Why the type annotation is changing in Scala for one val

Question

Why i am getting this type annotation difference in these below scenarios. For scenario 1

case class TestData(name : String , idNumber : Int)
val createRandomData : immutable.IndexedSeq[Int => TestData]= (0 to 2).map{
    _ => TestData("something",_)
  }

For scenario 2

case class TestData(name : String , idNumber : Int)
val createRandomData: immutable.Seq[TestData] = (0 to 2).map{
    i => TestData("something",i)
  }

Why in scenario 1 is return type is a function not a collection of Seq.

Dmytro Mitin · Answer 1 · 2019-04-29T10:04:40.727

3

Because TestData("something",i) has type TestData and TestData("something",_) has type Int => TestData.

The second underscore is used for lambda (while the first underscore means that argument doesn't matter).

What are all the uses of an underscore in Scala?

edited Apr 29 '19 at 10:04

answered Apr 25 '19 at 14:14

Dmytro Mitin

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score 3 · Accepted Answer · answered Apr 25 '19 at 16:00

When you do something like this:

    case class TestData(name : String , idNumber : Int)
    val createRandomData : immutable.IndexedSeq[Int => TestData]= (0 to 2).map{
        _ => TestData("something",_)
    }

the first underscore means that you ignore the value of the parameter, then you use another underscore in the body of the function passed to map so you are creation a lambda function that ends to be the return type.

What you wanted to in in first scenario was:

case class TestData(name : String , idNumber : Int)
val createRandomData = (0 to 2).map{
  TestData("something",_)
}

Which has TestData as return type.

Why the type annotation is changing in Scala for one val

2 Answers2