[racket] build-struct-generation

The `build-struct-generation' function is indeed broken. It originated
in the `mzscheme' days when an empty application produced an empty
list. 

I'll fix it. Meanwhile, you can work around the problem by supplying
the last two optional arguments (which don't have keywords --- another
sign of an old function) as '(list) or #'(list) instead of the default
'().

Alternatively, one reason that `build-struct-generation' rotted may be
that we switched to using `struct/derived' as a better way of writing
`struct'-like forms, so you might try that approach if it applies.

At Sat, 3 Sep 2011 08:06:11 -0400, Shriram Krishnamurthi wrote:
> build-struct-generation is a really useful utility, but I don't
> understand how to use it.  Specifically, see
> 
> Welcome to DrRacket, version 5.1.3 [3m].
> Language: racket; memory limit: 128 MB.
> > (require syntax/struct)
> > (build-struct-generation #'p (list #'x #'y) #f #f)
> '(let-values (((struct: make- ? acc mut)
>                (make-struct-type
>                 '.#<syntax:5:29 p>
>                 #f
>                 2
>                 0
>                 #f
>                 ()      ;; <---
>                 (current-inspector)
>                 #f
>                 ())))   ;; <---
>    (values
>     struct:
>     make-
>     ?
>     (make-struct-field-accessor acc 0 '.#<syntax:5:39 x>)
>     (make-struct-field-mutator mut 0 '.#<syntax:5:39 x>)
>     (make-struct-field-accessor acc 1 '.#<syntax:5:43 y>)
>     (make-struct-field-mutator mut 1 '.#<syntax:5:43 y>)))
> 
> Note the two empty lists in the output [()'s with the default printer;
> in constructor mode they come out as empty's].  They correspond to
> 
>   props : (listof (cons/c struct-type-property? any/c)) = null
>   immutables : (listof exact-nonnegative-integer?) = null
> 
> However, these are in code that it meant to subsequently be
> evaluated.  When used, for instance, in a macro, I get
> 
>   #%app: missing procedure expression; probably originally (), which
>   is an illegal empty application in: (#%app)
> 
> Is it possible to get a simple example of how to use this?
> 
> Shriram
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