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Notes for programming in Scala 2nd edition.

#Note for Programming in Scala


##Chp.0 SBT & Scala Interpreter

  1. Call scala interpreter by sbt.

    // enter scala interpreter
    console
    
    // exit scala interpreter
    :quit
    
    // help info
    :help
    
    // exit sbt
    exit
  2. You can escape wrong input by pressing enter two times.

    var oops =
    |
    |
    You typed two blank lines.  Starting a new command.

##Chp.1 A Scalable Language

  1. Scala is a hybrid of functional and object-oriented language. It is also statically typed.

    ####stackoverflow.com#Difference between strongly/statically typed language

  2. Functional programming has two main concepts:

    • Function is a first-class value.
    • Any function should map input values to output values, instead of changing input data in function scope.
  3. By above definition, Scala treat function as a value, and its input parameters are declared as immutable val constant rather than muttable var variables.

    // Because x is a val constant, so error will pop.
    def immutable(x: Int): Unit = {
      x = x + 1
      println(x)
    }
    // error: reassignment to val
    // x = x + 1

##Chp.2 First Step in Scala

  1. Define variables by constant val or variable var. Constant in Scala is immutable and unreassignable.

  2. Scala is able to figure out what types you leave off, it is called type inference.

    // In scala, String is acutally implemented by:
    // java.lang.String
    val msg = "Greeting!"
  3. In Scala you specify a variable type by :. Function is first-class so you can explicitly specify its type too.

    val msg: java.lang.String = "Hello"
    // msg: String = Hello
    
    // or use simple name String in Scala
    val msg: String = "Hello"
    
    def typed(x: Int, y: Int): Int = x
  4. Function must explicitly provide result type when it is recursive. If the function is not recursive, then scala compiler will be able to infer its result type.

    def recur(x: Int) = {
      if (x <= 10) recur(x + 1)
      else println(x)
    }
    // error: recursive method recur needs result type
    // if (x <= 10) recur(x + 1)
    //               ^
    
    def max(x: Int, y: Int) = if (x > y) x else y
    
    max(1,5)
    //res0: Int = 5
  5. We can return Unit type as void in Java, or C. The Unit type indicates that function returns no interesting value.

    def	hello(x: Int, y: Int): Unit = {
    	// println return Unit type
    	println("X: " + x + " Y: " + y);
    }
    
    hello(1, 2)
    // X: 1 Y: 2
  6. Same as Ruby, Scala use i += 1 or i = i + 1 instead of i++ or ++i in Java or C. That is because the assignment operator is actually create a new value from the copies of i and integer literal 1.

  7. Same as String literal or Integer literal, for the function, we cane have function literal too. If the function literal has only one parameter and consist of single statment which also takes single argument, then we can rephrase it as partial applied function.

    // Syntax of function literal
    (x: Int, y: Int) => function_body(x, y)
    
    args.foreach(arg => println(arg))
    
    // partial applied function
    args.foreach(println)
  8. The for-in loop:

    // Syntax for arg in args
    for(arg <- args) function_body
    
    for(arg <- 0 to 1) println(arg)
    // 0
    // 1

##Chp3. Next Step in Scala

  1. Parameterize arrays with types:

    val big = new java.math.BigInteger("12345")
    
    val greetings = new Array[String](3);
    greetings(0) = "Hello"
    greetings(1) = ", "
    greetings(2) = "World! \n"
    
    for(e <- 0 to 2) println(greetings(i))
    
    // or explicitly specify val type
    val greetings: Array[String] = new Array[String](3)
  2. Above array declare as val constant is because you should not reassign array variable, but operate its elements. Same idea as Object in Scala.

  3. Scala access index by parentheses, not square brackets as in Java.

  4. Same as Ruby, everything in Scala is object, so it can access some method by dot and parentheses.

    (0).equals(2)
    // Boolean = false

    We can also ignore parentehses and dot if the receiver of method call is explicitly specified. White space is not required too.

    // receiver is 0 object
    0 to 2 === (0).to(2)
    
    // cannot call println by this way,
    // println has no explicit receiver
    println 10
    
    Console println 10
  5. If a method takes only one parameter, you can call it without dot and parantheses.

    (0 to 2).equals(0.to(2))
    // Boolean = true
  6. If an Integer object call +, -, *, or / method (any arithmetic operators in Java) with dot, be sure to wrap it by parentheses. Otherwise Scala may treat it as numerical type instead(Double or Float).

