[plt-scheme] `shared' syntax confusing

Matthew Flatt <mflatt at cs.utah.edu> writes:

> The documentation for `scheme/shared' (or `scheme'), however, is
> considerably more detailed already:
>
>    http://docs.plt-scheme.org/reference/shared.html

Thanks, I had missed that.

> It was easy to support the second example when `cons' created mutable
> pairs. Now that it creates immutable pairs, `shared' must be able to
> predict more of the shape of the resulting data to be able to construct
> the circular version, and there will obviously be approximations in its
> analysis of the shape. Not being able to see through `(if #t ...)' is
> one of the approximations.

What about post-processing the values instead of predicting
the shape?  The following seems to work fine for me:

  (define-syntax shared
    (syntax-rules ()
      ((shared ((var expr) ...) body ...)
       (shared-build () ((var expr) ...) body ...))))

  (define-syntax shared-build
    (syntax-rules ()
      ((shared-build ((var tmp expr) ...) ((var2 expr2) rest ...) body ...)
       (shared-build ((var tmp expr) ... (var2 tmp2 expr2)) (rest ...) body ...))
      ((shared-build ((var tmp expr) ...) () body ...)
       (let ((tmp (make-placeholder #f)) ...)
         (let ((var #f) ...)
           (set! var (let-syntax
                         ((var (make-set!-transformer
                                (lambda (stx)
                                  (syntax-case stx (set!)
                                    ((set! id val) #'(set! id val))
                                    (id #'tmp)))))
                          ...)
                       expr)) ...
           (placeholder-set! tmp var) ...
           (set! var (make-reader-graph var)) ...
           body ...)))))

-- 
Alex