Sorry, that was very silly of me. That isn't what's happening at all,
because type soundness means we don't need to enforce the
parametricity at all.
The actual relevant program is:
(module m racket
(struct kons (a d))
(struct mt ())
(define MT (mt))
(define (FST v)
(when (eq? MT v) (error 'empty))
(kons-a v))
(define (RST v)
(when (eq? MT v) (error 'empty))
(kons-d v))
(define (LST . x)
(if (empty? x)
MT
(kons (first x) (apply LST (rest x)))))
(define (LST/C elem/c)
(define C (recursive-contract
(or/c (λ (v) (eq? v MT))
(struct/dc kons [a elem/c] [d C]))))
C)
(provide/contract
[LST (->* () #:rest any/c (LST/C any/c))]
[FST (-> (LST/C any/c) any/c)]
[RST (-> (LST/C any/c) (LST/C any/c))])
)
However, thinking about this more, it's an invariant that `kons`
structures are always correctly constructed, and we can rely on them
to have *some* instantiation that typechecks -- that's why the `any/c`
is ok. That suggests to me that contract generation for a struct type
applied to simple type variables can always be just the predicate for
that type, which would make Neil very happy. I want to think about
this more before I'm sure, though.
Thanks for being patient while I get this wrong in various ways ...
Sam
On Sun, Jan 6, 2013 at 4:13 PM, Robby Findler
<[email protected]> wrote:
> This has a non-chaperone contract being used in a struct/c, I think?
>
> (FST (LST 1 2 3)) => struct/dc: expected chaperone contracts, but field a
> has #<barrier-contract>
>
> Robby
>
>
> On Sun, Jan 6, 2013 at 2:40 PM, Sam Tobin-Hochstadt <[email protected]>
> wrote:
>>
>> On Sun, Jan 6, 2013 at 3:23 PM, Robby Findler
>> <[email protected]> wrote:
>> > On Sun, Jan 6, 2013 at 2:18 PM, Sam Tobin-Hochstadt <[email protected]>
>> > wrote:
>> >>
>> >> > The boundaries have the information; that's how the contracts got
>> >> > inserted
>> >> > in the first place.
>> >>
>> >> No, the contracts are parametric contracts using `parametric->/c`, and
>> >> thus don't have any information about the types used at all.
>> >
>> >
>> > I don't see why you can't tag them when something at a boundary and then
>> > check that something at another boundary instead of doing some deep
>> > check.
>>
>> The problem is that I don't know what to tag them *with*.
>>
>> Consider the following program:
>>
>> #lang racket
>>
>> (struct kons (a d))
>> (struct mt ())
>> (define MT (mt))
>> (define (FST v)
>> (when (eq? MT v) (error 'empty))
>> (kons-a v))
>> (define (RST v)
>> (when (eq? MT v) (error 'empty))
>> (kons-d v))
>> (define (LST . x)
>> (if (empty? x)
>> MT
>> (kons (first x) (apply LST (rest x)))))
>> (define (LST/C elem/c)
>> (define C (recursive-contract
>> (or/c (λ (v) (eq? v MT))
>> (struct/c kons elem/c C))))
>> C)
>> (provide/contract
>> [LST (parametric->/c (A) (->* () #:rest A (LST/C A)))]
>> [FST (parametric->/c (A) (-> (LST/C A) A))]
>> [RST (parametric->/c (A) (-> (LST/C A) (LST/C A)))])
>>
>> This is the essence of Neil's polymorphic list program, as implemented
>> by Typed Racket. I don't know how to change those contracts to not be
>> really expensive, because I can't pick the instantiation of A at
>> runtime to tag the structure instances with.
>>
>> Sam
>
>
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