    1.+(2)
    // warning: there were 1 deprecation warning(s);
    // re-run with -deprecation for details
    // Double = 3.0
    
    (1).+(2)
    // Int = 3
  7. When you apply parentheses surrounding one or more values to a variable, Scala will transform the code to call apply method instead. This is general rule.

    An application of an object to some arguments in parentheses will be transformed to an apply method call too. That object must define apply method so it can be successfully compiled.

    val greetings = new Array[Int](3);
    greetings(0) = 1
    greetings(1) = 2
    greetings(2) = 3
    
    
    println(greetings(0))
    
    // Above println statement is equivalent to
    greetings.apply(0)
  8. Similarly, when an assignment comes from a variable which take parentheses and one or more arguments have been applied, then the compiler transform code to call update method. This is the general rule too.

    val gret = new Array[Int](3)
    
    gret(0) = 1
    gret(1) = 2
    gret(2) = 3
    
    // the same as
    gret.update(0, 1)
    gret.update(1, 2)
    gret.update(2, 3)
  9. A more concise way to initialize an array:

    val int_ary = Array(1, 2, 3)
    
    val str_ary = Array("String", "In", "Array")
    
    // verbose ver
    val str_ary = Array.apply("String", "In", "Array")

    What this code actually doing is calling factory method apply, which creates and returns the new array. This apply is defined by companion object in Array class.

  10. List is an immutable sequence. Array is mutable sequence because its elements can be changed. You can infer that array's each element as var and list's each element as val.

    val list = List(1,2,3,4,5)
    val strs = List("S", "t", "r", "i", "n", "g")

    Differs from java.util.List type, scala.List is always immutable. Though its element cannot update value, but the element object can always changed its member field.

    class B {
    	var a = 0
    }
    
    val ary = Array(new B(), new B())
    // ary: Array[B] = Array(B@5967072b, B@7b07c5e7)
    
    ary(0) = new B()
    ary
    // Array[B] = Array(B@75c4957a, B@7b07c5e7)
    
    val list = List(new B(), new B())
    // List[B] = List(B@72dcecd9, B@641d579)
    
    list(0).a = 5
    // object's field is mutable
    // list(0).a: Int = 5
    
    list(0) = new B()
    // immutable element, error pops
    // error: value update is not a member of List[B]
  11. List concatenation method ::: returns a new list combined two lists.

    val one = List(1)
    val two_three = List(2, 3)
    
    val all = one ::: two_three
    //List[Int] = List(1, 2, 3)
  12. List prepend method ::

    val two = List(2)
    val one_two = 1 :: two
    // List(1, 2)

    In the expression 1 :: two, :: is actually right operand. If a method is invoked as operand notation, such as a * b, then the method is invoke as left operand as in a.*(b). If the method name ends in a colon :, then the method is invoked as right operand. So 1 :: two is the same as two.::(1).

  13. Nil is a short hand of empty list.

    val con = 1 :: 2 :: 3 :: Nil
    // List[Int] = List(1, 2, 3)

    Because :: is a right operand and defined in Nil or List, but not in 1 integer pbject. So we must put Nil at the end.

  14. Scala rarely use append method is because the time complexity grows linerly with size of list. Whereas :: takes constant time. Scala implement List as single pointer linked list. For prepend method ::, it only needs to create new list node then point and reset head node. For append method, it has to iterate to tail node and point to new node.

    ####programmers.stackexchange.com#append vs prepend ####docs.scala-lang.org#Collections performance characteristics

  15. Tuple is an immutaple but allowed multiple types collection.

    val pair = (99, "Luftballons")
    // Tuple2
    
    // access first elemenet by _1
    println(pari._1)
    println(pari._2)

    Tuple2 type is for the size 2 tuple, although conceptually you can create tuples of any length, Scala only defined the length up to 22 Tuple22 in library.

  16. Tuple's access method is one-based(_1 to _N) instead of zero-based(0 to n-1) in array's index. It is designed to distinguish ambiquity of syntax due to apply in array always return same type, but tuple may have different type in each element.

