[racket-dev] Lists aren't sets, but have set-like operations

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 <ianj at ccs.neu.edu> wrote:

> No, it doesn't seem to be using the fallback in this case.
>
> ianj at 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" <samth at cs.indiana.edu>
> To: "J. Ian Johnson" <ianj at ccs.neu.edu>, "Carl Eastlund" <cce at ccs.neu.edu>
> Cc: dev at racket-lang.org, "Matthew Flatt" <mflatt at 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 <ianj at 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" <samth at cs.indiana.edu>
> > To: "J. Ian Johnson" <ianj at ccs.neu.edu>
> > Cc: dev at racket-lang.org, "Matthew Flatt" <mflatt at 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" < ianj at 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" < mflatt at cs.utah.edu >
> > To: "Carl Eastlund" < cce at ccs.neu.edu >
> > Cc: "J. Ian Johnson" < ianj at ccs.neu.edu >, "dev" < dev at 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 < ianj at 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" < cce at ccs.neu.edu >
> >> > To: "J. Ian Johnson" < ianj at ccs.neu.edu >
> >> > Cc: "dev" < dev at 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 < ianj at 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
>
>
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