Re: Closure of lexical environment in Scheme closures

[Brian Casiello <bcasiello@...>]

On Thu, 11 Nov 2004 21:23:31 +0000, Dr. Rich Artym
<rartym@galacta.demon.co.uk> wrote:
> On Thu, Nov 11, 2004 at 03:54:36PM -0500, Aaron Brown wrote:
> 
> 
> 
> > I posted a Scheme program and an equivalent Lua program
> > earlier.  Here are improved versions.  (As I said, I'm not
> > fluent in Scheme, so they might not be exactly equivalent,
> > but they're close enough for our purposes.)
> >
> >   ;; Scheme version:
> >   (define func nil)
> >   (let ((foo 'bar))
> >     (set! func (lambda ()
> >                     foo))
> >     (set! foo 'baz)) ;; Alters the foo within func.
> >   ;; Outside the 'let':
> >   (define foo 'quux) ;; If this were a 'set!', we'd get an
> >                      ;; error.
> >   (func) ;; Returns baz.
> 
> Guile is not valid Scheme, since Scheme made the set! syntactic keyword
> illegal because it broke closure encapsulation.  This is *precisely*
> the issue --- if Scheme is our reference on this particular issue then
> we're illegally modifying our Lua closures.
> 
> I gave some history on this in the old (slightly hijacked) thread, so
> I'll repeat it here where it's more relevant:
> 
> ||  Admittedly it used to be legal in older versions of Scheme (up to 5c4),
> ||  and then Scheme r4rs referred to it as a "compatible extension but not
> ||  a compliant implementation" of Scheme (lol, Scheme had a lot of politics
> ||  in those days).  And finally they bit the bullet and simply made it
> ||  illegal Scheme starting from r5rs.  Scheme's always tried to move in
> ||  the direction of lambda calculus purity.
> ||
> ||  The Guile folks chose to be non-compliant on this point because they
> ||  needed the functionality of such a construct, which is fair enough.
> 
> Although Scheme makes a good reference, one should note too that there
> are many other implementations of closures in functional programming,
> and the property of creating pure functions is an absolutely key one.
> 
> It's what allows referential transparency --- Main Property #1 !! :-)
> 
> 
> 
> Rich Artym.
> --
> Existing media are so disconnected from reality that our policy debates
> spin around a fantasy world in which the future looks far too much like
> the past.   http://www.zyvex.com/nanotech/MITtecRvwSmlWrld/article.html
> 
> 

So it's the fact that assigments (i.e. set!) are allowed, not the
closure issue (since closures are handled the same way as scheme),
that make lua not a purely functional language?

-- 
Brian
"Seeking understanding through stupid questions"