[go-build-system] How to access properties or keys of a package on the build side?

[Leo Famulari <leo@famulari.name>]

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Recently I made some progress on finishing the prototype go-build-system
from Petter [0], and I need some advice.

AFAICT, building a Go program requires the program's Go dependencies to
have a particular filesystem layout, which corresponds to the way these
dependencies are imported by the program's code.

So, a Go build system needs to somehow create a symlink union of the
dependency graph, making the dependencies available at the correct paths
relative to the root of the build environment. AFAICT, we can't figure
these paths out programatically; they must be supplied by the packager
in advance, for example:

------
(arguments
  `(#:import-path "github.com/AudriusButkevicius/go-nat-pmp"))
------

Petter's prototype creates the symlink union, but instead of using the
#:import-path key, it instead duplicates the import path while listing a
package's dependencies, like this:

------
(inputs
 `(("github.com/AudriusButkevicius/go-nat-pmp"
    ,golang-github-com-audriusbutkevicius-go-nat-pmp)
------

... putting the import path in the car of the input, and using it as shown
below:

------
(define* (symlinking #:key inputs #:allow-other-keys)
  (for-each (lambda (input)
              (let ((import-path (car input))
                    (storepath (cdr input)))
                ; Don't deal with the compiler or source inputs
                (if (and (not (string=? import-path "go"))
                         (not (string=? import-path "source")))
                    (begin
                      (mkdir-p (string-append
                                "src/"
                                (string-take import-path
                                             (string-rindex import-path #\/))))
                      (let ((from (string-append storepath "/src/" import-path))
                            (to (string-append "src/" import-path)))
                        (if (file-exists? to) (delete-file-recursively to))
                        (symlink (string-append
                                  storepath "/src/" import-path)
                                 (string-append
                                  "src/" import-path)))))))
            inputs))
------

Are there any examples in the code base of accessing the inputs' keys? That
seems like a better solution to me, but I'm not sure how to achieve it.

[0]
https://lists.gnu.org/archive/html/guix-devel/2016-12/msg00399.html

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