As mentioned before, a function definition may contain an indefinite number of expressions in its body, one after the other. Take the following definition, which has two:
Notice that this function only returns the value of the last expression in its body. In this case the last expression in the body is (fourth-power x) so only the value 16 gets printed in the example above.> (defun powers-of (x) (square x) (fourth-power x)) POWERS-OF > (powers-of 2) 16
What is the point of having more than one expression in the body of a function if it only ever returns the last? The answer to this question is that we may be interested in side effects of the intermediate evaluations.
Powers-of does not have any side effects as it is, but change the definition as follows:
Watch what happens here:> (defun powers-of (x) (setq y (square x)) (fourth-power x)) POWERS-OF
The side effect of powers-of was to set the value of the variable y to 49. Since y did not appear in the parameter list of powers-of, it is treated as a global variable, and the effect of the set lasts beyond the life of your call to powers-of.> y 27 > (powers-of 7) 2401 > y 49
© Colin Allen & Maneesh Dhagat