  17. Scala provide 3 traits for the Set collection, two subtraits are scala.collection.immutable.Set and scala.collection.mutable.Set. By default scala returns immutable set when you invoke a Set. Immutable set cannot change element values.

    val x = Set(1, 2, 3)
    // x: scala.collection.immutable.Set[Int] = Set(2, 3)
    
    import scala.collection.mutable.Set
    var x = Set(1, 2, 3)
    // x: scala.collection.mutable.Set[Int] = Set(2, 3)
    
    // explicitly initiation
    val x = scala.collection.immutable.Set(2, 3)
    // x: scala.collection.immutable.Set[Int] = Set(2, 3)
  18. Set is a collection with unique elements. join method +=. For immutable set, that set += 1 is actually as set = set + 1 which reassign to a newly created set(so the set must be a var). For mutable set, that trait provides method += which add element to that set.

    var x = scala.collection.immutable.Set(1, 2)
    
    x.+(5)
    // returns new Set(1, 2, 5)
    // Set(1, 2, 5)
    
    (x.+(5)).equals(x)
    // Boolean = false
    
    x
    // Set(1, 2)
    
    x += 5
    // x = x + 5, x is a var
    // returns new Set(1, 2, 5)
    
    val y = scala.collection.mutable.Set(1, 2)
    (y += 5).equals(y.+(5))
    // Boolean = true
  19. There also has two other subtraits which are scala.collection.immutable.HashSet and scala.collection.mutable.HashSet. Default is immutable hashset too.

    import scala.collection.mutable.Map
    
    val map = Map[Int, String]()
    map += (1 -> "Hi")
    map += (2 -> "This")
    map += (3 -> "World")
    
    // For immutable Map
    val i_map = scala.collection.immutable.Map(
    	1 -> "Hi", 2 -> "This", 3 -> "World"
    )
    
    // Declare as var to make it reassignable.
    import scala.collection.immutable.Map
    var i_map = Map(1 -> "Hi")
    i_map += (2 -> "This", 3 -> "World")
    
    i_map
    // Map(1 -> Hi, 2 -> This, 3 -> World)
    
    println(i_map(1))
    // Hi

    Scala allows you invoke -> on any object which returns a two element tuple with key and value. This mechanism is called implicit conversion.


##Chp.4 Classes and Objects

  1. In Java:

    Modifier World Subclass Same package Self
    public x x x x
    protected x x x
    none (default) x x
    private x

    But in Scala, makes members public is by not explicitly specifying access modifier. It is different from Java. Scala set public as default access level.

  2. Parameters in a method are val not var. It is immutable inside the method.

  3. You can leave curly braces off if method computes only one singe result expression(not statement).

    def mth(x: Int): Unit = println(x)
  4. If a method's result expression has side effect(mutation, reassignment), then its result type will be Unit. We can leave off the equal sign and put only curly braces to declare its result type to Unit.

    var x = 10
    def add(y: Int): Int = x += y
    // Type mismatch
    // found   : Unit
    // required: Int
    
    def add(y: Int): Unit = x += y
    
    // declare its result type to Unit
    def add2(y: Int) { x += y }
    
    add(5)
    add(5)
    
    x
    // 20
  5. If you declare method result type to Unit, then every kinds of result type from expression/statement will be converted to Unit and its value lost. For example:

    def convertion() { "String" }
    
    convertion
    // prints nothing, Unit type returned
    
    def convertion(): String = { "String!" }
    
    convertion
    // String = String!
  6. The rules of Semicolon Inference:

    Semicolon will not apply if:

    1. If a line ends in a word that would not be a legal statment, such as . or infix operator.
    2. The next line begins with a word that cannot start a statment.
    3. The line ends with brackets, or parentehses because that brackets cannot contain multiple statements.
    // multiple statments in single line
    println("Hi"); println("This")
    
    // case 1
    x +
    y
    
    // case 2
    x =
    - 1
    
    // case 3
    (x
    + 5
    )

    Expression is a kind of statement, it produces result values.. Call a var, object, val, reassignment are all expressions. Declare a var, or val is not expressions(result type is Unit).

  7. Use keyword object to create singleton object.

  8. When a singleton object shares the same name as other class name, that singleton object is called class's companion object and that class is companion class.

