Re: Improving the speed of guix time-machine for development environments

[Richard Sent]

Hi Guix!

> I think channels listed in channels.scm files should optionally support
> using whatever channel version is already in the user's profile, if
> present. This would significantly speed up entering development
> environments composed of Guix channels.

> Unfortunately, *I may be delayed every time I enter the development
> environment by several minutes while Guix computes a new derivation
> due to an update in %default-channels*.

I wound up implementing a proof of concept for this idea here:

https://www.freakingpenguin.com/blog/guix-tricks-self-referential-channelsscm-files.html

--8<---------------cut here---------------start------------->8---
(define (soft-pin-channel input-channel)
  "Given @var{channel}, return a new version of channel that uses the
current profile's commit of said channel. If the channel is not present,
commit is #f."
  (define (desired-channel? channel)
    (equal? (channel-name input-channel)
            (channel-name channel)))

  (define (profile-commit input-channel)
    (call/ec
     (lambda (return)
       (map (lambda (x)
              (when (desired-channel? x)
                (return (channel-commit x))))
            (current-channels))
       #f)))

  (channel
   (inherit input-channel)
   (commit (profile-commit input-channel))))
--8<---------------cut here---------------end--------------->8---

If people think this is useful, I'm happy to flesh out the idea more and
submit a proper patch. In my experience this does improve the
development experience when setting up developer tools that rely on
custom channels. At least when there's a new Guix commit, I don't have
to wait several minutes for a new Guix channel collection to build, but
without pinning it to a specific commit.

At present I can think of soft-pinning a list of channels by name being
useful. I can also see an option to only fetch dependencies of a
channel, not the channel itself, being useful. I would definitely
appreciate feedback or use cases! :)

-- 
Take it easy,
Richard Sent
Making my computer weirder one commit at a time.