On Scheme cons and dots notation

Question

Given

#;> (cons (cons 1 2) 3)
((1 . 2) . 3)

When we try

#;> (cons 3 (cons 1 2))
(3 1 . 2)

What governs where the . is used? What would the memory representation of these constructs be?

Answer 1

Scheme implementations usually print things that look like lists in list form:

-> (cons 1 (cons 2 '()))
'(1 2)

In your example, (cons 3 (cons 1 2)) would be a list if it weren't for the last 2. So your implementation makes a best effort to print it as a list until the 2. The other example does not contain any part that looks like a list, so it just prints as nested pairs.