<Scala><For beginners>

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 + 1
  This 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

• Case classes are regular classes which export their constructor parameters and which provide a recursive decomposition mechanism via pattern matching.
• An example for a class hierarchy which consists of an abstract super class Term and three concrete case classes Var, Fun and App.
  

  abstract class Term
  case class Var(name: String) extends Term
  case class Fun(arg: String, body: Term) extends Term
  case class App(f: Term, v: Term) extends Term

• This class hierarchy can be used to represent terms of the untyped lambda calculus.
• To facilitate the construction of case class instances, Scala does not require that the new primitive is used.
• The main benefits of case class:
  • Dont need new when initialization;
  • better toString() method;
  • with equals() & hashCode() default;
  • with Serializable default;
  • The constructor parameters are public(can be access directly);
  • Support pattern matching;(It makes only sense to define case classes if pattern matching is used to decompose data stuctures.)
• For better understanding of case class, u should know pattern matching first:
  • For javaer, switch is some kind of pm, but it's easy for programmer to forget 'break';
  • But in scala: [Scala has a built-in general pattern matching mechanism. It allows to match on any sort of data with a first-match policy. ]
    

    object PatternMatchingTest extends App {
      for (i <- 1 to 100) {
        i match {
          case 10 => println(10)
          case 50 => println(50)
          case _ =>
        }
      }
    }

Case class can be seen as a special class that have been optimized for pattern matching

.

abstract class Person

case class Student(name: String, age: Int, studentNo: Int) extends Person
case class Teacher(name: Stirng, age: Int, teacherNo: Int) extends Person
case class Nobody(name: String) extends Person

object CaseClassDemo {
  def main(agrs: Array[String]): Unit = {
 	// case class will generate apply method, this can reduce 'new'
    val p: Person = Student("john", 18, 1024)
    
    // match case
    p match {
      case Student(name, age, studentNo) => println(name + ":" + age + ":" + studentNo)
      case Teacher(name,age,teacherNo)=>println(name+":"+age+":"+teacherNo)
      case Nobody(name)=>println(name)
    }
  }
}

当一个类被声明为case class时，scala会帮我们做以下几件事情：

• 自动创建伴生对象，同时在其内实现子apply方法，因为在使用时不用显式new；
• 伴生对象内同时实现了upapply()，从而可以将case class用于模式匹配，具体之后在extractor会介绍；
• 实现toString(), hashCode(), copy(), equals()

Extractor Objects

• In scala, patterns can be defined independently of case classess. To this end, a method named unapply is defined to yield a so-called extractor.
For instance, the following code defines an extractor object `Twice`.

object Twice {
  def apply(x: Int): Int = x * 2
  def unapply(z: Int): Option[Int] = if (z % 2) == 0 Some(z / 2) else None
}

object TwiceTest extends Application {
  val x = Twice(21)
  x match { case Twice(n) => Console.println(n) }
}

There are two syntactic conventions at work here:

• The pattern case Twice(n) will cause an invocation of Twice.unapply, which is used to match even number; the return value of the unapply signals whether the argument has matched or not, and any sub-values that can be used for further matching. Here, the sub-value is z/2.
• The apply method is not necessary for pattern matching. It is only used to mimick a constructor. val x = Twice(21) expands to val x = Twice.apply(21).
• The return type of an unapply should be chosen as follows:
  • If it is just a test, return a Boolean.
  • If it returns a single sub-value of type T, return a Option[T].
  • If u want to return several sub-values T1, ..., Tn, group them in an optional tuple Option[(T1, ..., Tn)].
• Extractor: 提取器是从传递给它的对象中提取出构造该对象的参数。Scala提取器是一个带有unapply方法的对象。unapply方法算是apply方法的反向操作：unapply方法接受一个对象，然后从对象中提取值，提取的值通常是用来构造该对象的值。

Scala is statically typed

• Scala is equipped with an expressive type system that enforces statically that abstractions are used in a safe and coherent manner. 
• In particular, the type system supports:
  • generic classes
  • variance annotations
  • upper and lower type bounds
  • inner classes and abstract types as object members
  • compound types
  • explicitly typed self references
  • vies
  • polymorphic methods

Generic classes

• Scala has built-in support for classes parameterized with types. Such generic classes are particularly useful for the development of collection classes.
• Eg:
  

  class Stack[T] {
    var elems: List[T] = Nil
    def push(x: T) { elems = x :: elems }
    def top: T = elems.head
    def pop() { elems = elems.tail }
  }
  

  The use of type parameters allows to check that only legal elements(that of type T) are pushed onto the stack.

• Note that subtyping of generic types is invariant. This means that if we have a stack of characters of type Stack[Char] then it cannot be used as an integer stack of type Stack[Int]. 

Variances

• Scala supports variance annotations of type parameters of generic classes.