[racket] Cyclic data structures in Typed Racket

Ah, if you want to create truly cyclic structure with no base case
like this, then you'll have to do more work. Either you'll need to
expand the type of the `x` field in `foo` to allow an initial value,
or you'll need to create a dummy `foo` with that extended type, and
then copy the cyclic data once you've created it to a new version of
`foo`, using hash tables to track cycles. That's basically how
`shared` works, although I haven't tried implementing it in TR and it
might not be possible.

Sam

On Mon, Sep 29, 2014 at 12:33 PM, Konrad Hinsen
<konrad.hinsen at fastmail.net> wrote:
> 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.