Scala list index

Question

Can someone please explain why an explicit call to apply() is needed after map()

scala> val l = List(1, 2, 3)
l: List[Int] = List(1, 2, 3)

scala> l(2)
res56: Int = 3

scala> l.map(x => x*2)
res57: List[Int] = List(2, 4, 6)

scala> l.map(x => x*2)(2)
<console>:9: error: type mismatch;
 found   : Int(2)
 required: scala.collection.generic.CanBuildFrom[List[Int],Int,?]
              l.map(x => x*2)(2)
                              ^

scala> l.map(x => x*2).apply(2)
res59: Int = 6

Thanks.

Answer 1

That is because map method takes a second, implicit argument list with a CanBuildFrom implicit parameter:

def map[B, That](f: (A) ⇒ B)(implicit bf: CanBuildFrom[List[A], B, That]): That

Scala compiler interprets your code as if you're trying to pass 2 where the implicit CanBuildFrom is required.

The usage of CanBuildFrom and ugly method signatures that come with it is a very controversial element of Scala collections library that is regularly discussed and criticized.

Another issue here is the fact that Scala allows explicit passing of implicit parameters. I would personally argue that it shouldn't be allowed and then we could avoid many issues similar to yours. But that is of course a matter of opinion.