Question

What is the different between + and ++ applying on a set?

scala> val set = Set[String]("a","b")
set: scala.collection.immutable.Set[String] = Set(a, b)

scala> set + "c"
res2: scala.collection.immutable.Set[String] = Set(a, b, c)

scala> set ++ "c"
res3: scala.collection.immutable.Set[Any] = Set(a, b, c)

The first return Set[String] while the second return Set[Any].
It looks like ++ is more general, but what exactly is the ++ additional value?

Answer 1

If you look at the API doc for Set’s ++ method, you’ll see it takes a GenTraversableOnce

scala> val c:GenTraversableOnce[Any] = "c"
c: scala.collection.GenTraversableOnce[Any] = c

That means that in this case:

scala> set ++ "c"
res3: scala.collection.immutable.Set[Any] = Set(a, b, c)

"c" is a GenTraversableOnce[Any], and then the ++ method adds all the elements of that collection.

A better example would be

scala> set ++ "cd"
res1: scala.collection.immutable.Set[Any] = Set(a, b, c, d)

(I suspect like many other answerers I just did a little fiddling in the Scala console to check and then double-check whether that’s through an implicit conversion or not. Yes it appears it is.)