Scheme: append to local environment

Question

Say, I want to implement "declarative" object system in Scheme by defining object symbol and then appending methods and fields to that object. While doing so, I want to exploit the local environment of this object to properly bind it's fields in methods (which are added later), for instance (a very "hacked-together" example):

(define myobj 
(begin
    (define x 5) ; some local field (hard-coded for example)
    (define (dispatch m d)
        (cond ((eq? m 'add-meth) (define localmethod1 d))
              ((eq? m 'inv-meth) (localmethod1 d))))
    dispatch
))
(myobj 'add-meth (lambda (y) (+ y x)) ;want this to bind to x of myobj
(myobj 'inv-meth 3) ;8

Don't mind silly dispatching mechanism and hard-coded "localmethod1" :) Also do mind, that x may not be available during definition of dispatcher. First of all, I get problems with using define in define (Bad define placement).
Then: how to make sure that x in lambda binds to the right one (inside of myobj) and not to some x in global environment?
Last: Is there a way to mutate such local enivornments (closures, right?) at all?

EDIT2: I know that you can make this with local lists like "fields" and "methods" and then mutate those by a dispatcher. I wanted to know if there is a possibility of mutating local environment (produced by a dispatch lambda).

mobiuseng mobiuseng · Accepted Answer · 2016-01-18T14:41:49

begin in Scheme does not produce local environment. To introduce local variables use let. Or, alternatively define an object, or rather a class to construct the object, as lambda-procedure, that will act as a constructor. This will solve your first two problems.

To mutate: you can mutate by having a right dispatch method. For example,

(define (make-object init-val)
  (define x init-val)
  (define (dispatch msg)
    (cond ((eq? msg 'inc)
           (lambda (y)
             (set! x (+ x y))))
          ((eq? msg 'x)
           x)
          (else
           (error "Unknown msg"))))
  dispatch)

> (define obj (make-object 10))
> (obj 'x)
10
> ((obj 'inc) 20)
> (obj 'x)
30

SICP, Chapter 3 provides good examples of how to make objects with local state.

Scheme: append to local environment

2 Answers