  9. A class and its companion object can access each other's private members.

  10. In Scala, companion object is a similar idea of static method in Java.

  11. Defind a singleton object does not define a type. Only campanion class defines a type.

  12. Singleton object cannot use new, but it can extend from superclass or mixin traits, which becomes to an instance of that superclass or traits, then able to called their instance methods.

    class G {
    	def g() { println("instance method g") }
    };
    
    object G {
    	def gg() { println("singleton method g") }
    }
    
    object V {}
    
    V.g
    // instance method g
    
    V.gg
    // cannot access G's singleton methods.
    // error: value gg is not a member of object V
  13. One difference between singlton object and class is that singleton object cannot take parameters, whereas class can. Each singleton object is implemented as an synthetic class referenced from a static variable. So they have the same initialization semantics as Java statics.

    The name of synthetic class is the singleton object's name plus dollar sign: object H to class H$.

  14. A singleton object does not share the name with other class is called standalone object.

  15. To run a Scala program, you have to supply a standalone singleton object with main method that takes one parameter Array[String] and has result type Unit. Then name the file should have the same name as that standalone object by convention.

    // Hello.scala
    object Helllo {
    	def main(args: Array[String]) {
    		println("Hello")
    	}
    }
  16. Scala implicitly imports java.lang and scala package and all the members of singleton object Predef into every scala source file.

  17. A Scala sciprt file must end with a result expression. Otherwise it is a Scala non-script file.

    // Script.scala have just single line
    println("Script file")

    Run scripts by scala REPL

    » scala Script.scala
    Script file.
  18. sbt project structure, and its compile, and run commands. Use sbt to manage project. Use scala REPL to run scala scripts.

    ####www.scala-sbt.org#Hello.html

  19. Scala provides a trait, scala.App(scala.Appliction trait has been deprecated since Scala 2.9), which save typing code for main method in your standalone object. The App trait actually declare main method. The code in curly braces is collected to primary constructor of singleton object. And is executed when the synthetic class is initialized.

    ####API: scala.App

    // AppTrait.scala
    object StandaloneObject extends App {
    for(arg <- args) println(arg)
    }

    Then compile by scalac and run scala:

    » scalac AppTrait.scala
    
    » ls
    StandaloneObject$$anonfun$1.class
    StandaloneObject$.class
    StandaloneObject$delayedInit$body.class
    StandaloneObject.class
    
    » scala StandaloneObject Hello World
    Hello
    World
  20. You can just run scala script by calling that standalone object:

    // AppTrait.scala
    object StandaloneObject {
    for(arg <- args) println(arg)
    }
    
    StandaloneObject

    Then run scala REPL(Read-Evaluate-Print Loop).

    » scala AppTrait.scala Hello World
    Hello
    World
  21. Caveat for scala.App or scala.Application:

    1. Use scala.App in single-threaded program, some JVM has restrictions of its threading model.
    2. Avoid overriden the main method in singleton object.
    3. scala.Application cannot access args, beware.

##Chp.5 Basic Types and Operations

  1. Basic types:

    Value Type Range
    Byte -(2^7) to 2^7 - 1
    Short -(2^15) to 2^15 - 1
    Int -(2^31) to 2^31 - 1
    Long -(2^63) to 2^63 - 1
    Char 16-bit unsigned Unicode char 0 to (2^16)-1
    String Sequence of Chars
    Float 32-bit IEEE single-precision float
    Long 64-bit IEEE double-precision float
    Boolean true or false

    15 bits => (2^15 - 1) because 0 holds one binary representation.

  2. Literals:

    // hex literal => prefix 0x
    val hex = 0x10F
    
    // octal literal => prefix 0
    val oct = 035
    
    // long literal => suffix L or l
    val long = 45L
    
    // short literal => cast Short type
    val short: Short = 367
    
    // byte literal => cast Byte type
    val byte: Byte = 127
    
    // double literal => decimal with/no suffix d or D
    val double = 3.14159
    val double = 3.14159d
    
    // double with power of 10
    val double = 3.14E5
    val double = 3.14E5D
    // 3.14 * 10^5
    
    // float literal => suffix f or F to decimal
    val float = 3.14.159f
  3. Character literals:

    // Char => single quote
    val char = 'a'
    // Char = A
    
    // Char => backslash with ASCII code value
    val char = '\101'
    // Char = A
    
    // Char => backslash with unicode value
    val char = '\u0041'
    // Char = A

    You can apply Unicde values to anywhere in Scala:

    val B\u0041\u0044 = 1
    // BAD: Int = 1
    
    v\u0061r B\u0061D = 1
    // BaD: Int = 1
  4. String Literals:

