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Scala list concatenation, ::: vs ++
In Scala, say I have two lists
scala> val oneTwo = List(1,2)
oneTwo: List[Int] = List(1, 2)
and
scala> val threeFour = List(3,4)
threeFour: List[Int] = List(3, 4)
I can concatenates Lists by doing:
scala> oneTwo ::: threeFour
res30: List[Int] = List(1, 2, 3, 4)
Or
scala> oneTwo ++ threeFour
res31: List[Int] = List(1, 2, 3, 4)
What is the difference between both approaches?
Thanks.
The ::: method is specific to List, while ++ is part of any Traversable.
The difference arise out of two things. First, List is one of the original Scala collections, used a lot in the compiler, and subject to special optimizations. The :: concatenation is the same as used in the ML family of languages, one of the big Scala inspirations, and ::: extrapolates from it.
On the other hand, ++ came along with the redesign of Scala collections on Scala 2.8.0, which made methods and inheritance uniform. I think it existed before that (on Set, for example), but the collections did not share a common superclass, so it was basically an ad hoc method for other collections.
In terms of performance, ::: should beat ++, but probably not significantly.
From the docs:
::: [B >: A](prefix : List[B]) : List[B]
++ [B >: A](that : Iterable[B]) : List[B]
You can see the ++ works for any Iterable while ::: is specifically for List's
scala> val oneTwo = List(1,2)
oneTwo: List[Int] = List(1, 2)
scala> val threeFour = List(3,4)
threeFour: List[Int] = List(3, 4)
scala> val fiveSix = Array(5,6)
fiveSix: Array[Int] = Array(5, 6)
scala> oneTwo ++ fiveSix
res2: List[Int] = List(1, 2, 5, 6)
scala> oneTwo ::: fiveSix
<console>:10: error: value ::: is not a member of Array[Int]
oneTwo ::: fiveSix
