>> As the practise shows, although guile documentation says something
>> different. In section 3.1.4.7 (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 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 :)
Best regards,
M
