球拍中的语法错误是否相等？

Question

I am trying pass in a list input to conversation and have the function check to see if the first element in another list (keyword) matches to the first element of the list that user passed in. If the two match then output a zero otherwise pass the tail of the inputted list recursively back to itself.

(define keyword '(am I))

(define (conversation input)
     (cond
      ((equal? (car keyword) (car input)) 0)
      (else (conversation (cdr input)))))

The error I get is:

car: contract violation
expected: pair?
given: '()

I understand that equal? compares two elements, a pair, but what I do not understand is why it would create an error when the car of both lists are both exactly an element. Any help would be much appreciated, I'm assuming the solution is rather simple but I can't seem to see it.

My goal is create several functions that pattern match and output appropriate dialog but without the use of regular expressions or other libraries. There is no mandate not to use the two mentioned above but I would like to do it without them to get a better understanding of the logic and the code. Thanks for the help!

Answer 1

The first thing to consider is that you have no condition of failure. You assume there's either a match now with the car , or there will be a match later with the cdr . But there may be no match at all, and you will cdr down your list until your list is '() . As there is no such thing as the car of '() you are getting an error when you try to extract it. Therefore the first thing to do is make sure you've handled this case. I don't know what you intend to do in this case, so I have made the procedure return #f .

Next you consider what to do if the symbols do match. In your case you are choosing to return 0 . This part seems to have no problems.

Finally, we consider what to do if the car s do not match. In this case we continue searching the input. This part seems to have no problems.

(define (conversation input)
  (cond ((null? input) #f)
        ((eq? (car input) (car keyword))
         0)
        (else
         (conversation (cdr input)))))