In SICP exercise 2.26 using DrScheme, why does cons return a list, instead of a pair of lists?

Question

In SICP exercise 2.26, this Scheme code is given:

(define x (list 1 2 3))
(define y (list 4 5 6))

Then this cons call is given:

<         


        

Answer 1

cons uses the first argument as head of the list, and the second as tail.

You give it a first list (1 2 3), which will constitute the head of the resulting list and a second list (4 5 6), to be used as tail of the list. Thus, you end with ((1 2 3) 4 5 6).

Thing of lists as left-to-right combs, ending with empty list (represented as o here), and see how they combine.

 X=      Y=
 /\      /\
1 /\  + 4 /\    
 2 /\    5 /\  
  3  o    6  o

You then build:

 /\
X  Y

Obtaining:

  /\
 /\ \
1 /\ \
 2 /\ \
  3  o/\
     4 /\
      5 /\
       6  o

which is ((1 2 3) 4 5 6 when represented with parenthesis. And this is a pair of lists.