How to cache the result of guix-for-channels?

[Tomas Volf]

Hi all,

I am finally trying to incorporate `guix-for-channels' into my system
configuration and cannot figure out how to make it cache the resulting
package.

Following snippet illustrates the problem:

--8<---------------cut here---------------start------------->8---
/tmp/repro $ cat x.scm
(use-modules (guix channels)
             (gnu packages package-management))

(guix-for-channels (list (channel
                          (name 'guix)
                          (url "https://git.wolfsden.cz/.git/guix";)
                          (commit
                           "b03eddc326ee4eb26b25743faee2080de6aded7e")
                          (introduction
                           (make-channel-introduction
                            "028e445a2028068e3c83996daa281057f19141a0"
                            (openpgp-fingerprint
                             "B783 49B3 8C14 7D36 2988  68A4 2FBF EE7D B67F 
C1A9"))))))
/tmp/repro $ guix build -f x.scm
Updating channel 'guix' from Git repository at 
'https://git.wolfsden.cz/.git/guix'...
Computing Guix derivation for 'x86_64-linux'... |
/gnu/store/9i597ckynirxy4w5iv2wbmy288vc7gf0-profile
/tmp/repro $ guix build -f x.scm
Updating channel 'guix' from Git repository at 
'https://git.wolfsden.cz/.git/guix'...
Computing Guix derivation for 'x86_64-linux'... |
/gnu/store/9i597ckynirxy4w5iv2wbmy288vc7gf0-profile
--8<---------------cut here---------------end--------------->8---

Notice that the work (updating channel & computing the derivation) is
done both times (resulting in the same profile).  It is fairly expensive
and, since I am specifying commit, does not seem necessary.

I have tried few variations, including attempting to misuse
cached-channel-instance, (origin (method git-fetch) ...) and few other,
but none ended up with desired result.



So, have can I make guix-for-channels cache the package?  Or rather, how
can I get package suitable for guix-configuration (does not have to be
via guix-for-channels) that is cached instead of being built every time?

Maybe something from (guix inferior) would help?
cached-channel-instance seems to do pretty much what I want I presume,
however not sure how to convert store path back into a package (that I
could stuff into guix-configuration's guix field).

Have a nice day,
Tomas

-- 
There are only two hard things in Computer Science:
cache invalidation, naming things and off-by-one errors.

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