The following are experiments that I ran during my read-through of Land of Lisp… I am starting this file from chapter 7, so earlier experiments are omitted.
It would seem as though you can make the cdr of a list point to the
car of the beginning of the list… This is a circular reference that
can be quite dangerous if we try to print it out, so it was
recommended that we print this out after setting a variable that
warns Lisp that we are going to be trying something wacky with
circular references:
(setf *print-circle* t)(defparameter foo '(1 2 3))
(setf (cddr foo) foo)(1 2 1 . #1)
The printer appears to reference the first element by the notation:
#<index>, so #1. Of course, since the cdr of the last element is
not nil, it prints out in dot notation.
Fancy!
You can return multiple values from a function, this happens with the
”values” function. For example, to return two values, the numbers 2
and 3 at the same time, we can do the following:
(values 2 3)2
When a function like this is called, the first value is considered to
be the correct value to work with and the second value is
discarded… However, if you wish to use this second return, you can
use multiple-value-bind to assign all of the return values from a
function call to variables.
So:
(multiple-value-bind (x y) (values 2 3)
y)3
Of course you can use this for functions too.
In Common Lisp, there is a handy command, time, that allows you to
measure the speed of an operation and gauge how expensive a call
is. For example:
(let ((*trace-output* *standard-output*))
(time (expt 999 9999)))Real time: 0.0080005 sec. Run time: 0.0156001 sec. Space: 36632 Bytes
Versus:
(let ((*trace-output* *standard-output*))
(time (expt 999 99999)))Real time: 0.1340077 sec. Run time: 0.1404009 sec. Space: 334176 Bytes