What is the meaning of this Scheme

Question

I am new to Scheme, by looking at the Exercise 1.5 of SICP, what is the meaning/usage of this expression?

(define (p) (p))

Thanks!

Answer 1

(define (p) (p))

The above defines a function p that takes no arguments and calls itself recursively (infinitely).

The exercise 1.5 is about applicative-order vs normal-order evaluation.

(define (test x y)
  (if (= x 0)
       0
       y))

(test 0 (p))

In applicative-order all arguments are evaluated and then they are applied to the test, so the program will freeze if the interpreter uses that kind of evaluation in this specific case.