Re: [plt-scheme] Defines in lets

[Max Hailperin <max@gac.edu>]

   From: pocm@rnl.ist.utl.pt (Paulo J. Matos)
   Date: 18 May 2002 20:21:59 +0100
   Hi all,

   Why can't I do something like this:
   > (let ((x 0)) (define (set-x y) (set! x y)) (define (read-x) x))
   begin (possibly implicit): no expression after a sequence of internal definitions

   I'm used to do it in Common Lisp like a way to avoid global
   variables and use data encapsulation. How can I do it in Scheme?

In Scheme define only has local scope; if you change your example to

(let ((x 0))
  (define (set-x y) (set! x y))
  (define (read-x) x) 
  SOME-EXPRESSION-HERE)

then SOME-EXPRESSION-HERE can use set-x and read-x as well as x, but
nothing outside the let can use any of the three names.  This
explains, then, why Scheme doesn't allow you to leave the
SOME-EXPRESSION-HERE out (as you did) and have a let where the body is
nothing but defines: there would be no point in it.

There is nothing in standardized Scheme that neatly corresponds to what
you are used to in Common Lisp.  Various Scheme implementations have
non-standard extensions to the language that do what you want.  If you
want to stay within the standardized language, the closest you can
come is:

(define set-x 'not-yet-set)  ; or any other placeholder value
(define read-x 'not-yet-set)

(let ((x 0))
  (set! set-x (lambda (y) (set! x y)))
  (set! read-x (lambda () x)))


 -Max Hailperin
  Associate Professor of Computer Science
  Gustavus Adolphus College
  800 W. College Ave.
  St. Peter, MN 56082
  USA
  http://www.gustavus.edu/~max/