Wsrtjtyk

In Scala, is there a way to specify that a function should have default parameter values declared?

For example, in the code below, is there a way to specify in the signature of indirectHelloName that the provided function must provide a default value for the second parameter?

def helloName(name: String, greating: String = "hello"): Unit = { 

  println(s"$greating $name")

}



def indirectHelloName(name: String, function: (String,String) => Unit): Unit = {

  if (name == "Ted") {  

    function(name, "Custom Greeting for Ted!")

  } else {

    function(name) //This would use the default value for the second argument.

  }

}

In Scala, is there a way to specify that a function should have default parameter values declared?

For example, in the code below, is there a way to specify in the signature of indirectHelloName that the provided function must provide a default value for the second parameter?

A function cannot have an optional parameter with default argument, therefore there is no way to specify one:

val f = (a: Int, b: Int) => a + b

//⇒ f: (Int, Int) => Int = $$Lambda$1073/0x000000080070c840@6cd98a05



val g = (a: Int, b: Int = 5) => a + b

// <console>:1: error: ')' expected but '=' found.

//        val g = (a: Int, b: Int = 5) => a + b

//                                ^



val h = new Function2[Int, Int, Int] { 

  override def apply(a: Int, b: Int) = a + b

}

//⇒ h: (Int, Int) => Int = <function2>



val i = new Function2[Int, Int, Int] {

  override def apply(a: Int, b: Int = 5) = a + b

}

//⇒ i: (Int, Int) => Int{def apply$default$2: Int @scala.annotation.unchecked.uncheckedVariance} = <function2>



i(3, 5)

//⇒ res: Int = 8



i(3)

// <console>:13: error: not enough arguments for method apply: (v1: Int, v2: Int)Int in trait Function2.

// Unspecified value parameter v2.

//        h(3)

//         ^

One thing you could do is move the default value from the parameter list to inside the method.

def helloName(in :String*) :Unit =

  println(in.lift(1).getOrElse("hello") + ", " + in.head)



def indirectHelloName(name: String, function: (String*) => Unit): Unit =

  if (name == "Ted") function(name, "Custom Greeting")

  else               function(name) //use default

usage:

indirectHelloName("Ted", helloName)  //Custom Greeting, Ted

indirectHelloName("Tam", helloName)  //hello, Tam

I tried and come up with the below answer

    def helloName(name: String, greeting: String = "hello"): Unit = {

        println(s"$greeting $name")

    }



    val helloNameFn = (x: String, y :String) =>  println(x + y)



    type fn = (String, String) => Unit



    def indirectHelloName(name: String, function:fn = helloNameFn): Unit = {

        if (name == "Ted") {

        function(name, "Custom Greeting for Ted!")

        } else {

        helloName(name) //This would use the default value for the second argument.

        }

    }



    helloNameFn("123", "123")

Please try and let me know your comments.