[racket] what is this thing: '#(struct:foo 3 4) ?

On Fri, Sep 10, 2010 at 11:40 AM, John Clements
<clements at brinckerhoff.org> wrote:
>
> On Sep 10, 2010, at 9:34 AM, Matthew Flatt wrote:
>
>> At Fri, 10 Sep 2010 09:21:28 -0700, John Clements wrote:
>>> I've spent long enough with the docs on this not to feel too embarrassed about
>>> this question: what kind of value is produced by
>>>
>>> '#(struct:foo 3 4)
>>>
>>> (N.B. the leading quote)
>>
>> Are you by chance using the PLAI language that is distributed with 5.0.1?
>>
>> I ask because this looks like something that would be produced by the
>> broken printer in that version.
>
> No:
>
> #lang racket
>
> (define-struct foo (a b) #:transparent)
>
> (define o (open-output-string))
> (write (make-foo 3 4) o)
> (read (open-input-string (get-output-string o)))
>
> =>
>
> Welcome to DrRacket, version 5.0.1.5--2010-09-08(93ee20e/g) [3m].
> Language: racket; memory limit: 256 MB.
> (foo 3 4)
> '#(struct:foo 3 4)
>>
>
>
> And to everyone else: D'oh! Okay, I do feel embarrassed.
>

I've been confused by this too. For some reason, '#(struct:foo 3 4)
just looks like it MUST be special.