Scala, placeholders and partial functions

A couple of Scala examples to demonstrate the use of place-holder syntax and partial functions. These two things are related but partial functions goes beyond syntactical conciseness to provide new and powerful constructions.

Substitution:

package test

object Placeholders {

  def main(args : Array[String]) {

    val myNumbers = 1 to 10 //List(1, 2, 3, 4, 5, 6, 7, 8, 9);

    println("Greater than 3")

    var result = myNumbers.filter((x : Int) => x > 3)

    result.foreach(println) // result.foreach((x) => println(x))

    println("Greater than 4")

    result = myNumbers.filter(x => x > 4)

    result.foreach(println)

    println("Greater than 5")

    result = myNumbers.filter(_ > 5) // _ placeholder _ for

    result.foreach(println)
  }
}

Partially applied functions:

Partial functions are functions defined with only some of the arguments (or none), as the following example demonstrates:

package test

object PartiallyAppliedFunctionsTest {

  def main(args : Array[String]) {

    val partiallyAppliedFunctions = new PartiallyAppliedFunctions();

    println("sum 4 arg 1 + 2 + 3 + 4 = " + partiallyAppliedFunctions.getSum4arg()(1, 2, 3, 4))                  // expect 10

    println("partiallyAppliedFunction 3 arg, 1 + 2 + 3 = " + partiallyAppliedFunctions.getSum3arg()(1, 2, 3))   // expect 6

    println("partiallyAppliedFunction 2 arg, 1 + 2 = " + partiallyAppliedFunctions.getSum2arg()(1, 2))          // expect 3
  }
}

class PartiallyAppliedFunctions() {

  private def sum(i1 : Int, i2 : Int, i3 : Int, i4 : Int) = {

    i1 + i2 + i3 + i4
  }

  def getSum4arg() = {

    sum _
  }

  def getSum3arg() : (Int, Int, Int) => Int = {

    sum(0, _ : Int, _ : Int, _ : Int)
  }

  def getSum2arg() : (Int, Int) => Int = {

    sum(0, 0,  _ : Int, _ : Int)
  }
}

The above example shows the function _ syntax to reference a function as a value.

Also, functions getSum3arg() and getSum4arg() show the use of partial functions to re-use an existing function by partially defining and supplying some of the arguments.


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