From: Maxime Devos <maximedevos@telenet.be>
To: Taylan Kammer <taylan.kammer@gmail.com>, 48318@debbugs.gnu.org
Subject: bug#48318: (ice-9 match) does not allow distinguishing between () and #nil
Date: Thu, 13 May 2021 22:39:45 +0200 [thread overview]
Message-ID: <26e0536a8f0c16f53002b045cb7e9a30a1e8a09e.camel@telenet.be> (raw)
In-Reply-To: <d68fdbea-533d-2444-9fde-85775d3bf8de@gmail.com>
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Taylan Kammer schreef op do 13-05-2021 om 21:14 [+0200]:
> Hi Maxime,
>
> I believe that match conflating () and #nil is the right thing,
> even if the current implementation does it unintentionally.
>
> Those two values should be considered "the same" in most situations
> even though (eqv? #nil '()) is false.
Conflating #nil and () is reasonable for my use case,
though this conflation should be documented.
> In fact I think they should be equal? to each other. It feels
> wrong that (equal? '(foo . #nil) '(foo . ())) evaluates to false,
> even though both arguments represent the list '(foo).
The guile manual has some information on this.
(6.24.2.1 Nil, under 6.24.2 Emacs Lisp).
> Please note that #nil is not ever supposed to be used intentionally.
I know, but ...
> It's there purely as an Elisp compatibility trick, and the only time
> Scheme could should receive it is when receiving data generated by
> Elisp code. For instance when Elisp code generates a list, it would
> be terminated by #nil. (Which is why I think it should equal? '().)
I have been porting some common lisp code to Guile Scheme. I replaced
'() with #nil, which allows me to largely ignore whether Lisp nil is used
as end-of-list or as boolean for now (I'm in the process of replacing it
with '() or #f where appropriate).
Being able to directly refer to #nil, to perform equality checks like
(eq? #f #nil) --> #f, (eq? '() #nil) --> #f, ... can be useful for writing
Scheme code that could be called from both elisp and Scheme when the
compatibility isn't transparent. For example, suppose (ice-9 match) actually
did not conflate #nil and () (which is what I initially thought; I expected
(ice-9 match) to compare atoms with eqv?), then the
following code ...
;; Somewhat contrived (untested), but based on some real code
(define
(match-lambda
((_ . stuff) stuff)
(() 0)))
... would need to be rewritten to something like ...
;; Somewhat contrived (untested), but based on some real code
(define
(match-lambda
((_ . stuff) stuff)
(() 0)
(#nil 0)))
Also, consider the 'case' syntax.
Greetings,
Maxime.
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next prev parent reply other threads:[~2021-05-13 20:39 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-09 16:42 bug#48318: (ice-9 match) does not allow distinguishing between () and #nil Maxime Devos
2021-05-13 19:14 ` Taylan Kammer
2021-05-13 20:39 ` Maxime Devos [this message]
2021-05-13 21:21 ` Taylan Kammer
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