[racket] Cyclic data structures in Typed Racket

Sam Tobin-Hochstadt writes:

 > I recommend doing the mutation yourself, and not using `shared`.
 > That's the most obvious solution.

Not for me, unfortunately, but probably I am missing something
obvious.

Here's explicit mutation in untyped Racket:

   #lang racket

   (struct foo (x) #:mutable)
   (struct bar (x))

   (define f (foo (void)))
   (define b (bar f))
   (set-foo-x! f b)

   (eq? (bar-x b) f)
   (eq? (foo-x f) b)

That works fine. Moving to Typed Racket, this is rejected by the type
checker because (void) is of type Void.  Since I need a bar to make a
foo and a foo to make a bar, I don't see how I can ever initialize my
foo structure.

 > The reason that your `require/typed` doesn't work is that it creates
 > new versions of the placeholder functions, but those aren't the ones
 > that `shared` uses internally, so Typed Racket doesn't use the types
 > you've given.

That makes sense, thanks for the explanation!

Konrad.