[racket] Syntax objects in prefab structures

Prefab structures are tested like pairs, vectors, and boxes in syntax
objects. For example, `datum->syntax` and `syntax->datum` can recur
into a prefab, but they don't go into transparent structures.

If you add a list to your list of two structures,

 (set! structs (list (list #'foox) (foo #'foox) (bar #'barx)))

then you'll see that the prefab's content is treated the same way as
the list content.

At Fri, 28 Nov 2014 15:38:47 +0100, Jens Axel Søgaard wrote:
> The following program shows a difference between syntax objects stored
> in prefab structures versus
> transparent structures.
> 
> Is this a bug - or am I missing something about prefab structures?
> 
> #lang racket
> (begin-for-syntax
>   (struct foo (baz) #:prefab)
>   (struct bar (baz) #:transparent)
>   (define structs '()))
> 
> (define-syntax (macro1 stx)
>   (set! structs (list (foo #'foox) (bar #'barx)))
>   #'(void))
> 
> (begin-for-syntax (displayln (list 1 structs)))
> (macro1)
> (begin-for-syntax (displayln (list 2 structs)))
> 
> (define-syntax (with stx)
>   (with-syntax ([s structs])
>     #'(begin
>         (begin-for-syntax
>           (set! structs 's))
>         (void))))
> 
> (with)
> 
> (begin-for-syntax (displayln (list 3 structs)))
> 
> The output is:
> 
> Welcome to DrRacket, version 6.1.1.4--2014-11-08(b7d2722d/d) [3m].
> Language: racket; memory limit: 256 MB.
> (1 ())
> (2 (#s(foo .#<syntax:8:29 foox>) #(struct:bar .#<syntax:8:42 barx>)))
> (3 (#s(foo foox) #(struct:bar .#<syntax:8:42 barx>)))
> (1 ())
> (2 ())
> (3 (#s(foo foox) #(struct:bar .#<syntax:8:42 barx>)))
> (1 ())
> (2 ())
> (3 (#s(foo foox) #(struct:bar .#<syntax:8:42 barx>)))
> 
> The syntax object in the prefab structure turned into a symbol !
> 
> --
> Jens Axel Søgaard
> 
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