Question

We are using scheme version r5rs trying to check a procedure pointer for the type it is pointing to.

We’ve created a object using:

(define (%macro name exp env)
  (define (set-exp new_exp) (set! exp new_exp))
  (define (set-env new_env) (set! env new_env))
  (define (set-name new_name) (set! name new_name))

  (define (%dispatch call)
    (cond 
      ((eq? call 'get-exp) exp)
      ((eq? call 'get-env) env)
      ((eq? call 'get-name) name)

      ((eq? call 'set-exp) set-exp)
      ((eq? call 'set-env) set-env)
      ((eq? call 'set-name) set-name)
  ))

  %dispatch
)

We then want to check a procedure reference to determine if it points to the dispatch function in another part of the code:

(define (macro? exp)
  (eq? %macro exp)
  )

This doesn’t seem to work, how do we properly check the pointer if it points to a macro object (really the dispatch procedure)?

Any help is greatly appreciated 🙂

Answer 1

Perhaps add one more method to your object that responds to ‘macro?. In the terms of a Java programmer: avoid instanceof calls. If all your objects follow a common protocol like implementing a macro? method, would that solve the problem?

In more featureful languages like full Racket, using a structure type property would also work. Since you’re deliberately in r5rs, your options are more limited.