Subject: bug#42688: Running a script with `guix repl` doesn't "see" additional channels using (%package-module-path)

From: Ludovic Courtès <[email-masked]>
Date: Thu, 17 Sep 2020 17:31:05 +0200

Hi Leo,

Leo Prikler <[email-masked]> skribis:

> I've finally figured out, what causes this issue.
>
> Guix repl uses the following code to call scripts:
> ```
>     (unless (null? script)
>       ;; Run script
>       (save-module-excursion
>        (lambda ()
>          (set-program-arguments script)
>          (set-user-module)
>          (load-in-vicinity "." (car script)))))
> ```
>
> But `guix describe` (which is used to initialize %package-module-path)
> has the following:
>
> ```
> (define current-profile
>   (mlambda ()
>     "Return the profile (created by 'guix pull') the calling process
> lives in,
> or #f if this is not applicable."
>     (match (command-line)
>       ((program . _)
>        (and (string-suffix? "/bin/guix" program)
>            [...])))))
>
> (define current-profile-entries [...])
> (define current-channel-entries [...])
> (define package-path-entries [...])
> ```
>
> Each of these procedures depends on the previous, building up a chain
> that fails exactly in the case where we (set-program-arguments [...])
> with a script other than the current channel's guix (which is probably
> the way you'd want to use `guix repl`).  

Good catch!

> There are some ways of resolving this.  One would be to access earlier
> versions of "command-line" – it does resolve to a fluid, but that fluid
> itself is not exposed to Guile.  Perhaps there might be some FFI magic
> to access it.

‘scm_program_arguments_fluid’ is marked as SCM_INTERNAL, so it’s really
inaccessible.

However, perhaps we could save the initial value of (program-arguments)
in (guix ui) and use that in (guix describe)?

> On the other hand, we could patch `guix repl` to initialize %package-
> module-path earlier (still leaving `guix describe` broken) or somehow
> try to work around that issue in `guix describe`.

Initializing (%package-module-path) earlier sounds like a good idea too,
maybe like this:

diff --git a/guix/scripts/repl.scm b/guix/scripts/repl.scm
index 7d4e474e92..b672489ed6 100644
--- a/guix/scripts/repl.scm
+++ b/guix/scripts/repl.scm
@@ -22,6 +22,7 @@
   #:use-module (guix ui)
   #:use-module (guix scripts)
   #:use-module (guix repl)
+  #:autoload   (gnu packages) (%package-module-path)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-37)
@@ -173,6 +174,10 @@ call THUNK."
   (with-error-handling
 
     (unless (null? script)
+      ;; Before running SCRIPT, initialize %PACKAGE-MODULE-PATH so that it
+      ;; contains the user's channels (the statement triggers an autoload).
+      (%package-module-path)
+
       ;; Run script
       (save-module-excursion
        (lambda ()

?

Thanks!

Ludo’.