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 <[email protected]>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 > <[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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