Use method with default parameter in map

Question

I want to map a stream of Doubles to a method which takes two parameters, one of them has a default value. I want to use the default parameter so my method has only 1 parameter which I need to pass:

  def pow(x:Double, exponent:Double=2.0) = {
    math.pow(x,exponent)
  }

I've found out that the following works, but I do not understand why:

  val res = (1 to 100).map(_.toDouble).map(pow(_))

I'm especially confused because the following does not work (compiler error because of missing type information):

  val pow2 = pow(_)
  val res = pow2(2.0)
  println(res) // expect 4.0

The compiler is not able to interfere the type provided for pow2 clearly. If you say val pow2 = pow(_:Double) the example works. — sascha10000
– sascha10000, Commented Aug 25, 2016 at 8:54
What scala version do you use (mine is 2.11.8 REPL) ? Unable to reproduce behavior, val pow2 = pow(_) gets compile error missing parameter type. If I write val pow2: Double => Double = x => pow(x), everything works. — dveim
– dveim, Commented Aug 25, 2016 at 8:56

sebszyller · Accepted Answer · 2016-08-25 08:54:14Z

3

The compiler is not able to infer the type that you will provide to pow2. In the res mapping you explicitly feed it a collection of Doublesand therefore pow(_) does not complain. However, in the case of val pow2 = pow(_) it complains that type parameter is missing. Change it to

val pow2 = pow(_: Double)
val res = pow2(2.0)
println(res)

and it will work just fine. pow(_) will be expanded two x => pow(x) and at this point the compiler cannot infere what's x without the type annotation.

answered Aug 25, 2016 at 8:54

sebszyller

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3 Comments

Raphael Roth Over a year ago

I do not really understand what pow(_) does because pow takes two arguments. Can you comment that?

dveim Over a year ago

val pow2 = pow(_) is shorthand for val pow2: Double => Double = x => pow(x), if you write types explicitly.

sebszyller Over a year ago

What @dveim said. Without the type annotation pow(_) you tell the compiler "Here is pow function, I will give you some _ parameter later" but you don't tell it what type it will be. You might give it a conflicting one, say String or a correct one Double. The error is a safeguard against the former case.

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