Exercise solution:
The get-prop function for property lists

(define (get-prop key property-list)
  (cond ((null? property-list) #f)
        ((null? (cdr property-list))
           (error "Malformed property list"))
        ((equal? key (car property-list))
           (cadr property-list))
        (else (get-prop key (cdr (cdr property-list))))))

I see a number of problems and issues with property lists and get-prop:

• We need to take care of ill-formed property lists, where there is an odd number of elements in the list.
• R5RS offer three different variants of assoc, for different equality funtions. We could do the same here, but it would be tedious. Alternatively, we could go for a solution based higher-order function solution, where we generate get-prop with an approprite comparison function. See another exercise.
• If the property list relates the key to boolean false, ambiguities arrise. The design of assoc avoids this (to some degree, at least) because it returns the cons pair of (key . value) in case of success.

The last observation could give rise to an alternative solution, which returns a list of (key value) in case key is found, and #f if the key is not found:

(define (get-prop key property-list)
  (cond ((null? property-list) #f)
        ((null? (cdr property-list))
           (error "Malformed property list"))
        ((equal? key (car property-list))
           (list key (car (cdr property-list))))
        (else (get-prop key (cdr (cdr property-list))))))

However, this requires allocation of two new cons cells (in contrast to association lists, where no new cons cell allocation is necessary). Please understand this.