On 31-Jan-09, at 7:49 PM, lowly coder wrote:

is there anyway i can obtain dynamic scoping in gambit? lexically scoping is really cool, and I want it as the default behavior; i just want the ability to go:

(define (foo x) (* x 2))

(define (g x) (foo x))

(pp (list (g 1) (dynamic-let ( (foo (lambda (x) (* x 3)))) (g 1))))

to get me:

(2 3)

This is the way you can do dynamic scoping in Gambit (and other Schemes too):

(define foo (make-parameter (lambda (x) (* x 2))))

(define (g x) ((foo) x))

(pp (list (g 1) (parameterize ((foo (lambda (x) (* x 3)))) (g 1))))

You can also use dynamic-wind to simulate dynamic scoping but it does not work properly with threads.

Marc

On 31-Jan-09, at 11:44 PM, lowly coder wrote:

Great, the following works:

cat test.scm
(define (foo x) (* x 2))

(define (g x) (foo x))


(pp (g 2))
(pp
 (let ((old-func '()))
  (dynamic-wind
    (lambda ()
      (set! old-func foo)
      (set! foo (lambda (x) (* x 3))))
    (lambda () (g 2))
    (lambda () (set! foo old-func)))))

(pp (g 2))

4
6
4

two questions:
1) can anything go wrong with variable capture / aliasing? [intuitively, I believe no, so long as I don't use 'old-func' in my thunk
2) is there a more elegant/idiomatic way to do this?

In terms of elegance I prefer this (which eliminates all variable capture problems):

 (let ((thunk (lambda () (g 2)))
      (new-foo (lambda (x) (* x 3)))
      (old-foo foo))
  (dynamic-wind
    (lambda () (set! foo new-foo))
    thunk
    (lambda () (set! foo old-foo))))

But once again this will not work right if multiple threads are dynamically scoping foo simultaneously, but the parameterize based approach will work fine.

Marc