Adding two Set[Any]

Question

Adding two Set[Int] works:

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scala> Set(1,2,3) ++ Set(4,5,6)          
res0: scala.collection.immutable.Set[Int] = Set(4, 5, 6, 1, 2, 3)

But adding two Set[Any] doesn't:

scala> Set[Any](1,2,3) ++ Set[Any](4,5,6)
<console>:6: error: ambiguous reference to overloaded definition,
both method ++ in trait Addable of type (xs: scala.collection.TraversableOnce[Any])scala.collection.immutable.Set[Any]
and  method ++ in trait TraversableLike of type [B >: Any,That](that: scala.collection.TraversableOnce[B])(implicit bf: scala.collection.generic.CanBuildFrom[scala.collection.immutable.Set[Any],B,That])That
match argument types (scala.collection.immutable.Set[Any])
   Set[Any](1,2,3) ++ Set[Any](4,5,6)
           ^

Any suggestion to work around this error?

Flapper

It was fixed in scala2.9. Addable was removed. See [ticket4059][lampsvn.epfl.ch/trac/scala/ticket/4059] for more information. – Twinflower

Answer 1

This works:

Set[Any](1, 2, 3).++[Any, Set[Any]](Set[Any](4, 5, 6))

But is ugly as sin. The compiler is confused as to whether to use the method on Addable or the one on TraversableLike, which has an implicit parameter. They don't have the same sig, but syntactic sugar makes it appears as if they do. Tell it which one to use and the compiler's happy.

I imagine the reason it works for Ints is that they don't have any subtypes.

This will call the method on Addable, if that's important to you:

Set[Any](1, 2, 3).asInstanceOf[collection.generic.Addable[Any, Set[Any]]] ++ Set[Any](4, 5, 6)

Overstretch

@Landei: You're right. The value's a Set, but it would have to be cast. That's interesting. – Overstretch

Answer 2

Looks like using the alias union works,

scala> Set[Any](1,2,3) union Set[Any](4,5,6)
res0: scala.collection.immutable.Set[Any] = Set(4, 5, 6, 1, 2, 3)

I'm still curious if there's a way to use ++ instead.

Flapper

Answer 3

That works, but won't win the "Beautiful Code Contest":

Set[Any](1,2,3).++[Any,Set[Any]](Set[Any](4,5,6))

Blakley

Answer 4

val s:scala.collection.TraversableLike[Any, Set[Any]] = Set(1,2,3)
val t:Set[Any] = Set(3,4,5)
s ++ t

Consider this yet another entry in the ugliest code contest. ;)

Andreandrea