Re: Define in let

[Panicz Maciej Godek]

2013/8/21 Ralf Mattes <address@hidden>

On Wed, Aug 21, 2013 at 08:52:02AM +0200, Panicz Maciej Godek wrote:
> 2013/8/20 David Pirotte <address@hidden>
>
> > 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