Pascal Bourguignon <[EMAIL PROTECTED]> writes:
| leon <[EMAIL PROTECTED]> 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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