Find installed library version of GNU Guix commit x

[Zelphir Kaltstahl <zelphirkaltstahl@posteo.de>]

Hello Guix users!

I would like to know a command line way of finding the version of a
library, which I install, when I am using time-machine as follows:

~~~~
guix time-machine \
     --channels="${DIR}/channels.scm" -- \
     environment \
     --manifest="${DIR}/manifest.scm"
~~~~

Where DIR is of course some directory in which a channels.scm and
manifest.scm file are.

I initially tried:

~~~~
guix time-machine --channels=/home/xiaolong/dev/guile/fantasy-vacation-planner/guix-env/channels.scm -- package -I
~~~~

But that does not give the expected output and lists versions of things
not specified in the manifest file, as well as not showing the versions
of the things specified in the manifest file. It apparently relates to
the base profile.

Then I looked at the websites:

http://data.guix.gnu.org/repository/1/branch/master/package/guile-json

OK here I see the library I am specifying in my manifest file. But how
to know, between which 2 commits the revision (or commit id) is, which
is specified in my channels file? Then I tried changing the URL:

http://data.guix.gnu.org/repository/1/commit/8518a3692cbb9cd96d69c03e9de9ad6fdcfebbee/package/guile-json

Nope, this does not exist. But hey, I can click on one of the displayed
links to a revision and then change that URL!

http://data.guix.gnu.org/revision/f9e1baa8933ea2ed385d0272dbfbe4e22f192bde

Becomes:

http://data.guix.gnu.org/revision/8518a3692cbb9cd96d69c03e9de9ad6fdcfebbee

And there I found a link "View package derivations":

http://data.guix.gnu.org/revision/8518a3692cbb9cd96d69c03e9de9ad6fdcfebbee/package-derivations

Perhaps those "Package derivations" are what I am looking for. So I
search for guile-json in the first input field "Search query".
Unfortunately now several versions come up and now I am stuck. Which
version am I installing?

Here are contents of my manifest and channels files:

~~~~
(specifications->manifest
 '("guile"
   "guile-json"))
~~~~

(no versions specified, as those are probably redundant, because the
channel is specified with a precise commit)

~~~~
(list (channel
        (name 'guix)
        (url "https://git.savannah.gnu.org/git/guix.git")
        (commit
          "8518a3692cbb9cd96d69c03e9de9ad6fdcfebbee")
        (introduction
          (make-channel-introduction
            "9edb3f66fd807b096b48283debdcddccfea34bad"
            (openpgp-fingerprint
              "BBB0 2DDF 2CEA F6A8 0D1D  E643 A2A0 6DF2 A33A 54FA")))))
~~~~

So I already learned today:

1. Revisions seem to be the same thing as commit ids in the version
control system. Just a different name for the same thing.

2. Apparently package derivations are "the versions following from a git
commit of Guix", but there can be more shown, than I expected.

Can you help me Guix noob out?^^

Thank you!
Zelphir

-- 
repositories: https://notabug.org/ZelphirKaltstahl