[racket-dev] Motivation for polymorphic opaque types

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 <samth at ccs.neu.edu>wrote:

> On Sun, Jan 6, 2013 at 3:23 PM, Robby Findler
> <robby at eecs.northwestern.edu> wrote:
> > On Sun, Jan 6, 2013 at 2:18 PM, Sam Tobin-Hochstadt <samth at ccs.neu.edu>
> > 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
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.racket-lang.org/dev/archive/attachments/20130106/1984e543/attachment.html>