Question

Forgive me if this has already been asked elsewhere. I have a Scala syntax question involving function-values and implicit parameters.

I’m comfortable using implicits with Scala’s currying feature. For instance if I had a sum function and wanted to make the second argument an implicit:

scala> def sum(a: Int)(implicit b: Int) = a + b
sum: (a: Int)(implicit b: Int)Int

Is there a way to do this using the function-value syntax? Ignoring the implicit for a moment, I typically write curried function-values like this:

scala> val sum2 = (a: Int) => (b: Int) => a + b
sum: (Int) => (Int) => Int = <function1>

However, the function signature in the second approach is much different (the currying is being expressed explicitly). Just adding the implicit keyword to b doesn’t make much sense and the compiler complains as well:

scala> val sum2 = (a: Int) => (implicit b: Int) => a + b
<console>:1: error: '=>' expected but ')' found.
       val sum2 = (a: Int) => (implicit b: Int) => a + b
                                              ^

Furthermore partially-applying sum from the very first approach to get a function-value causes problems as well:

scala> val sumFunction = sum _
<console>:14: error: could not find implicit value for parameter b: Int
       val sumFunction = sum _
                         ^

This leads me to believe that functions that have implicit parameters must have said parameters determined when the function-value is created, not when the function-value is applied later on. Is this really the case? Can you ever use an implicit parameter with a function-value?

Thanks for the help!

Answer 1

scala>  val sum2 = (a: Int) => {implicit b: Int => a + b}
sum2: (Int) => (Int) => Int = <function1>

This will just make b an implicit value for the scope of the function body, so you can call methods that expect an implicit Int.

I don’t think you can have implicit arguments for functions since then it is unclear what the function is. Is it Int => Int or () => Int?

The closest I found is:

scala> case class Foo(implicit b: Int) extends (Int => Int) {def apply(a: Int) = a + b}
defined class Foo

scala> implicit val b = 3
b: Int = 3

scala> Foo()
res22: Foo = <function1>

scala> res22(2)
res23: Int = 5