Slide 8.d: COND

COND

COND is an unusual function which may take any arbitrary number of arguments. Each argument is called a clause, and consists of a list of exactly two S-expressions. We will call the first S-expression in a clause a condition, and the second S-expression a result. Thus, a call to COND looks like this:

   (COND
      (condition1   result1)
      (condition2   result2)
      ...
      (T            resultN))

The value returned by COND is computed as follows:

if condition1 is true (not NIL), then return result1;

else if condition2 is true then return result2;

else if ...; else return resultN.

It is an error if none of the conditions are true, and the result of the COND is undefined. For this reason, T is usually used as the final condition.

READ:
(cond ((null (read)) 'empty) (T 'not-empty))

The above expression tests whether or not the input is empty by using the following two instructions:

NULL: null is true if its argument is ( ), and otherwise is false.

READ: Lisp stops and waits for the user to type an expression. That expression, without evaluation, becomes the value of (READ).