I wonder if #union: of Bags should use Set semantics or Bag semantics

Currently, and with your suggestion -
#(1 2 1) asBag union: #(1 2) asBag -> a Set(1 2)

But maybe it should be 
#(1 2 1) asBag union: #(1 2) asBag -> a Bag(1 1 1 2 2)

It would be more meaningful, and one can always convert to Sets first to
get the usual functionality.

Daniel

"Richard A. O'Keefe" <ok at cs.otago.ac.nz> wrote:
> In Squeak 3.0, 3.2, and 3.4alpha(5108) we find the same implementation
> of #union:
> 
>     Collection>>
>     union: aCollection
>         ^self asSet addAll: aCollection; yourself
> 
> This is the _only_ implementation of #union: in those versions of Squeak.
> There are three implementations of #asSet,
> 
>     Collection>>
>     asSet
> 	"Answer a Set whose elements are the unique elements
> 	 of the receiver."
> 	^Set withAll: self
> 
>     Bag>>
>     asSet
>         ^contents keys
> 
>     Set>>
>     asSet
> 	^self
> 
> The method comment in Collection says that the answer is a Set,
> but for bags it may be a Set or an IdentitySet, and for sets it
> may be a Set, an IdentitySet, or a PluggableSet.
> 
> Collections other than sets always answer a new object to #asSet;
> sets of all kinds never answer a new object.
> 
> The problem is that the point of #union: is to return a new object
> without mutating any existing objects.  But, irony of ironies, if
> the receiver already is some kind of set, it IS mutated.
>     x := #(3 1 4 1) asSet<print it>
> =>  a Set(1 3 4)
>     x union: #(5 9 6)<print it>
> =>  a Set(1 3 4 5 6 9)
>     x<print it>
> =>  a Set(1 3 4 5 6 9)
> 
> The first problem to sort out is what the semantics of #asSet
> is supposed to be.  #asIdentitySet always returns an IdentitySet,
> not some other kind of set.  "as: Set" always returns a new object
> that is precisely a Set, not any other kind of set.  The method
> comment in Collection suggests that the author thought #asSet
> always returned a Set.
> 
> The simplest change that could possibly work is "add a colon".
> 
>     Collection>>
>     union: aCollection
>         ^(self as: Set) addAll: aCollection; yourself
> 
> This would eliminate the unintended mutation bug.
> It would also make the result always be a Set, not any other kind of set.
> Less happily, it would be less efficient for bags.
> 
> What I actually propose is a "bigger" change that makes much less
> difference to the observable behaviour.  In particular, the change
> I propose
> + does not alter any existing method
> + does not change the costs or results of #union: for anything except sets.
> ? still allows the kind of set you get to depend on the class of the
>   receiver, and in the same way as before.
> 
> The correction is to add a new method:
> 
>     Set>>
>     union: aCollection
>         "Answer the set theoretic union of the receiver and aCollection,
>          using the receiver's notion of equality and not side effecting
>          the receiver at all."
> 
> 	^self copy addAll: aCollection; yourself