[plt-scheme] writing scheme application extensions in scheme

Paul Graunke wrote:

>
> On Monday, June 9, 2003, at 05:16 PM, Peter Santoro wrote:
>
>> ;------------------------------------
>> ; sample hook-mgr.scm
>>
>> (define _hooks (list))
>
> (define _hooks empty)
> ; or
> (define _hooks null)
> ; work, too.
>
>> (define _hook-key 0)
>
>
> ;If you only need to compare keys for eq?-ness (i.e. you don't need to 
> multiply, add,
> ;subtract, etc the keys) then you might want to use symbols instead.
> ;Then you can eliminate _hook-key.
>
>>
>> (define (get-next-hook-key)
>>   (begin0 _hook-key (set! _hook-key (+ _hook-key 1))))
>
>
> (gensym) ; generates a new, unique symbol
>
> (define (get-next-hook-key)
>   (set! _hook-key (add1 _hook-key))
>   _hook-key)
> ; will work if you are stuck with numbers
>
>>
>> (define (get-hook-key ls)
>>   (car ls))
>>
>> (define (get-hook-desc key)
>>   (let ((ls (assq key _hooks)))
>>     (if ls
>>       (caadr ls)
>>       #f)))
>>
>> (define (get-hook-proc key)
>>   (let ((ls (assq key _hooks)))
>>     (if ls
>>       (cadadr ls)
>>       #f)))
>>
>> (define (set-hook! proc menu-text)
>>   (set! _hooks (append _hooks (list (list (get-next-hook-key) (list 
>> menu-text proc))))))
>>
>
> ;You might want to use a hash table with symbols for keys.  Search 
> help-desk.
>
> ;Use structures instead of lists when you know how many pieces there are.
> (define-struct extension (menu-text proc))
> ; Then replace
> (list some-menu-text some-proc)
> ; with
> (make-extension some-menu-text some-proc)
> ; You can retrieve the pieces
> (extension-menu-text (make-extension x f)) = x
> (extension-proc (make-extension x f)) = f
> ; and recognize extensions
> (extension? (make-extension x f)) = #t
> (extension? 5) = #f
>
> Also, if you are trying to keep _hooks private by starting the name 
> with an _, you
> should search help-desk for "module" .  They can hide private 
> variables like _hooks.
> (I would then drop the underscore.)
>
> Enjoy!
>
> Paul
>
>
Thanks Paul.  I appreciate your suggestions.  I did put this code 
together perhaps too quickly (I know about and I'm already using structs 
and modules, but I will investigate using mzscheme's dynamic module 
access.).  My main concern was whether there was any "show-stopper" with 
writing scheme extensions and hooking the code together as I did (so 
that I could make rapid modifications "in the field").  To do this in 
the past with C/C++, I usually created and loaded dynamically linked 
libraries (a slightly less simple and more time consuming process than 
using scheme).  With scheme, many things seem almost too good to be true.

Peter