1.5.1 Special Functions

LISP treats ‘setq’ differently from other functions because ‘setq’ is an instance of a special function. Whereas LISP causes the arguments to an ordinary function to be evaluated, LISP may handle the arguments of each special function rather idiosyncratically. There are only a small number of special functions in LISP.

Note that when we assigned ‘x’ a value using ‘setq’, LISP also printed out that value. That is, the expression ‘(setq x 5)’ returned the value ‘5’ in addition to assigning ‘x’ the value ‘5’. This is important in that we can use the value returned by ‘setq’ to continue the computation. Consider the following:

-> (+ 2 (setq x (* 3 4)))
14
-> x
12

Apply the rule of LISP evaluation:

• LISP tries to evalute arguments first, so first it evaluates ‘2’ to ‘2’.
• The second argument then gets evaluated.
• This is ‘setq’, so to evaluate it, LISP only evaluates its second argument.
• This causes the ‘*’ function to be applied to ‘3’ and ‘4’, and the value ‘12’ is computed.
• Then ‘setq’ is applied to ‘x’ and ‘12’.
• ‘setq’ sets ‘x’ to ‘12’ and also returns ‘12’ as its value.
• ‘+’ is now applied to ‘2’ and ‘12’, and the answer is ‘14’ returned.
• Since ‘x’ was changed in the process, we can query LISP for its value afterwards.