Re: NIL cons cell

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.