Re: what does '.' do in (visible . t)?

[leon]

Pascal Bourguignon <spam@mouse-potato.com> writes:

 | leon <sdl.web@gmail.com> writes:
 | > Here is an expression of lisp that I don't know what it means.
 | >      (visible . t)
 | >
 | > Could someone explain the function of '.' a bit?
 | 
 | This is the notation for the basic structure named cons (or pair in
 | scheme), which contains two fields: one named cdr and another named
 | car.  Conses can be allocated and initialized with (cons car cdr).
 | Conses are printed (and can be read back) with the format:
 | 
 |       ( car . cdr )
 | 
 | (cons 'a 'b) --> (a . b)
 | 
 | 
 | Now, since in lisp we use conses to build lists, putting an element in
 | the car, and the rest of the list in the cdr (which can be another
 | cons, or the symbol nil, indicating the end of the list), the printer
 | writes conses specially when their cdr contains another cons.
 | 
 | A list containing the symbols a, b, c, and d, can be built with:
 | 
 |       (cons 'a (cons 'b (cons 'c (cons 'd nil))))
 | 
 | this is what the function list does: (list 'a 'b 'c 'd) executes the
 | above cons function calls.
 | 
 | It could be printed as:
 | 
 |       (a . (b . (c . (d . nil))))
 | 
 | 
 | With:
 | 
 | (defun print-conses (x)
 |    (if (consp x)
 |        (progn (princ "(")
 |               (print-conses (car x))
 |               (princ " . ")
 |               (print-conses (cdr x))
 |               (princ ")"))
 |        (princ x)) 
 |     x)
 | 
 | (print-conses (cons 'a (cons 'b (cons 'c (cons 'd nil)))))
 | 
 | prints:
 | 
 |     (a . (b . (c . (d . nil))))
 | 
 | But since conses are often used to store lists, the printer prints
 | them by default as:
 | 
 |     (a b c d)
 | 
 | 
 | (print (cons 'a (cons 'b (cons 'c (cons 'd nil)))))
 | 
 | Prints:
 | 
 |     (a b c d)
 | 
 | 
 | 
 | Finally, if the last cons of a list contains another atom than nil in
 | the cdr, it's not a proper list, and print prints a dot to indicate
 | this, followed by the cdr, instead of eliding the nil:
 | 
 | (print (cons 'a (cons 'b (cons 'c (cons 'd 'e)))))
 | 
 | Prints:
 | 
 |     (a b c d . e)
 | 
 | 
 | 
 | 
 | Note: for all x, (eq (not (consp x)) (atom x))
 | -- 
 | __Pascal Bourguignon__                     http://www.informatimago.com/
 | Our enemies are innovative and resourceful, and so are we. They never
 | stop thinking about new ways to harm our country and our people, and
 | neither do we. -- Georges W. Bush

Thank you for this wonderful explanation:-)

-- 
.:*~*:._.:*~*:._.:*~*:._.:*~*:._.:*~*:._.:*~*:.
.     *                                       .
.    /.\  excuses are the easiest things      .
.   /..'\     to manufacture                  .
.   /'.'\   and the hardest things to sell    .
.  /.''.'\                                    .
.  /.'.'.\                                    .
. /'.''.'.\                                   .
. ^^^[_]^^^                                   .
.                                             .
.Leon                                         .
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