Re: Puzzled by Typed Racket

On Thu, Mar 3, 2016 at 4:26 AM, Antonio Menezes Leitao
<[email protected]> wrote:
> Hi,
>
> Here is a MWE for something that is puzzling me. In file testtyped.rkt, I have
>
> #lang typed/racket
>
> (provide foobar)
>
> (struct (T) foo
>   ([x : (T -> T)]))
>
> (define (foobar [a : Boolean])
>   (if a
>       (foo (lambda ([x : Number]) x))
>       (foo (lambda ([x : String]) x))))
>
> In the REPL, I can test this file and it works fine:
>
> Language: typed/racket; memory limit: 2048 MB.
>> foobar
> - : (-> Boolean (U (foo Number) (foo String)))
> #<procedure:foobar>
>> (foobar #t)
> - : (U (foo Number) (foo String))
> #<foo>
>> (foobar #f)
> - : (U (foo Number) (foo String))
> #<foo>
>
>
> However, using it from non-typed racket causes an error.  If, in file
> test.rkt we have:
>
> #lang racket
> (require "testtyped.rkt")
>
> ;;Note that this is _not_ typed/racket
>
> Then the same example stops working:
>
>> (foobar #t)
> . . foobar: broke its own contract
>   two of the clauses in the or/c might both match: (struct/c foo (->
> Number any)) and (struct/c foo (-> String any))
>   produced: #<foo>
>   in: the range of
>       (->
>        (or/c #f #t)
>        (or/c
>         (struct/c foo (-> Number any))
>         (struct/c foo (-> String any))))
>   contract from:
>       C:\Users\aml\Dropbox\AML\Projects\Rosetta\testtyped.rkt
>   blaming: C:\Users\aml\Dropbox\AML\Projects\Rosetta\testtyped.rkt
>    (assuming the contract is correct)
>   at: C:\Users\aml\Dropbox\AML\Projects\Rosetta\testtyped.rkt:3.9
>
> I know that Typed Racket has limitations in the generation of
> contracts. Is this one of them? Is there a workaround?

Unfortunately, this is a real limitation. You end up with a union type
in the result, and it's one that can't be decided immediately, since
there are function contracts inside the struct contract.

In many cases, you can avoid a union of functions by pushing the union
type inside the function type. But that's not the case here, because
the type T appears in both the domain and the range of the function.

If you can say more about your actual use case, maybe I can be more helpful.

Sam

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