Scheme empty list expression

Question

I'm using http://rextester.com/runcode to do some scheme testing.

When I run

(define x (cons 3 null))
(write x)

it has an ERROR: Unbound variable: null.

How do I refer to an empty list in the above context?

Answer 1

(define x (cons 3 '()))
(write x)

Alternatively, you can define null first:

(define null '())
(define x (cons 3 null))
(write x)

Answer 2

In scheme, the empty list is (), not null. In other lisps it is also called nil.