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!

Answer 1

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.