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

[Maxime Devos <maximedevos@telenet.be>]

Philip McGrath schreef op wo 22-12-2021 om 23:25 [-0500]:
> 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:
> [
> ;...]
> 
> ; This one fails!
> (check-equal? (gexp->json-string #~'(@ ("k" . #nil)))
>                "{\"k\":null}"
>                "gexp: null in object")

A simpler test:

Compare this:
  (cdr (gexp->approximate-sexp #~("stuff" . #nil)))
  ; output: #nil --- seems like everything is ok?

with:
  (gexp->approximate-sexp #~("stuff" . #nil))
  ; output: ("stuff") --- where did the #nil go?

I think the idea is that, if you construct a list (a b c . #nil)
in elisp, and pass it to Scheme, then Scheme should treat it as a
Scheme list, so it should be printed as (a b c) when using Scheme's
'write' or 'display'.

Greetings,
Maxime.