(use-modules (srfi srfi-1)
             (ice-9 match)
             (guix packages)
             (gnu packages))

(define (package+propagated-inputs package)
  (match (package-transitive-propagated-inputs package)
    (((labels packages) ...)
     (cons package packages))))

(define (something)
  (list 42 43 44))

(define (an-example)
  (define build
    (with-extensions (package+propagated-inputs
                      (specification->package "guile-json"))
      (with-imported-modules '((guix build utils))
        #~(begin
            (use-modules (guix build utils)
                         (json))

            (define that
              ;; 'something' returns a list of integers; we need to quote
              ;; that list: it's data, not code, that we are inserting here.
              '#+(something))

            (define file-name
              (string-append #$output "/example.json"))

            (mkdir-p (dirname file-name))
            (with-output-to-file file-name
              (lambda ()
                (scm->json (list->vector that))))))))

  (computed-file "example-json"
                 build))

(manifest
 (list (manifest-entry
         (name "bang")
         (version "0")
         (item (an-example)))))