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[plt-scheme] Resources in the transformer environment

From: Jon Zeppieri (zeppieri at gmail.com)
Date: Fri Oct 27 17:52:21 EDT 2006		 
Hi,

So, I've attempted to implement the macro suggested below:

On Fri, 27 Oct 2006 08:13:22 0800, Matthew Flatt <mflatt at cs.utah.edu> wrote:
> Would it make sense to have a form like
>
>  (with-database
>   "host.domain.foo" 3306 "my_username" "my_password" "my_schema"
>   <body>)
>
> that opened the database both a compile time and at run time? The macro
> might be implemented as mostly
>
>   (dynamic-wind
>    (lambda () <...open the database...>)
>    (lambda () ... (local-expand <body> ...) ...)
>    (lambda () <...close the database...>))
>
> but the expansion of <body> would also be wrapped with a similar
> `dynamic-wind' to perform the same DB open and close.

Here is the code:

  (define-syntax (with-database stx)
    (syntax-case stx ()
      ((_ (host port username password schema) body-expr ...)
       (dynamic-wind (lambda ()
                       (display "opening transformer DB connection\n")
                       (current-connection (connect
(syntax-object->datum #'host)

(syntax-object->datum #'port)

(syntax-object->datum #'username)

(syntax-object->datum #'password)

(syntax-object->datum #'schema))))
                     (lambda ()
                       (local-expand #'(dynamic-wind (lambda ()
                                                       (display
"opening runtime DB connection\n")

(current-connection (connect host port username password schema)))
                                                     (lambda ()
                                                       body-expr ...)
                                                     (lambda ()
                                                       (display
"closing runtime DB connection\n")
                                                       (close
(current-connection))))
                                     (syntax-local-context)
                                     '()))
                     (lambda ()
                       (display "closing transformer DB connection\n")
                       (close (current-connection)))))))


When I attempt to use it, however, there is a problem:

(module foo mzscheme

  (require (lib "dbo.ss" "dbo"))

  (with-database (<host> <port> <username> <password> <schema>)
    (define project%
      (dbo-class* "project" object% ()
                  (super-new)))

    (define o (make-object project%))
    (define ps (send o find #:all))
    (display ps))
)

...results in...

opening transformer DB connection
closing transformer DB connection
compile: identifier used out of context in: local-accessor


Perhaps I'm expanding the body in the wrong context?

-Jon
Posted on the users mailing list.