    Special Literal Meaning
    \n new line \u000A
    \b backsapce \u0008
    \t tab \u0009
    \f form feed \u000c
    \r carriage return \u000d
    " double quote \u0022
    ' single quote \u0027
    \ back slash \u005c
    // use char literal
    val hello = "hello"
    // hello
    
    val escapes = "\\\"\'"
    // \"'

    Or use """ to make multiple lines Strings.

    println("""Welcome to Multiple Lines:
    				"Hello\"
    				\World\""")

    To indent the leading space, use pipe | character then call stripMargin.

    println("""|Hello\
               |World\""".stripMargin)
  5. Symbol literals: If you declare same symobl literal twice, it will refer to same symbol object.

    // syntax: prefix `
    val s = `aSymbol
  6. Operators are methods, those operators may have serveral overloaded methods that take different parameter types.

    1 + 2
    
    // overload to Long parameter type
    1 + 2L
  7. Infix operators can concise the syntax as in 3.4

    "String".indexOf('s')
    
    "String" indexOf 'S'
  8. Prefix and postfix operators are unary, which means they only take one operand.

    The only prefix operators are +, -, !, ~, you can call method unar_ append these four operators to prepend it to receiver.

    -2.0 == (2.0).unary_-
    +2.0
    ~0xfff == (0xfff).unary_~
    !true

    p.unary_* cannot prepend * as *p because * is not the prefix operators, instead, it actually call customed unary_* method. Note that *p will be invoked as (*).p in Scala.

  9. Postfix operators are methods that take no parameters.

    "String" toLowerCase
  10. To get IEEE 754 remainder, call scala.math.IEEEremainder.

    math.IEEEremainder(11.0, 4.0)
    // Double = -1.0
  11. For 16 signed bits:

    1 		// 0000000000000001
    32767	// 01111..........1
    -1		// 11111..........1
    -2		// 1111111111111110
    
    ~1 == -2
    // true
  12. Operator precedence

    Operator precedence(high to low)
    * / %
    + -
    :
    = !
    < >
    &
    ^
    |
    (all letters)
    (assignment operators) ops all end in an equal sign.

    If the methods end in : they are group right to left, otherwise group from left to right.

    a ::: b ::: c == a ::: (b ::: c)
    
    a * b * c == (a * b) * c
    
    // assignment operatos
    ==
    >=
    *=
    +=
  13. RichWrapper provides more methods, then basic types.


##Chp.6 Functional Objects

  1. Class can have class parameters, Scala compiler will grap class parameters then create a priamry constructor that takes the same two parameters.

    class Params(x: Int, y: Int) {
    	val a = x
    	val b = y
    }
    
    Console println (new Param(5, 10)).a
    // 5
  2. You cannot access class parameter directly as object fields from outside, but you can only access their value in its method definition of same object. You can access it by defining fields and assigning its value to class parameter.

    class Params(x: Int, y: Int) {
      val a = x + y
    }
    
    (new Param(5, 10)).x
    // value x is not a member of Param
    
    (new Param(5, 10)).a
    // 15
    
    class Params(x: Int, y: Int) {
      def add(that: Params): Params = {
        new Params(x + that.x, y + that.y)
      }
    }
    
    // value x is not a member of Params
    // new Params(x + that.x, y + that.y)
    //                     ^
    
    class Params(x: Int, y: Int) {
      val that_x = x;
      val that_y = y;
      def add(that: Params): Params = {
        new Params(x + that.that_x, y + that.that_y)
      }
    }
    
    new Params(5, 10).add(new Params(5, 10)).that_x
    // 10
  3. Scala will compile any code you place in the class body which isn't part of a field or a method definition to primary constructor.

    class Part(x: Int, y: Int) {
    	println("Create" + " x: " + x + " y: " + y)
    }
    
    new Part()
    // Create x: 5 y: 10
  4. Pros & Cons of immutable object:

    Pros:

    1. Readable.
    2. You can pass immutable freely, while passing mutable object require defensive copies.
    3. Two threads may corrupt mutable object, but there has no way to corrupt immutable object's state because its immutable.
    4. Immutable objects makes safe hash keys, such as HashSet's key may not found if that mutable object mutate after it has been placed into HashSet.

