What kind of a structure can the cadar procedure be applied
to? What must its operand look like?
The operand must be a pair (pr1) in which the first component
is also a pair (pr2); moreover, the second component of
pr2 must also be a pair (pr3).
Which of the elements of such a structure does cadar
return?
It returns the first component of pr3.
> (define example-1 (cons (cons #f (cons #t #f)) #f))
> (cadar example-1)
#t
> (define example-2 (cons (cons (cons 1 2) (cons 3 4))
(cons (cons 5 6) (cons 7 8))))
> (cadar example-2)
3
What happens if the structure has the wrong shape? It is an error, so under most implementations the program crashes:
> (cadar (cons (cons 1 2) (cons 3 4))) ERROR: caaar: Wrong type in arg1 2
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