[racket] importing signatures

On Mon, Jul 26, 2010 at 1:55 PM, Shriram Krishnamurthi <sk at cs.brown.edu> wrote:
> Here's the file "sig":
>
>  #lang racket
>  (define-signature Foo (make-foo foo?))

(provide Foo)

> Here's another file:
>
>  #lang racket/unit
>  (require "sig")
>  (import)
>  (export Foo)
>  (define-struct foo ())
>
> When I execute this, I get
>
>  define-unit: unknown signature in: Foo
>
> I would have thought that the the signature is precisely what the
> (require "sig") line would have imported.  I must be missing something
> in the grammar -- anyone?
>
> Shriram