Scala Overview

Scala is object-oriented

• Scala is a pure object-oriented language in the sense that every value is an object. Types and behavior of objects are described by classes and traits. Classes are extended by subclassing and a flexible mixin-based composition mechanism as a clean replacement for multiple inheritance.

• Every user-defined class in Scala implicitly extends the trait scala.ScalaObject.
• If Scala is used in the context of a Java runtime environment, then scala.AnyRef corresponds to java.lang.Object.

Scala is Functional

• Scala is also a functional language in the sense that every function is a value.
• Scala provides a lightweight syntax for defining anonymous functions, it supports higher-order functions, it allows functions to be nested, and supports currying.
• Scala‘s case classes and its built-in support for pattern matching model algebraic types used in many functional programming languages.

Anonymous Function Syntax

• eg:

  (x: Int) => x + 1This is a shorthand for the following anonymous class definition:

  new Function1[Int, Int] {
    def apply(x: Int): Int = x + 1
  }

• It is also possible to define functions with multiple parameters:

  (x: Int, y: Int) => "(" + x + ", " + y + ")"
  

• or even with no parameter:

  () => { System.getProperty("user.dir") }

Higher-Order Functions

• Higher-order functions are those who can take functions as parameters, or whose result is a function.
• Eg: Function apply which takes another function f and a value v and applies function f to v:
  def apply(f: Int => String, v: Int) = f(v)
• A more complicated example:

  class Decorator(left: String, right: String) {
    def layout[A](x: A) = left + x.toString() + right
  }
  
  object FunTest extends Application {
    def apply(f: Int => String, v: Int) = f(v)
    val decorator = new Decorator("[", "]")
    println(apply(decorator.layout, 7))
  }

　　In this example, the method decorator.layout is coerced automatically to a value of type Int => String as required by method apply. Please note that the method decorator.layout is a polymorphic method(i.e. it abstracts over some of its signature types) and the Scala compiler has to instantiate its method type first appropriately.

Nested Functions

• In Scala it is possible to nest function definitions.
• Eg:

  object FilterTest extends Application {
    def filter(xs: List[Int], threshold: Int) = {
      def process(ys: List[Int]): List[Int] =
        if (ys.isEmpty) ys
        else if (ys.head < threshold) ys.head :: process(ys.tail)
        else process(ys.tail)
      process(xs)
    }
    println(filter(List(1, 9, 2, 8, 3, 7, 4), 5))
  }

Currying

• Methods may define multiple parameter lists. When a method is called with a fewer number of parameter lists, then this will yield a function taking the missing parameter lists as its arguments.
• Eg:

  object CurryTest extends Application {
    def filter(xs: List[Int], p: Int => Boolean): List[Int] =
    	if (xs.isEmpty) xs
    	else if (p(xs.head)) xs.head :: filter(xs.tail, p)
    	else filter(xs.tail, p)
  
    def modN(n: Int)(x: Int) = ((x % n) == 0)
  
    val nums = List(1, 2, 3, 4, 5, 6, 7, 8)
    println(filter(nums, modN(2)))
    println(filter(nums, modN(3)))
  }
  

  Note that method modN is partially applied in the two filter calls; i.e. only its first argument is actually applied. The term modN(2) yields a function of type Int => Boolean and is thus a possible candidate for the second argument of function filter.

Case Classes