On Fri, Jul 6, 2012 at 11:59 AM, Neil Toronto <neil.toro...@gmail.com> wrote: > On 07/05/2012 05:54 PM, Sam Tobin-Hochstadt wrote: >> >> On Jul 5, 2012 8:50 PM, "Neil Toronto" <neil.toro...@gmail.com >> <mailto:neil.toro...@gmail.com>> wrote: >> > >> > (define-predicate boxof-integer? (Boxof Integer)) >> >> This is the bug -- there's no way to write the boxof-integer? predicate, >> and define- predicate shouldn't think it can. > > > Meaning "no way that ensures preservation when some other piece of the > program has a pointer to that box," right?
Right. > Anticipating a bug fix, I've started converting my recent TR code so that it > doesn't define predicates for mutable container types. Instead of using > `define-predicate', I need to *copy* the mutable data structure, using > occurrence typing on each immutable containee. It's kind of a pain, but I > only have one type that needs this treatment. Can you not do the checking *after* extracting the elements from the container? That may well be faster. > It's actually a good thing to have to do this. Copying happens at the > boundary between private code and user code, and ensures that user code > can't violate the private code's invariants by mutating private data. > > If I had more to change, it'd be a major hassle. Would a `define-converter' > be possible for mutable container types? Here's an example: > > (define-converter (make-vectorof T) (Vectorof T)) > > #;(: make-vectorof (All (A B) ((A -> Boolean : B) > (Vectorof A) -> (U False (Vectorof B))))) > ;; Receives a predicate that indicates `B' instances and a vector > ;; Returns #f when every `A' in the vector isn't also a `B' > > (: ds (Vectorof Integer)) > (define ds (build-vector 10 identity)) > > (: idx-ds (U False (Vectorof Index))) > (define idx-ds (make-vectorof index? ds)) ; possibly makes a copy > > (cond [idx-ds ...] ; idx-ds is a (Vectorof Index) > [else ...]) ; it's not I'm confused about what this does. When does it copy, and when does it produce `#false`? > Of course, this wouldn't leverage the contract system nicely like > define-predicate does. And in general, implicitly copying mutable data seems like a bad idea -- mutable data is for sharing. > I could also maintain invariants easily if I had an Immutable-Vectorof type. > But that doesn't help with the fact that subtyping and occurrence typing > don't work nicely on parameterized mutable types. Right. Unfortunately, that's a pretty fundamental feature of languages with mutation and aliasing. -- sam th sa...@ccs.neu.edu _________________________ Racket Developers list: http://lists.racket-lang.org/dev
