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(librep.info)Cons Cells

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Cons Cells
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   A "cons cell" is an ordered pair of two objects, the "car" and the
"cdr".

   The read syntax of a cons cell is an opening parenthesis followed by
the read syntax of the car, a dot, the read syntax of the cdr and a
closing parenthesis. For example a cons cell with a car of 10 and a cdr
of the string `foo' would be written as,

     (10 . "foo")

 - Function: cons car cdr
     This function creates a new cons cell. It will have a car of CAR
     and a cdr of CDR.

          (cons 10 "foo")
              => (10 . "foo")

 - Function: consp object
     This function returns true if OBJECT is a cons cell.

          (consp '(1 . 2))
              => t
          
          (consp '())
              => ()
          
          (consp (cons 1 2))
              => t

   The strange syntax `'(1 . 2)' is known as "quoting" (Note:
Quoting), it tells the evaluator that the object following the
quote-mark is a constant, and therefore should not be evaluated. This
will be explained in more detail later.

   In Lisp an "atom" is any object which is not a cons cell (and is,
therefore, atomic).

 - Function: atom object
     Returns true if OBJECT is an atom (not a cons cell).

   Given a cons cell there are a number of operations which can be
performed on it.

 - Function: car cons-cell
     This function returns the object which is the car (first element)
     of the cons cell CONS-CELL.

          (car (cons 1 2))
              => 1
          
          (car '(1 . 2))
              => 1

 - Function: cdr cons-cell
     This function returns the cdr (second element) of the cons cell
     CONS-CELL.

          (cdr (cons 1 2))
              => 2
          
          (cdr '(1 . 2))
              => 2

 - Function: rplaca cons-cell new-car
     This function sets the value of the car (first element) in the cons
     cell CONS-CELL to NEW-CAR. The value returned is CONS-CELL.

          (setq x (cons 1 2))
              => (1 . 2)
          (rplaca x 3)
              => (3 . 2)
          x
              => (3 . 2)

 - Function: rplacd cons-cell new-cdr
     This function is similar to `rplacd' except that the cdr slot
     (second element) of CONS-CELL is modified.