It isn't clear if the "correct" input for the procedure is an arbitrary list or a two-element list. If it's strictly a two-element list, this will work:
(define (is-two-element-list? lst)
(and (list? lst)
(= (length lst) 2)))
(define (zipper? lst)
(and (is-two-element-list? lst)
(is-two-element-list? (first lst))
(is-two-element-list? (second lst))))
… And if it's an arbitrary-length list whose elements we want to check, this will work in Racket, using andmap
:
(define (zipper? lst)
(andmap is-two-element-list? lst))
If you are not using Racket, then this solution using every
will work in any interpreter with SRFIs:
(require srfi/1)
(define (zipper? lst)
(every is-two-element-list? lst))
Either way, notice that the trick was defining the is-two-element-list?
procedure, which verifies the two-element-list property, after that we can apply it as needed.