Re: Cycle Org Shift Select

[Michael Heerdegen <michael_heerdegen@web.de>]

Christopher Dimech <dimech@gmx.com> writes:

> Many thanks Tomas, have gone though the Elisp Manual yesterday
> and I am getting to understand this list Ouroboros thing. :)

If you want a simple way to think about it starting from a syntax point
of view:

A very simple way to think about the dotted syntax is to start from
regular lists.  You can write

  (elt1 elt2 . rest)

to describe a list of the elements elt1 elt2 (any positive number of
starting elements will do) with the elements in the list `rest'
appended. For example try to eval

  '(x y . ())

or

  '(x y . (z))

(You need the quote "'" because we don't want to evaluate the lists as
an expression.)

Of course the dot syntax is ambiguous, e.g.

  '(x y z)
  '(x . (y z))
  '(x . (y . (z)))
  '(x . (y . (z . ())))

all describe equal three element lists containing the symbols x, y and z.

Then you need to know that the "rest" can be actually anything.  If the
`rest' doesn't describe a regular list, you get a "dotted" list (it will
be printed using the dot syntax).  In the simplest case

  (x . y)

you have a pair, a `cons' cell which is the building block lists are
constructed from in Lisp.  A cons with a list cdr is also a list.

And when `rest' refers to the list itself (possible using the # reader
syntax):

  '#1=(nil t always . #1#)

you get a circular list.

HTH,
Michael.