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 <[email protected]> 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. ____________________ Racket Users list: http://lists.racket-lang.org/users
