Scala - currying function with implicit value

Question

I've approached a problem with scala implicits and I am posting a simplified version here.

I have a class called SomeClass that contains a implicit val a = 3;

I have a created a trait that I intend to use with SomeClass like below:

 trait TestSum {
    def sum(a:Int)(b:Int)(implicit c: Int): Unit = {
      a + b + c
    }

    val sum_a = sum(1) _
  }

Sum_a obviously will not work, because it requires an implicit value c, which is accessible in SomeClass scope. Moving sum_a to SomeClass solves the issue but messes a bit in my code. Is there any good solution to this?

Angelo Genovese · Accepted Answer · 2015-08-13 15:52:41Z

6

You could require the implicit to exist in the implementor like this:

trait TestSum {
  implicit def c: Int

  def sum(a:Int)(b:Int): Unit = {
    a + b + c
  }

  val sum_a = sum(1) _
}

edited Aug 13, 2015 at 15:52

answered Aug 12, 2015 at 20:29

Angelo Genovese

3,39820 silver badges23 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

adelbertc · Accepted Answer · 2015-08-12 20:29:44Z

5

Functions in Scala cannot have implicit parameters, so when you do eta-expansion (the _) on a method to turn it into a function, it will look right then and there for a valid implicit parameter in scope to fill it in. As it stands, your code does not have an implicit Int in scope so the expansion fails. If you move it into SomeClass which does have an implicit Int, it will use that Int where the implicit is expected.

For instance:

scala> def foo(i: Int)(implicit s: String): String = i.toString ++ " " ++ s
foo: (i: Int)(implicit s: String)String

scala> foo _
<console>:10: error: could not find implicit value for parameter s: String
              foo _
              ^

scala> implicit val s: String = "number"
s: String = number

scala> foo _
res1: Int => String = <function1>

scala> res1(5)
res2: String = 5 number

More info can be found here: http://tpolecat.github.io/2014/06/09/methods-functions.html

answered Aug 12, 2015 at 20:29

adelbertc

7,34011 gold badges50 silver badges71 bronze badges

1 Comment

TheMP Over a year ago

Yes, I know why it does not work, I am just curious if there is any way to walk around this problem and keep the code more or less clean.

Collectives™ on Stack Overflow

Scala - currying function with implicit value

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related