[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:
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>
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