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Re: Closure?

From: Maciek Godek
Subject: Re: Closure?
Date: Sun, 13 Jul 2008 08:57:20 +0200

> Hi Maciek,
> Just picking up another point from your original email.  You may have
> already worked this out, but just in case...

OK, it's always good to say to much than not to say enough
(at least if it comes to hacking ;])

>> Additionaly, it would be nice to see the possibility
>> of explicit definitions of environments, like:
>> (define env (make-closure (a . 1)(b . 2))
>> (with env (define c 3))
> This is equivalent to:
> (define env (let ((a 1) (b 2)) (the-environment)))
> (local-eval '(define c 3) env)
> except that the last line fails with a "Bad define placement" error.
> That's because there are special rules for defines inside lexical
> scopes.

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

So one might conclude that it _is_ possible to use define inside
a 'let' form.

>> so that we could define the aforementioned
>> counter as:
>> (define counter-env (make-closure (c . 0)))
>> (define ++ (with counter-env (lambda()(set! c (1+ c))c)))
> This one really works:
> (define counter-env (let ((c 0)) (the-environment)))
> (define (++) (local-eval '(begin (set! c (+ c 1)) c) counter-env))
> So, in summary, make-closure wouldn't provide anything more than what
> we already have.

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)

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