Question

I have read the answer to my question about scala.math.Integral but I do not understand what happens when Integral[T] is passed as an implicit parameter. (I think I understand the implicit parameters concept in general).

Let’s consider this function

import scala.math._

def foo[T](t: T)(implicit integral: Integral[T]) { println(integral) }

Now I call foo in REPL:

scala> foo(0)  
scala.math.Numeric$IntIsIntegral$@581ea2
scala> foo(0L)
scala.math.Numeric$LongIsIntegral$@17fe89

How does the integral argument become scala.math.Numeric$IntIsIntegral and scala.math.Numeric$LongIsIntegral ?

Answer 1

The parameter is implicit, which means that the Scala compiler will look if it can find an implicit object somewhere that it can automatically fill in for the parameter.

When you pass in an Int, it’s going to look for an implicit object that is an Integral[Int] and it finds it in scala.math.Numeric. You can look at the source code of scala.math.Numeric, where you will find this:

object Numeric {
  // ...

  trait IntIsIntegral extends Integral[Int] {
    // ...
  }

  // This is the implicit object that the compiler finds
  implicit object IntIsIntegral extends IntIsIntegral with Ordering.IntOrdering
}

Likewise, there is a different implicit object for Long that works the same way.