Question

I’m poking around with Scala again and have what I hope will be a basic question regarding duck typing, or maybe it is really with function definitions. Let me explain:

Given the following code:

package johnmcase.scala.oneoffs

object DuckTyping extends App {

  def printIt(x:Int, y:Int) = println("Value with " + x + " = " + y);

  // This method will accept ANY object that has a method named foo of type (Int) => Int
  def duckTyped(duck: {def foo: (Int) => Int}) = {
    List(1,2,3,4,5) foreach (i => printIt(i, duck.foo(i)))
  }

  println(new DoublerThatWontWork().foo(5))
  println(new Doubler().foo(5))
  println("DOUBLER:");
  duckTyped(new Doubler());
  println("Squarer:");
  duckTyped(new Squarer());
  println("AlwaysSeven:");
  duckTyped(new AlwaysSeven());
  println("DoublerThatWontWork :");
  duckTyped(new DoublerThatWontWork ()); // COMPILER ERROR!!
}

class DoublerThatWontWork { // WHY??
  def foo(x:Int) = x*2
}

class Doubler {
  def foo = (x:Int) => x*2
}

class Squarer {
  def foo = (x:Int) => x*x
}

class AlwaysSeven {
  def foo = (x:Int) => 7
}

So basically I have a method “duckTyped” that will accept ANY object as long as that object has a method named “foo” that is a Int=>Int function.

Why does the function declaration of foo in the class “DoublerThatWontWork” not satisfy the parameter type of function duckTyped?

Answer 1

Your signature says there is a method foo with no parameters, which returns a function from Int to Int. What you have is a method with an Int parameter and an Int Result.

you want

duck: {def foo(i: Int) : Int}

(parameter name does not have to match)