Scala map explicit type

Question

I'm new in Scala and programming in general.. I have troubles with the Scala map function..

The simple signature of the map function is: def map[B](f: (A) ⇒ B): List[B]

So i guess the B of map[B] is generic and i can explicit set the type of the result?

If i try to run the code:

 val donuts1: Seq[Int] = Seq(1,2,3)
 val donuts2: List[Int] = {
    donuts1.map[Int](_ => 1)
  }

i got the error message "expression of type int doesn't conform to expexted type B"

I don't understand the problem here.. Could someone explain the problem?

Thank you!

jwvh jwvh · Accepted Answer · 2018-09-17T09:59:49

The map() signature quoted in your question is a simplified/abbreviated version of the full signature.

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

So if you want to specify the type parameters (which is almost never needed) then you have to specify both.

val donuts1: List[Int] = List(1,2,3)
val donuts2: List[Int] = donuts1.map[Int,List[Int]](_ => 1)
//donuts2: List[Int] = List(1, 1, 1)

and i can explicit set the type of the result?

Not really. The type parameter has to agree with what the f function/lambda returns. If you specify the type parameter then you're (usually) just asking the compiler to confirm that the result type is actually what you think it should be.

Scala map explicit type

1 Answers