Path: eternal-september.org!news.eternal-september.org!feeder3.eternal-september.org!fu-berlin.de!uni-berlin.de!not-for-mail
From: ram@zedat.fu-berlin.de (Stefan Ram)
Newsgroups: comp.lang.lisp
Subject: Re: NIL cons cell
Date: 27 Jan 2024 14:49:19 GMT
Organization: Stefan Ram
Lines: 22
Expires: 1 Dec 2024 11:59:58 GMT
Message-ID: <NIL-20240127154615@ram.dialup.fu-berlin.de>
References: <8734uj0x2m.fsf@brilhante.top> <NIL-20240127140420@ram.dialup.fu-berlin.de> <87y1cazz9d.fsf@brilhante.top> <7w7cjuq2mk.fsf@junk.nocrew.org>
Mime-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
X-Trace: news.uni-berlin.de 3ojyyaKfpfBYuFPncjgjxg+DF4yBfAAr63ss2CSjxIUslW
Cancel-Lock: sha1:bytCnVFlzKrSV1wvbaQQYFaaQR4= sha256:RXqyiPH8PaATJmF1mVA9/fPcsdqi7kr8s7Sm+ik6NCw=
X-Copyright: (C) Copyright 2024 Stefan Ram. All rights reserved.
	Distribution through any means other than regular usenet
	channels is forbidden. It is forbidden to publish this
	article in the Web, to change URIs of this article into links,
        and to transfer the body without this notice, but quotations
        of parts in other Usenet posts are allowed.
X-No-Archive: Yes
Archive: no
X-No-Archive-Readme: "X-No-Archive" is set, because this prevents some
	services to mirror the article in the web. But the article may
	be kept on a Usenet archive server with only NNTP access.
X-No-Html: yes
Content-Language: en-US
Accept-Language: de-DE-1901, en-US, it, fr-FR

Lars Brinkhoff <lars.spam@nocrew.org> writes:
>NIL is a symbol (and also an (empty) list).  Its internal representation
>is not specified by the standard.

  A symbol is an entity of the source-code model.

  A list is an entity of the runtime model.

  So, NIL "is" a symbol and a list (and also an atom), but on two
  different layers: In the source code it's a symbol, and the value of
  that symbol when it is being evaluated at runtime is the empty list.

  Models always contain entities which are not defined by
  being a kind of a more fundamental entity, because it is
  not possible to define every entity in that manner. But such
  entities are defined by their relations to other entities.

  Not defining the representation of entities gives Lisp the
  flexibility to be implemented in various ways on various machines.
  But it takes some time to get used to this kind of abstract thinking
  where abstract models replace specific internal representations.