[racket] Completely disallow numbers, quoted and all

Ahhh, so simple! Thanks, Matthew. I'll give it a shot.


On Wed, Jun 4, 2014 at 1:11 AM, Matthew Flatt <mflatt at cs.utah.edu> wrote:

> At Wed, 4 Jun 2014 00:38:11 -0400, Daniel Brady wrote:
> > So I'm playing around with the DrRacket reader, and I'm decided to see
> if I
> > could disallow certain types of literal data. I figured I'd start with
> > numbers, and maybe see if I can come up with a tiny numberless language
> > like pure lambda calculus.
> >
> > This is a simple module that I came up with:
> >
> > (module numberless racket
> >   (provide (rename-out [no-nums #%datum]))
> >
> >   ;; Raises a syntax error when a number literal is encountered.
> >   ;; Note that the datum must be quoted to prevent infinite expansion.
> >   (define-syntax (no-nums stx)
> >     (syntax-case stx ()
> >       ((_ . datum) #'(if (number? (quote datum)) (raise-syntax-error
> 'Sorry
> > "number literals disallowed" (quote datum)) (quote datum))))))
> >
> > I don't know if that's the right way to go about it, but it gets most of
> > the job done.
>
> That's the right idea, but I think you want to check for numbers at
> expansion time instead of generating code to check at run time, which
> means moving the test out of the #' like this:
>
>   (define-syntax (no-nums stx)
>     (syntax-case stx ()
>       ((_ . datum)
>        (if (number? (syntax-e #'datum))
>            (raise-syntax-error 'Sorry
>                                "number literals disallowed"
>                                #'datum)
>            #'(quote datum)))))
>
> Some compound values are can be sent to `datum`, such as vectors like
>
>  #(1 2 3)
>
> If you want to disallow numbers in things like vectors, you'll need to
> traverse `datum` looking for numbers. See the documentation for
> `syntax-e` for the description of values that you'd have to traverse.
>
> > But it doesn't catch quoted number literals:
> >
> > > '3
> > 3
> >
> > I don't know what part the reader deals with quoted values.
>
> The reader turns
>
>   '3
>
> into
>
>  (quote 3)
>
> So, you don't want to change the reader, but instead define a new
> `quote` macro that expands to `no-nums`:
>
>   (provide (rename-out [no-nums-quote quote]))
>
>   (define-syntax-rule (no-nums-quote v)
>     (no-nums . v))
>
>


-- 
*SEE YOU SPACE COWBOY...*
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