DrRacket：应用程序：不是R5RS语言的过程错误

Question

this is my first day on Dr.Racket and R5RS language. I'm trying to modify the existing code I share below.

;; This is an internal helper procedure.
;;  - it gets the method out of "in-object"
;;  - it invokes the method, passing "for-object" as the
;;    "self" for the method.                    
;;                  
(define (apply-method in-object for-object message args)
  (let ((method (get-method message in-object)))
    (cond ((method? method)                 
           (apply method for-object args))
          ((eq? in-object for-object)
       (display method)                 
           (error "No method for" message 'in
          (safe-ask 'UNNAMED-OBJECT
                in-object 'NAME)))
          (else (error "Can't delegate" message
                       "from" (safe-ask 'UNNAMED-OBJECT
                    for-object 'NAME)
               "to" (safe-ask 'UNNAMED-OBJECT
                      in-object 'NAME))))))


(define (get-method message object) ; single-inheritance
  (object message))

This snippet is the part of considerably huge project. Therefore, I share only the related part. Inside the project, when this apply-method procedure is called. I got error on the line that starts with let expression. The error message is as the following:

objsys.scm:53:2: application: not a procedure; expected a procedure that can be applied to arguments
given: #f
arguments...:

So, I wonder if is there anyone who can help me to solve the problem. EDIT

I suspect that there is a syntactic error like misused or unaligned paranthesis above but I cannot find the exact place that cause error.

Answer 1

When you have used apply-method the value of in-object is #f . When apply-method tries to do (get-method message in-object) get-method then applies (in-object message) that turns into (#f 'some-unknown-message) and since #f is not a procedure racket has problems treating it as one and ends up with an error telling you that during application the given value #f is not a procedure.

Since you are using racket, perhaps you should just start the debugger and put a breakpoint on get-method and you'll see it. Note that Scheme and Racket languages has only one namespace for all bindings so you cannot have a procedure and a variable by the same name since a named procedure would occupy the very same variable.