Interacting with the Listener

Evaluating expressions

? (+ 10 20 30)
60
? (aref #(a b c d) 0)
A
? "Simple String"
"Simple String"

Every kind of Lisp expression has its own rules for evaluation. The first two examples given above are function calls, which evaluate to the result of calling the function on the arguments. The third example is a string constant. String constants (like other constants) evaluate to themselves.

Figure 2-3 A simple Listener interaction

You can also type more complex expressions into the Listener. Many expressions will contain nested subexpressions. It is also common to test out function and class definitions in the Listener.

? (+ 25 (* 10 10) (* 5 5))
150
? (defun factorial (n)
    (if (= n 0)
      1
      (* n (factorial (- n 1)))))
 FACTORIAL
? (factorial 10)
3628800

• Before going on, try out using the Listener by typing in some simple expressions and seeing how they evaluate. (If you get into any error situations, you can always get out by typing command-period.)

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