> Hi,
> 
> Looking at this definition in gnu/system.scm [1] I am trying to figure
> out what 'os' is:
> 
>   (define* (operating-system-kernel-arguments
>             os root-device #:key (version %boot-parameters-version))
>     "Return all the kernel arguments, including the ones not specified directly
>   by the user.  VERSION should match that of the target <boot-parameters> record
>   object that will contain the kernel parameters."
>     (append (bootable-kernel-arguments os root-device version)
>             (operating-system-user-kernel-arguments os)))
> 
> The same file also contains a record definition for <operating-system>
> so it seemed reasonable to assume that 'os' referred to such a
> record. In fact, the second procedure inside the 'append' above,
> operating-system-user-kernel-arguments, is one of the accessors [2]
> (even though the name does not match the field).

Yes, it is supposed to be an <operating-system> record (of course,
scheme does not enforce that in any way and it may end up being any
other type during runtime if there is a bug in some other code).

> In the first procedure bootable-kernel-arguments [3] however, 'os'
> (which is called 'system' there) is used like a string, although inside
> a gexp:
> 
>   (define* (bootable-kernel-arguments system root-device version)
>     "Return a list of kernel arguments (gexps) to boot SYSTEM from ROOT-DEVICE.
>   VERSION is the target version of the boot-parameters record."
>     ;; If the version is newer than 0, we use the new style initrd parameter
>     ;; names, otherwise we use the legacy ones.  This is to maintain backward
>     ;; compatibility when producing bootloader configurations for older
>     ;; generations.
>     (define version>0? (> version 0))
>     (list (string-append (if version>0? "root=" "--root=")
>                          ;; Note: Always use the DCE format because that's what
>                          ;; (gnu build linux-boot) expects for the 'root'
>                          ;; kernel command-line option.
>                          (file-system-device->string root-device
>                                                      #:uuid-type 'dce))
>           #~(string-append (if #$version>0? "gnu.system=" "--system=") #$system)
>           #~(string-append (if #$version>0? "gnu.load=" "--load=")
>                            #$system "/boot")))
> 
> I know objects in the store become paths when unquoted via '#$'. Does
> that also work for Guix records declared via define-record-type* [4]
> (please note the asterisk)? Thanks!

The <operating-system> record is expanded to a store path because there
is a (define-gexp-compiler ...) form for it in gnu/system.scm[1]. The
gexp expansion mechanism is explained quite well in a "Dissecting Guix"
blog post[2].

[1]: https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/system.scm#n1623
[2]: https://guix.gnu.org/en/blog/2023/dissecting-guix-part-3-g-expressions/