I can see why an eq-set and an eqv-set can't be union-ed without additional information about the desired result type.
But if we had two different implementation of the eq-set API we could easily define unions as derived code; that's the point of OO. On Aug 22, 2013, at 12:14 PM, Carl Eastlund wrote: > Set-union never worked for even different hash set representations. Even > before I touched the code, the union of an eq set and an eqv set, for > instance, caused a runtime error. > > Generics do not have multiple dispatch. That's not a mechanism we have right > now. And "fallbacks" are for when there's no method implemented for a given > receiver value, they're not particularly related to a question of "are these > things the same type". > > I chose to keep the semantics that union and operations like it only work > with the same representation. Partly because that's how things already were, > and partly to set the precedent that generics authors don't have to write two > versions of every method. Perhaps this wasn't the best idiom, but it's what > I wrote. Perhaps there's a better idiom for fallbacks that makes this work > more cleanly. Anyway, right now, generic sets are designed so you can use > any one representation, but they don't combine representations. > > Carl Eastlund > > > On Thu, Aug 22, 2013 at 9:03 AM, J. Ian Johnson <i...@ccs.neu.edu> wrote: > No, it doesn't seem to be using the fallback in this case. > > ianj@sampson:~/racket/racket/bin$ ./racket -il xrepl > Welcome to Racket v5.90.0.8. > -> (set-union '() (set)) > ; in-list: contract violation > ; expected: list? > ; given: (set) > ; [,bt for context] > -> > > -Ian > ----- Original Message ----- > From: "Sam Tobin-Hochstadt" <sa...@cs.indiana.edu> > To: "J. Ian Johnson" <i...@ccs.neu.edu>, "Carl Eastlund" <c...@ccs.neu.edu> > Cc: dev@racket-lang.org, "Matthew Flatt" <mfl...@cs.utah.edu> > Sent: Thursday, August 22, 2013 8:51:30 AM GMT -05:00 US/Canada Eastern > Subject: Re: [racket-dev] Lists aren't sets, but have set-like operations > > Wait, `set-union` of two different set representations doesn't work? > > Sam > > On Thu, Aug 22, 2013 at 8:07 AM, J. Ian Johnson <i...@ccs.neu.edu> wrote: > > You misunderstand. I used set-union, but single dispatch saw '() and used > > list-union for the '() (set) combination. > > -Ian > > ----- Original Message ----- > > From: "Sam Tobin-Hochstadt" <sa...@cs.indiana.edu> > > To: "J. Ian Johnson" <i...@ccs.neu.edu> > > Cc: dev@racket-lang.org, "Matthew Flatt" <mfl...@cs.utah.edu> > > Sent: Thursday, August 22, 2013 8:02:50 AM GMT -05:00 US/Canada Eastern > > Subject: Re: [racket-dev] Lists aren't sets, but have set-like operations > > > > > > > > But 'list-union' is not a generic operation so it isn't surprising that > > this didn't work. To do this generically, you'd need to use 'set-union'. > > > > Sam > > On Aug 22, 2013 7:59 AM, "J. Ian Johnson" < i...@ccs.neu.edu > wrote: > > > > > > The problem manifested itself when I got an exception that in-list can't be > > called on (set), which really confused me. (set? '()) answered true, so it > > tried to do (list-union '() (set)), which failed. > > Generic sets as they are don't work generically. Some action should be > > taken. Either set? means what it once did, or we do some awfully slow > > multiple dispatch for set operations. My bias shows. > > -Ian > > ----- Original Message ----- > > From: "Matthew Flatt" < mfl...@cs.utah.edu > > > To: "Carl Eastlund" < c...@ccs.neu.edu > > > Cc: "J. Ian Johnson" < i...@ccs.neu.edu >, "dev" < dev@racket-lang.org > > > Sent: Thursday, August 22, 2013 7:22:25 AM GMT -05:00 US/Canada Eastern > > Subject: Re: [racket-dev] Lists aren't sets, but have set-like operations > > > > How much should we prioritize backward compatibility in this case? > > > > One possibility is to make `set?' mean `hash-set?', and add > > `generic-set?' in place of the current `set?'. That's uglier, > > obviously, but it would be better if we want to prioritize backward > > compatibility. > > > > At Wed, 21 Aug 2013 19:14:06 -0400, Carl Eastlund wrote: > >> Ah, yes. The set? predicate no longer distinguishes a representation. > >> There are several predicates for the original set type, now called "hash > >> sets": set-eq?, set-eqv?, set-equal?, set-mutable?, set-immtuable?, and > >> set-weak?. I didn't add the basic "hash-set?", but perhaps I should. It's > >> a weird name, since "hash-set" and "hash-set!" are already existing, > >> unrelated functions. > >> > >> Carl Eastlund > >> > >> > >> On Wed, Aug 21, 2013 at 7:08 PM, J. Ian Johnson < i...@ccs.neu.edu > wrote: > >> > >> > Okay, I can abide. However, that doesn't really get at my frustration. > >> > I'm > >> > using the set constructor, that appears to now be an immutable-custom-set > >> > with make-immutable-hash as its make-table. So what I'm looking for is > >> > not > >> > set?, but set-immutable?, as it's a distinct (family of) struct types > >> > that > >> > won't clash with the primitive data that I'm otherwise using. > >> > -Ian > >> > ----- Original Message ----- > >> > From: "Carl Eastlund" < c...@ccs.neu.edu > > >> > To: "J. Ian Johnson" < i...@ccs.neu.edu > > >> > Cc: "dev" < dev@racket-lang.org > > >> > Sent: Wednesday, August 21, 2013 6:58:56 PM GMT -05:00 US/Canada Eastern > >> > Subject: Re: [racket-dev] Lists aren't sets, but have set-like operations > >> > > >> > > >> > Ian, sets are now a generic datatype, like dictionaries. Association > >> > lists > >> > are dictionaries, and lists are now sets. They're also streams and > >> > sequences. They're not just "set-like". > >> > > >> > > >> > > >> > Carl Eastlund > >> > > >> > > >> > On Wed, Aug 21, 2013 at 6:56 PM, J. Ian Johnson < i...@ccs.neu.edu > > >> > wrote: > >> > > >> > > >> > I just wasted about 2 hours tracking down a bug that ended up being due > >> > to > >> > (set? '()) now evaluating to #t. I have no problems with set-union, > >> > intersection, etc. being defined for lists, but to treat lists as sets > >> > always is perverse to me. The contracts for set operations should use > >> > set-like? for (or/c set? list?) and keep the two constructions separate. > >> > > >> > This conflation is almost as bad as treating empty list as false. > >> > > >> > -Ian > >> > _________________________ > >> > Racket Developers list: > >> > http://lists.racket-lang.org/dev > >> > > >> > > >> > > >> > > >> _________________________ > >> Racket Developers list: > >> http://lists.racket-lang.org/dev > > _________________________ > > Racket Developers list: > > http://lists.racket-lang.org/dev > > > _________________________ > Racket Developers list: > http://lists.racket-lang.org/dev
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