bug#52749: G-expressions don't consistently preserve #nil

[Philip McGrath]

G-expressions currently do not consistently preserve the distinction 
between #nil and '(), which causes trouble for programs that rely on 
that distinction. In particular, the issue affects programs that use 
(guix build json), because that library uses #nil to represent the JSON 
value `null', whereas it uses '() to represent an empty JSON array.

The following program exposes the error:

--8<---------------cut here---------------start------------->8---
(use-modules (guix build json)
             (guix gexp)
             (guix monads)
             (guix store)
             (guix derivations)
             (rnrs conditions)
             (ice-9 textual-ports))

(define (check-equal? actual expected message)
  (define who 'check-equal?)
  (unless (equal? actual expected)
    (raise-exception
     (condition
      (make-assertion-violation)
      (make-who-condition 'check-equal?)
      (make-irritants-condition (list actual))
      (make-message-condition
       (format #f "~a: ~a\n  ~a: ~s\n  ~a: ~s\n  ~a: ~s"
               who "test failed"
               "message" message
               "expected" expected
               "actual" actual))))))

(define (sexp->json-string sx)
  (call-with-output-string
    (lambda (out)
      (write-json sx out))))

(define (gexp->json-string gx)
  (run-with-store (open-connection)
    (mlet* %store-monad ((drv (gexp->derivation "example.json"
                                (with-imported-modules `((guix build json))
                                  #~(begin
                                      (use-modules (guix build json))
                                      (call-with-output-file #$output
                                        (lambda (out)
                                          (write-json #$gx out)))))))
                         (_built (built-derivations (list drv))))
      (return (call-with-input-file (derivation->output-path drv)
                get-string-all)))))

(check-equal? (sexp->json-string '())
              "[]"
              "sexp: empty array")

(check-equal? (gexp->json-string #~'())
              "[]"
              "gexp: empty array")

(check-equal? (sexp->json-string #nil)
              "null"
              "sexp: null")

(check-equal? (gexp->json-string #~#nil)
              "null"
              "gexp: null")

(check-equal? (sexp->json-string '(@ ("k" . #nil)))
              "{\"k\":null}"
              "sexp: null in object")

;; This one fails!
(check-equal? (gexp->json-string #~'(@ ("k" . #nil)))
              "{\"k\":null}"
              "gexp: null in object")
--8<---------------cut here---------------end--------------->8---

-Philip