Question

A newcomer to Lisp. I know that

(mapcar #'list '(1 2) '(3 4)) 

will give

'((1 3) (2 4))

and based on my understanding of how apply works, I expect

(apply #'(lambda (&rest x) (mapcar #'list x)) '((1 2) (3 4)))

to return the same result. Instead, I am getting

'(((1 2)) ((3 4)))

I am confused because

(apply #'append '((1 2) (3 4)))

gives me

'(1 2 3 4)

as expected. What is going on?

Answer 1

Simplify it. Suppose you used A instead of (1 2), and B instead of (3 4):

(apply #'(lambda (&rest x) (mapcar #'list x)) '(A B))

Because &rest x takes all the arguments and packs them up as a list, so x has the value
(A B).
Then mapcar iterates twice, passing A to list, producing (A), then it does the same with B. Then mapcar makes a list of those, producing ( (A) (B) )

It’s useful to put print statements in there to see what’s going on.