[plt-scheme] eval and servlets

The other solution is to put the code you need in a module, and
namespace-attach-module it to the namespace you create for running
eval in.

But basically it's all about namespaces.

sam th


On Sun, 3 Oct 2004 14:55:53 +0200, ifconfig nslookup
<configurator at gmail.com> wrote:
>   For list-related administrative tasks:
>   http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> 
> This has to do with environments. If I am not mistaken, when you use
> eval, it uses the top-level environment. When you define 'square' in
> the top-level (i.e. when running outside of the servlet), you can
> reference it inside the evaluated quote. When you define it in a
> module or unit, however, you would have to use the module or servlet's
> environment instead.
> One solution that *might* work is to transfer the definitions into the
> top-level. You can do that simply by calling (eval `(define square
> ,square)).
> 
> Yours,
> ifconfig
> 
> 
> 
> 
> On Sun, 3 Oct 2004 08:14:47 -0400, David J. Neu <djneu at att.net> wrote:
> >  For list-related administrative tasks:
> >  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> >
> > Hi all,
> >
> > I was wondering if someone could explain why in the following code,
> > the commented out version of _start_ fails, with error:
> >
> >    Servlet exception: "reference to undefined identifier: square"
> >
> > in both a module-based and a unit-based servlet.
> >
> > The "same" function works fine outside of a servlet.
> >
> > Something with eval and namespaces?
> >
> > Many thanks!
> > --David
> >
> > (module act-test
> >  mzscheme
> >
> >  (provide interface-version timeout start)
> >
> >  (require (lib "servlet-sig.ss" "web-server")
> >           (lib "servlet.ss" "web-server")
> >           (lib "servlet-helpers.ss" "web-server"))
> >
> >  (define interface-version 'v1)
> >
> >  (define timeout +inf.0)
> >
> >  (define square (lambda (x) (* x x)))
> >
> >  (define ops
> >    `((s . ,(lambda (x) (square x)))
> >      (i . ,(lambda (x) (add1 x)))))
> >
> >   ; this fails
> > ;  (define start
> > ;    (lambda (arequest)
> > ;      (let ((lfile (open-input-file "act.ss")))
> > ;        (let ((ops (eval (read lfile))))
> > ;          (close-input-port lfile)
> > ;          `(html
> > ;            ,(format "~a" ((cdr (assq 's ops)) 2)))))))
> >
> >  ; this works
> >  (define start
> >    (lambda (arequest)
> >      `(html
> >        ,(format "~a" ((cdr (assq 's ops)) 2))))))
> >
> > ;; start: act.ss
> > `((s . ,(lambda (x) (square x)))
> >  (i . ,(lambda (x) (add1 x))))
> > ;; end: act.ss
> >
> 



-- 
sam th
samth at ccs.neu.edu