Re: modify environments to make sandboxes

[Jon Wilson]

Hi all,
How about having the ability to capture the current lexical environment and 
use it as an environment for eval?  Something like call/cc, but I guess 
call/env.  In essence, this would make environments useful as first class 
objects (perhaps).

(define my-env #f)

(let ((a 0) (b 1) (c 2))
  (call/env (lambda (x) (set! my-env x))))

(eval '(begin
        (display a) (newline)
        (display b) (newline)
        (display c) (newline)
        (display my-env) (newline))
      my-env)

>> prints:
0
1
2
#<environment 0xDEADBEEF>


Perhaps an unlet would also be useful.  This would create a lexical 
environment in which several variables were unbound.

(unlet (eval read load)
 (call/env (lambda (x) (set! my-env x))))

(eval '(eval '(+ 1 2)) my-env)
=>
#error

Hmmm, perhaps this idea (unlet) couldn't work in any kind of a sensible, 
simple, lexical scoping system.  Maybe unlet could just set the variables 
given to it to #<unspecified> or #f or something.

So, which extant language feature have I overlooked that provides more or 
less exactly this? :)

Regards,
Jon

Mildred wrote:
> Hi,
>
> I'm new to this mailing list and also new to the scheme language ...
> I'm used to Lua but I try to search about functionnal programming, and
> I found scheme. It looks like a good language but before using it in my
> projects I would like to know if tere is an easy way to create
> sandboxes.
>
> In lua, it is easy, you create a table containing functions ... and the
> table can be made environment for a function. So, you can easily create
> secure sandboxes by loading lua code from file and changing the
> environment of the loaded code.
>
> I do not know how to do that in scheme. Apparetly the function
> null-environment can return an environment and eval can evaluate some
> code in an environment. But the question is how to define a variable in
> an environment and also how to undefine a variable that you don't want
> to appear.
> I didn't found anything about modifying an environment. Is it
> possible ? If not, why not ? and is it possible to create sandboxes ?
> Thanks
>
> Mildred