[racket] Cyclic data structures in Typed Racket

I guess the real problem is to create suitable default values so 
that the pass the type checker or to figure out a cast that forces
TR to accept some other value as the default value (and making sure
you never use the object). 

Undefined not completely solved. Argh. -- Matthias





On Sep 29, 2014, at 12:52 PM, Sam Tobin-Hochstadt <samth at cs.indiana.edu> wrote:

> 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.
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