Question

Scala’s Map and Set define a + operator that returns a copy of the data structure with a single element appended to it. The equivalent operator for Seq is denoted :+.

Is there any reason for this inconsistency?

Answer 1

Map and Set has no concept of prepending (+:) or appending (:+), since they are not ordered. To specify which one (appending or prepending) you use, : was added.

scala> Seq(1,2,3):+4
res0: Seq[Int] = List(1, 2, 3, 4)

scala> 1+:Seq(2,3,4)
res1: Seq[Int] = List(1, 2, 3, 4)

Don’t get confused by the order of arguments, in scala if method ends with : it get’s applied in reverse order (not a.method(b) but b.method(a))