Question

(define str '("3" "+" "3"))
(define list '(3 + 4))


(define (tokes str)
  (case (car str)
    ((or "+" "-" "*" "/")(write "operand")
                         (tokes (cdr str)))

                         (else (write "other"))
    ))

(define (tokelist)
  (case (car list)
    ((or "+" "-" "*" "/")(write "operand"))
    (else (write "other"))))

Answer 1

You are trying to compare a String "+" with the procedure + when you’re working with the list. These are different types, and they are not equal.

Try this:

> (string? "+")
#t
> (procedure? +)
#t
> (string? +)
#f

This should give you a good idea of how to solve the problem, but note:

> (= + +)
=: expects type <number> as 1st argument, given: #<procedure:+>;
other arguments were: #<procedure:+>

You need:

> (equal? + +)
#t
> (equal? + "+")
#f
> (equal? "+" "+")
#t

Using these ideas, this should get your code working:

(define (plus? s)
    (if (procedure? s) (equal? + s) (equal? "+" s)))