2013/8/21 Ralf Mattes <[email protected]>
> On Wed, Aug 21, 2013 at 08:52:02AM +0200, Panicz Maciej Godek wrote:
> > 2013/8/20 David Pirotte <[email protected]>
> >
> > > Hello,
> > >
> > > > It seems following is invalid:
> > > >
> > > > (let ((a 2))
> > > > (define (foo x) (+ a x)))
> > > >
> > > > I prefer to reduce scope of variable as much as possible, so
> > > > I find this restriction unconvinent. Is is part of standard or
> technical
> > > > limitation? Is it any workaround?
> > >
> >
> > The Scheme's idiomatic way to achieve the effect that you
> > probably want would be
> > (define foo #f)
> > (let ((a 2))
> > (set! foo (lambda (x) (+ a x))))
>
> I'd say this is extremly contorted and non-schemish.
> What's wrong with:
>
> (define foo
> (let ((a 2))
> (lambda (arg) (+ a arg))))
>
> This is the basic let-over-lambda closure ....
>
>
You're right, but it only works if you want to export only one symbol
from a lexical scope. If you wanted a few procedures accessing
a single scope, you'd either need to use the solution with 'set!',
or -- as Taylan suggested -- have a "define-values" form.
Actually, I think should also be possible to write a "define-values"
macro using the method I presented
(define-macro (define-values symbols . body)
(let ((value-identifiers (map gensym (map symbol->string symbols))))
`(begin
,@(map (lambda(x)`(define ,x #f)) symbols)
(let-values ((,value-identifiers ,@body))
,@(map (lambda(symbol value)`(set! ,symbol ,value))
symbols
value-identifiers)))))
So we can say that guile already has that ;]
I don't know however how to write this using define-syntax, the
idea is to transform
(define-values (symbol1 symbol2 ...) body ...)
into
(begin
(define symbol1 #f)
(define symbol2 #f)
...
(let-values (((value1 value2 ...) (begin body ...))
(set! symbol1 value1)
(set! symbol2 value2)
...)
but somehow we need to generate identifiers for symbol1 symbol2 ...
(here I wrote symbolically value1 value2 ..., but I'd appreciate if someone
more competent could provide a syntax-rules-based solution)
regards
