Question

I have the following code and its evaluation as comment:

(require scheme/mpair)

(list)                   ;; '()
(mlist)                  ;; '()

(cons 'un (list))        ;; '(un)
(list 'un (list))        ;; '(un ())

(mcons 'un (mlist))      ;; (mcons 'un '())
(mlist 'un (mlist))      ;; (mcons 'un (mcons '() '()))

My questions:

1. why doesn’t the 3rd expression return '(), instead of '(un . ())?

2. why don’t the 5th expression and the the 6th expression return the same thing?

Answer 1

As a general example, (list x y z) is an abbreviation for (cons x (cons y (cons z empty))). And (list) is an empty list, i.e. empty. So (list 'un (list)) is just an abbreviation for (cons 'un (cons empty empty)) – I.e. a list with two elements in it: the symbol 'un and an empty list. mlist similarly is an abbreviation for a sequence of mcons‘es onto empty at the very end.