Re: Closure?

[Kjetil S. Matheussen]

"Maciek Godek"
> > I don't know exactly how it works out that using a define in
> > local-eval falls foul of the define placement rule, but it is not hard
> > to imagine that it could do.
>
> The other question is: is it really necessary to impose such
> limitations on "define". Why is it required to make its position
> inside let privileged?

The scheme standard is a bit pedantic. The above I would write
like this:

(let ()
  (define a)
  (display a)
  (newline)
  (let ()
    (define b 2)
    (+ a b)))



> > > Yes, since there's local-eval and the-environment, everything I've
> > > ever dreamed of is possible :)
> > > But as I've concluded from the discourse,  neither of these is
> > > defined in R5RS (and it makes me wonder)
> >
> > Well I've never thought this through before, but perhaps that is
> > because in many cases it is equivalent to create a lambda at the point
> > where you would call the-environment, containing the code that you
> > would later pass to local-eval.
> >
> > For example, the ++ example then becomes:
> >
> > (define ++ (let ((c 0)) (lambda () (begin (set! c (+ c 1)) c))))
> >
> > - which is the traditional way of writing this example.
>
> You didn't focus :>
> The whole idea of accessing a closure environment
> was in fact to make scheme object oriented
> programming more intuitive.
>
> In guile info pages there's an oo closure example:
>
> (section 3.1.4.9 "Example 4: Object Orientation")
> "
>     (define (make-account)
>       (let ((balance 0))
>         (define (get-balance)
>           balance)
>         (define (deposit amount)
>           (set! balance (+ balance amount))
>           balance)
>         (define (withdraw amount)
>           (deposit (- amount)))
>
>         (lambda args
>           (apply
>             (case (car args)
>               ((get-balance) get-balance)
>               ((deposit) deposit)
>               ((withdraw) withdraw)
>               (else (error "Invalid method!")))
>             (cdr args)))))
>
>     (define my-account (make-account))
> "
> Notice the ugly "case" statement that requires
> the variables to be accessed in the following manner
> (the same example, a few lines later):
> "
>     (my-account 'get-balance)
>     =>
>     0
>
>     (my-account 'withdraw 5)
>     =>
>     -5
>
>     (my-account 'deposit 396)
>     =>
>     391
>
>     (my-account 'get-balance)
>     =>
>     391
> "
>
> This is ugly as it requires doubling the names of functions.
> Perhaps it could be overcome with some sort of macro,
> but the "with" I proposed allows to avoid the whole "case"
> and to write (after slight modifications in the "let" form):
>
> (with my-account (get-balance))
>
> Or maybe I think wrong; I'm new in the world of lisp,
> so please forgive me my mistakes :)

I think local-eval is necessary for making
a namespace system of the below type without having to use
codewalking macros to expand the bodies of functions:

(define-namespace bank)
(def-bank sum 0)
(def-bank (add n)
  (set! sum (+ n sum)) ;; Note that it's enough to write "sum".

(bank.add 50)
bank.sum
=> 50


But for implementing a message passing OO system,
it's easier to use macros and hash tables, plus
that it probably performs much better:

(def-class <bank>
   (def-var sum 0)
   (def-method (add n)
      (set! sum (+ n sum)))

(define bank (new <bank>))
(-> bank add 50)
(-> bank sum)
=> 50

There's a bunch of these systems for scheme.
The syntax above is used from
http://snd.cvs.sourceforge.net/snd/cvs-snd/oo.scm?view=log

Guile's own OO system called GOOPS is also very nice.
GOOPS a quite verbose but very powerful and a lot
more interactive. It's similar to CL's CLOS, which
you should look at if you are not familiar with
already.