How to lower a record to the build code ?

[edk]

Hi,

I've been stuck for a few days on the following:
Let's say I have a record type:

(define-record-type* <my-record> my-record make-my-record
  my-record?
  this-record
 (first-field my-record-first-field)
 (second-field my-record-second-field))

And a function that uses such a record, but needs to run on the build
side, because it also needs the store path of a package (I can't edit
this function):

(define (function-of-a-record-and-a-build-time-path rec path)
  "Concatenate the path, first, and second field"
  (string-append path " " (my-record-first-field rec) " " (car
  (my-record-second-field rec)) " " (cdr (my-record-second-field rec))))

How can I use this record in the build side. For example, I'm unable to
build the following G-exp:
(define a-record (my-record
              (first-field "first")
              (second-field '("second" . "third"))))


#~(with-output-to-file (string-append #$output "/file.txt")
     (lambda _
       (display (function-of-a-record-and-a-build-time-path #$a-record
     #$bash)))))

I expect the gexp to compile to a file in the store whose contents are
/gnu/store/....bash/ first second third

The error is   1. &gexp-input-error: #<<my-record> first-field: "first"
second-field: ("second" . "third")>

I tried to define a gexp-compiler, but I have clearly no idea of what
I'm doing and I could sometimes make it work if I convert the record
into a list, but not if there is a list within that list.

I've read the relevant sections of the manual, and the code, and
basically all instances of a call to define-gexp-compiler in the source
code, but nothing comes close to what I need.

I really could use some help,

Thank you very much,

Edouard.

P.S. here is a minimal not-working example that you can copy paste in
guix repl in order to reproduce my problem:


(use-modules
 (guix gexp)
 (guix store)
 (guix derivations)
 (guix records)
 (guix monads)
 (gnu packages bash))
(define (build-gexp gexp)
  "Build GEXP using the local daemon."
  (let ((derivation
         (run-with-store (open-connection)
           (gexp->derivation
            "noname"
            gexp))))
    (build-derivations
     (open-connection)
     (list derivation))
    (derivation-output-path (assoc-ref (derivation-outputs derivation) "out"))))
;; There is a record type that I need on the build side
(define-record-type* <my-record> my-record make-my-record
  my-record?
  this-record
 (first-field my-record-first-field)
 (second-field my-record-second-field))
(define a-record (my-record
              (first-field "first")
              (second-field '("second" . "third"))))
(define (function-of-a-record-and-a-build-time-path rec path)
  "Concatenate the path, first, and second field"
  (string-append path " " (my-record-first-field rec) " " (car 
(my-record-second-field rec)) " " (cdr (my-record-second-field rec))))

;;   1. &gexp-input-error: #<<my-record> first-field: "first" second-field: 
("second" . "third")>
(build-gexp
 #~(with-output-to-file (string-append #$output "/file.txt")
     (lambda _
       (display (function-of-a-record-and-a-build-time-path #$a-record 
#$bash)))))