Exercise 1.4.6

More CAR and CDR

cons, car and cdr are purely functional. Which means they never mutate their arguments.

* (defvar *a* (cons 1 2))
*A*

* *a*
(1 . 2)

* (cdr *a*)
2

* *a*
(1 . 2)

* (cons 3 (cdr *a*))
(3 . 2)

* *a*
(1 . 2)

It is an error to use car and cdr on something other than a Cons-Cell.

* (car 1)
; Evaluation aborted on #<TYPE-ERROR expected-type: LIST datum: 1>

* (cdr 'a)
; Evaluation aborted on #<TYPE-ERROR expected-type: LIST datum: A>.

This includes other compound values such as strings and vectors

* (car "a")
; Evaluation aborted on #<TYPE-ERROR expected-type: LIST datum: "a">.

* (cdr #(1 2))
; Evaluation aborted on #<TYPE-ERROR expected-type: LIST datum: #<(SIMPLE-VECTOR 2) {1007D4C76F}>>.

but not the empty list, also represented as NIL

* (car nil)
NIL

* (cdr nil)
NIL

* (car ())
NIL

* (cdr ())
NIL