How to install previous versions?

[Zelphir Kaltstahl <zelphirkaltstahl@posteo.de>]

Hello Guix Users!

I have not figured out yet how to install a specific version of a
package, that is not the newest available version.

For example I do the following:

~~~~
guix environment --ad-hoc 'python-redis@3.3.8' 'python@3.8.2' -- python3.8
~~~~

I had this version of python-redis in a manifest.scm file. However,
there seems to have been an update and now this manifest file cannot be
used any longer, as it references an old version. It will tell me the
same error as the one-liner above:

~~~~
guix environment: error: python-redis: package not found for version 3.3.8
~~~~

However, I think that is strange, because I can definitely see, that
version does exist on
http://data.guix.gnu.org/repository/1/branch/master/package/python-redis.
There are now the versions:

'(3.5.3 3.3.8 3.2.1 3.2.0 2.10.6)

So how would I need to write my manifest.scm or my one-liner to get back
to that older version?

I would like to have reproducible environments and I think that is also
a goal of GNU Guix package manager? Otherwise how could it ever
guarantee, that on 2 different machines at 2 different points in time, I
can build the same environment? A package could have been updated in
between the 2 setups and then the previously latest version would become
unavailable. I must be missing something.

Best regards,
Zelphir