[plt-scheme] verifying whether a symbol is defined at compile time?

Hi Jens -

Thanks for the example ;)

yinso

On 4/22/07, Jens Axel Søgaard <jensaxel at soegaard.net> wrote:
>
> Hi Yin-So,
>
> Matthew's hint about identifier-binding is probably what you need now.
> In more general situations you can keep track of the names (and/or
> associated informations) by a define-like macro with the help
> of a little bookkeeping.
>
> (module names mzscheme
>    (provide register-name defined?)
>
>    (require (prefix srfi: (lib "1.ss" "srfi")))
>
>    (define defined-names '())
>
>    (define (defined? name)
>      (srfi:member name defined-names module-identifier=?))
>
>    (define (register-name name)
>      (cond [(defined? name)
>             (raise-syntax-error
>                'define-once
>                 format "the name '~a' is already defined"
>                        (syntax-e name))
>                 name)]
>            [else
>             (set! defined-names (cons name defined-names))])))
>
> (module define-once mzscheme
>    (provide define-once)
>    (require-for-syntax names)
>
>    (define-syntax (define-once stx)
>      (syntax-case stx ()
>        [(define-once name expr)
>         (register-name #'name)
>         #'(begin
>             (require names)
>             (define name expr))])))
>
> (module test mzscheme
>    (provide foo bar)
>    (require define-once)
>    (define-once foo 1)
>    (define-once bar 2)
>    ;(define-once bar 3)
>    )
>
> (require test)
> foo
> bar
>
>
> --
> Jens Axel Søgaard
>
>


-- 
http://www.yinsochen.com
...continuous learning...
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