcig8g2l")))
   (inputs
     (list fftwf fftw openblas boost gcc-toolchain git gsl lapack 
gfortran-toolchain readline flex bison cfitsio wcslib python python-numpy))
   (build-system cmake-build-system)
   (arguments (list
               #:configure-flags #~(list "-DHELLO_WHO=Guix" "-DBUILD_DYSCO=ON")
               #:build-type "Release"
               #:tests? #f ;; some test are failing due to lack of data that 
are downloable via ftp; for now testing is disabled
               #:parallel-build? #t ;; TODO : check if it makes a difference in 
building time
               #:phases
               #~(modify-phases %standard-phases
                   (add-after 'install 'install-data
                     (lambda* (#:key inputs outputs #:allow-other-keys)
                       (let* ((out (assoc-ref outputs "out"))
                              (data #$WSRT_Measures)  ; <- ===== See here! 
================
                              (share-dir (string-append out 
"/share/casacore/data/")))
                         (mkdir-p share-dir)
                         (invoke "tar" "-xzf" data "-C" share-dir)
                         #t))))))
   (home-page "http://casacore.github.io/casacore/";)
   (synopsis "")
   (description "A suite of C++ libraries for radio astronomy data processing.")
   (license license:gpl2+)))
```

Yeah, this is not well-explained in the documentation right now.

(Also took the opportunity to reformat the code from the email. There seems to 
be quite a few mismatching parentheses, which would've led to some very 
confusing error messages... my advice is turning on the Lisp / Scheme mode of 
your editor and turning on the "rainbow parentheses" option.)