[racket-dev] Motivation for polymorphic opaque types

Oh-- I think you're right that the type parameter can matter (it could go
over to R as an Integer list and come back as a Boolean list or something).

Robby


On Sun, Jan 6, 2013 at 4:08 PM, Sam Tobin-Hochstadt <samth at ccs.neu.edu>wrote:

> 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
> <robby at eecs.northwestern.edu> 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 <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
> >
> >
>
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