Question

When I make a list in Scheme, what does the cdr of the last pair point to? Is it an ’empty word’ or an ’empty list’? DrScheme doesnt mind (cons 'a empty) nor (cons 'a '()). Finally what is the difference between the empty word and the empty list?

Answer 1

DrScheme doesn’t mind your cons-ing, because cons can do more than just build lists. From the reference,

(cons a d) → pair?
  a : any/c
  d : any/c

which means that cons builds pairs. It’s also perfectly legal to do

> (cons 1 2)
(1 . 2)

which is not a list, but which still lets you do

> (car (cons 1 2))
1
> (cdr (cons 1 2))
2

But anyways, the “empty word” and the “empty list ” are the same object, as can be checked by

> (eq? empty '())
#t