[plt-scheme] dynamically varying the definitions used by a procedure

When is it a good idea to use dynamic-require instead of units for
problems like this one?  The web-server servlet interface (a similar
problem) uses both, right?

Daniel


On Wed, 29 Sep 2004 22:59:44 -0400, Matthias Felleisen
<matthias at ccs.neu.edu> wrote:
>   For list-related administrative tasks:
>   http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> 
> Here is one way -- Matthias
> 
> ;; *** start: fxy-sig.ss
> (define-signature fxy^ (f x y))
> ;; *** end: fxy-sig.ss
> 
> ;; *** start: test.ss ****
> (load "fxy-sig.ss")
> 
> (define dbms
>    '((euclidean . "euclidean.ss")
>      (hamming . "hamming.ss")))
> 
> (define (execute arequest) (run (lookup arequest)))
> 
> (define run
>    (lambda (lfilename)
>      (invoke-unit/sig
>       (compound-unit/sig
>         (import)
>         (link
>          [FILE : fxy^ ((load  lfilename))]
>          [MAIN : () ((unit/sig () (import fxy^)
>                        (printf "~a~n" (f x y)))
>                      FILE)])
>         (export)))))
> 
> (define lookup
>    (lambda (arequest)
>      (cond
>        [(assq arequest dbms) => cdr]
>        [else (error (format "request ~a not found~n" arequest))])))
> 
> (execute 'euclidean)
> (execute 'hamming)
> ;; *** end: test.ss ****
> 
> ;; *** start: hamming.ss ****
> (unit/sig fxy^
>    (import)
>    (define x '(1 0 0))
> 
>    (define y '(1 0 1))
> 
>    (define f ;hamming
>      (lambda (ax ay)
>        (+  (- (car ax)  (car ay)))
>        (- (cadr ax)  (cadr ay))
>        (- (caddr ax)  (caddr ay)))))
> ;; *** end: hamming.ss ****
> 
> ;; *** start: euclidean.ss ****
> (unit/sig fxy^
>    (import)
>    (define x '(2 5 7))
> 
>    (define y '(1 0 2))
> 
>    (define square
>      (lambda (ax)
>        (* ax ax)))
> 
>    (define f
>      (lambda (ax ay)
>        (sqrt
>         (+ (square (- (car ax)  (car ay)))
>            (square (- (cadr ax)  (cadr ay)))
>            (square (- (caddr ax)  (caddr ay))))))))
> ;; *** end: euclidean.ss ****
> 
> 
> 
> 
> On Sep 29, 2004, at 7:31 PM, David J. Neu wrote:
> 
> >
> > ;; *** start: test.ss ****
> > (define dbms
> >   '((euclidean . "euclidean.ss")
> >     (hamming . "hamming.ss")))
> >
> > (define execute
> >   (lambda (arequest)
> >     (let ((lfilename (assq arequest dbms)))
> >       (if lfilename
> >           (begin
> >             (load (cdr lfilename))
> >             (printf "~a~n" (f x y)))
> >           (error (format "request ~a not found~n" arequest))))))
> >
> > (execute 'euclidean)
> > (execute 'hamming)
> > ;; *** end: test.ss ****
> >
> > ;; *** start: euclidean.ss ****
> > (define x '(2 5 7))
> >
> > (define y '(1 0 2))
> >
> > (define square
> >   (lambda (ax)
> >     (* ax ax)))
> >
> > (define f
> >   (lambda (ax ay)
> >     (sqrt
> >      (+ (square (- (car ax)  (car ay)))
> >         (square (- (cadr ax)  (cadr ay)))
> >         (square (- (caddr ax)  (caddr ay)))))))
> > ;; *** end: euclidean.ss ****
> >
> > ;; *** start: hamming.ss ****
> > (define x '(1 0 0))
> >
> > (define y '(1 0 1))
> >
> > (define hamming
> >   (lambda (ax ay)
> >     (+  (- (car ax)  (car ay)))
> >     (- (cadr ax)  (cadr ay))
> >     (- (caddr ax)  (caddr ay))))
> > ;; *** end: hamming.ss ****
> 
>