Question

I am currently playing around with LISP. Everything is fine, but I can’t understand the following issue.

I have the this append-operation:

(define (append l1 l2)
   (if (eq? l1 null)
      l2
      (cons (first l1)
            (myappend (rest l1) l2))))

I use it like this:

(myappend (cons (cons 1 2) null) '(4 5))

And the result in Racket is:

 '((1 . 2) 4 5)

But why? In my oppinion it should be ‘(1 2 4 5), because cons returns a list and myappends appends two lists. Can anybody help me? What is LISP doing?

Answer 1

cons returns a dotted pair, not necessarily a list.

(cons 1 2) returns (1 . 2)

(cons 1 null) returns (1)

(cons 1 (cons 2 null)) returns (1 2)