bug#42164: Combining file-append with gexps results in incomprehensible errors

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hi Jakub!

Jakub Kądziołka <kuba@kadziolka.net> writes:

> Consider this minimal operating-system definition:
>
> (use-modules (gnu))
> (use-package-modules gcc)
>
> (operating-system
>   (host-name "test")
>   (timezone "UTC")
>   (bootloader (bootloader-configuration
>                 (bootloader grub-efi-bootloader)
>                 (target "/boot/efi")))
>   (file-systems (cons*
>                   (file-system
>                     (device (file-system-label "root"))
>                     (mount-point "/")
>                     (type "ext4"))
>                   %base-file-systems))
>   (packages '())
>   (services (cons*
>               (service special-files-service-type
>                        `(("/lib64" ,(directory-union "rustup-libs"
>                                       (list
>                                         (file-append glibc "/lib")
>                                         (file-append #~#$gcc:lib "/lib"))))))
>               %base-services)))
>
> I would expect this way of specifying a specific output of a package to
> work, but it results in the following error:
>
> ice-9/boot-9.scm:1515:18: object is not an exception of the right type #<&gexp-input-error input: #<gexp #<gexp-input #<package gcc@7.5.0 gnu/packages/gcc.scm:520 7f06c996c960>:lib> 7f06c6b06990>> #<record-type &package-input-error>

[...]

> I have figured out that the following does work:
>
>               (service special-files-service-type
>                        `(("/lib64" ,#~(directory-union "rustup-libs"
>                                         (list
>                                           (string-append #$glibc "/lib")
>                                           (string-append #$gcc:lib "/lib"))))))
>
> However, I would expect the other variants to work as well. The
> documentation and error messages are lacking in this regard.

This seems to come up every time I have to specify a non-default package
output; I've investigated the issue a bit and it seems to be caused by
the lack of a gexp-compiler for the gexp object resulting from
#$package:output, which is of type gexp-input as represented by, for
example:

#<gexp-input #<package opendht@2.2.0rc4 /home/maxim/src/guix/gnu/packages/networking.scm:3426 7f2dbcae1aa0>:tools>

From the REPL

(lower-object (gexp (ungexp opendht "tools")))
$17 = #<procedure 7f2db56ed100 at guix/gexp.scm:246:2 (state)>
scheme@(gnu services jami)> ,run-in-store $17
While executing meta-command:
ERROR:
  1. &gexp-input-error: #<gexp #<gexp-input #<package opendht@2.2.0rc4
  /home/maxim/src/guix/gnu/packages/networking.scm:3426
  7f2dbcae1aa0>:tools> 7f2db7427690>
  
So if we were to define/register a gexp-compiler for this kind of
gexp-input, we could make it work, but I haven't yet investigated how
easy/difficult that would be.

Seems worthwhile though, this issue is quite annoying when attempting to
work with specific outputs!

Thanks for the detailed report.

Maxim