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Re: Closure?

From: Maciek Godek
Subject: Re: Closure?
Date: Mon, 14 Jul 2008 03:15:32 +0200

>> As the practise shows, although guile documentation says something
>> different. In section (A Shared Persistent Variable)
>> "An important detail here is that the `get-balance' and `deposit'
>> variables must be set up by `define'ing them at top level and then
>> `set!'ing their values inside the `let' body.  Using `define' within
>> the `let' body would not work: this would create variable bindings
>> within the local `let' environment that would not be accessible at top
>> level."
>> So one might conclude that it _is_ possible to use define inside
>> a 'let' form.
> Which would be correct!  For example:
> (let ((a 1))
>  (define b 2)
>  (+ a b))
> =>
> 3
> Whereas:
> (let ((a 1))
>  (display a)
>  (newline)
>  (define b 2)
>  (+ a b))
> =>
> ERROR: Bad define placement
> The "special rules" are just that  any defines have to come before
> anything else in the body of the let.

Yeah, guess you're right (under certain circumstances :P)

> 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?

>> 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 "Example 4: Object Orientation")
     (define (make-account)
       (let ((balance 0))
         (define (get-balance)
         (define (deposit amount)
           (set! balance (+ balance amount))
         (define (withdraw amount)
           (deposit (- amount)))

         (lambda args
             (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)

     (my-account 'withdraw 5)

     (my-account 'deposit 396)

     (my-account 'get-balance)

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 :)

Best regards,

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