Abbé <gnu.bonfire@p.atriar.ch> writes:

> (define-configuration/no-serialization home-rbw-configuration
>   (pinentry-program
>    (file-like (file-append pinentry "/bin/pinentry-curses"))
>    "Pinentry program to use.")
>   (email
>    (string "")
>    "Email address of the user")

...

>   (define (rbw-config config)
>     (match-record config <home-rbw-configuration>
>                   (pinentry-program email base-url identity-url notifications-url
>                                     client-cert-path sync-interval lock-timeout)
>                   `((pinentry-program . ,pinentry-program)
>                     (email . ,email)
>                     (base_url . ,base-url)
>                     (identity_url . ,identity-url)
>                     (notifications_url . ,notifications-url)
>                     (client_cert_path . ,client-cert-path)
>                     (sync_interval . ,sync-interval)
>                     (lock_timeout . ,lock-timeout))))
>   (define (home-rbw-configuration-file config)
>     (let ((xformed-config (rbw-config config)))
>       (computed-file "rbw-config.json" 
>
>                      #~(call-with-output-file #$output
>                          (lambda (port)
>                            (display #$(scm->json-string xformed-config) port))))))

Here you're calling scm->json-string on an alist containing the
file-append record for pinentry-program. scm->json-string doesn't know
how to handle that, it's invalid. Hence the exception you're getting.

>   `(("rbw/config.json" ,(home-rbw-configuration-file config))))

...

> While applying this module, I end up with following obvious error, but
> I'm not quite sure how to go about resolving this:
>
> ----------------8<----------------------8<-----------------------
> ice-9/boot-9.scm:1685:16: In procedure raise-exception:
> Throw to key `json-invalid' with args `(#<file-append #<package pinentry@1.2.1 gnu/packages/gnupg.scm:1016 7df6139219a0> "/bin/pinentry-curses">)'.
> ----------------8<----------------------8<-----------------------

If you want to use scm->json-string, you need to be working with a data
structure it can understand, and it can't understand gexp's. That
suggests you should run it on the build side, once the file-append
record has been transformed in to a string.

I think this would be similar to how the transmission service in Guix
builds it's settings.json file.