What (define (p) (p)) does?

Question

I'm reading the "Structure and interpretation of computer programs" 2nd edition in the exercise 1.5, I found a combination that I didn't understand what it does exactly (define (p) (p)).

When I called the procedure (p) I had the cursor blinking in the next line without the ability to write anything .

(define (p) (p))
(p)

I don't know what to expect from this procedure because I defined it by itself.

Hmm, what could running the expression `(p)` do? – Rainer Joswig Jul 07 '19 at 10:32 — Rainer Joswig, Jul 07 '19 at 10:32

score 8 · Accepted Answer · answered Jul 07 '19 at 10:47

p is a procedure with no parameters. Its body is (p). In Scheme, we call procedures by surrounding them in brackets together with their arguments. Given that p doesn't have parameters, (p) simply calls p. Which calls p. Which calls p... and so on. So what does it do? an infinite loop! and that is all.

score 2 · Answer 2 · answered Jul 08 '19 at 12:22

2

(define (p) (p))
(p)

This is syntactic sugar for this

(define p (lamnda () (p))
(p)

After you deepen SICP you will learn that this infinite recursion can also be done so:

((lambda(s) (s s))
 (lambda(s) (display ".") (s s)))

answered Jul 08 '19 at 12:22

alinsoar

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What (define (p) (p)) does?

2 Answers2