    Cons:

    1. Copy an large immutable object is wasted, Scala provide alternative way to use mutable object instead.
  5. Override with override modifier.

    class Rational(n: Int, d: Int){
    // override default toString method
    	override def toString = n + "/" + d
    }
  6. You can specify precondition by require to check class parameters' input value.

    class Rational(n: Int, d: Int){
      require(d!= 0)
      override def toString = n + "/" + d
    }
    
    new Rational(5, 6)
    // Rational = 5/6
    
    new Rational(5, 0)
    //	java.lang.IllegalArgumentException:
    // 	requirement failed
    //  at scala.Predef$.require(Predef.scala:207)
  7. this refer to current instance, it is the same as Ruby's self.

  8. You can provide auxiliary constructor this(...) which has different class parameters than the primary constructor. Every auxiliary constructor must invoke another primary or auxiliary constructor in first statement.

    class Rational(n: Int, d: Int) {
    	def this(n: String, d: String) = {
    		this(5, 6)
    		println("String for n: " + n + " d: " + d)
    	}
    }
    
    new Rational("5", "6")
    // String for n: 5 d: 6

    The net effect is that every constructor will finally call to primary constructor. Then the primary constructor is the single point of entry of class.

  9. Private fields or methods are only callable inside that class, subclass cannot call them.

  10. Initializer is the statement that initialize a variable. Scala initialize the fields first, then methods, if the field init by other method, then init that method first.

  11. Operators(*, +, -) are methods, we can define it in a custom class.

    class Sum(x: Int) {
    	val value = x;
    	def + (that: Sum) = {
    		new Sum(x + that.value)
    	}
    
    	def | () = println(value)
    }
  12. Scala use camal-case and alphanumeric identifier. It starts with a underscore, or letter. The $ is reserverd for Scala compiler. Avoding using trailing underscore:

    // This will lead compiler trat as declare a name:_ variable.
    val name_: Int = 1
    
    // It proper define type to Int
    val name_ : Int = 1
  13. Though val is a constant. It still a variable, such as method's parameters are vals, but still can hold different values each time the method is called. Constant in scala is more permanent. It starts with an uppercase letter by convention.

    val Constant = 10
    
    // java style
    val CONSTANT = 10
  14. +, ++, :::, <?>, and :-> are some example of operators. Scala compiler will internally mangle operators to turn them into legal Java identifier ex: :-> would be represented as $colon$minus$greater internally. If you want to access this identifier from Java code, then you have to use this internal representation.

    Operators are 7-bit ASCII chars that are not digits and letters.

  15. Beware if you don't leave space between operator letters, then it will be treat as a single identifier:

    // In Java, this count as 4 lexical symbols
    x <- y == x < - y
    
    // In Scala, <- count as single identifer.
    x <- y != x < - y
    
    // If you want to seperate <-, add space
    
    x < - y
  16. Scala also provide literal identifier, so you can dynamically call the identifier in runtime.

    // syntax:  wrap by back ticks `
    
    `x`
    `<client>`
    `yield`
    
    // You can not directly call java thread's yield,
    // because Scala reserve this word.
    
    // call yield at runtime instad.
    Thread.`yield`()
  17. Implicit conversion, Scala allows you define customed conversion method with modifier implicit, it will be called when the conversion may occur by its method name.

    // In Scala interpreter scope
    
    // method name decide when intToRational occur.
    scala> implicit def intToRational(x: Int) = new Rational(x, 1)
    
    scaka> val r = new Rational(3, 2)
    scala> 2 * r
    // Rational: 3

    The implicit conversion only works when it is in scope. If you place convertion method definition to Rational class it won't work. Because when implicit conversion occurs in later, the method is not defined in current execution scope so there is no way to tell compiler automatically convert it in runtime.


##Chp.7 Built-in Control Structures

  1. if...else condition:

    println(if (!args.isEmpty) args[0] else "default.txt")
    
    val filename =
    	if (!args.isEmpty)
    		args[0]
    	else
    		"default.txt"
  2. Look for opportunities to use vals. They can make your code both easier to read and easier to refactor.

  3. while and do...while loop both are loops, not expressions, so the result type is always Unit. Unit type has return value as ().

    println("Void, return Unit value") == ()
    // Boolean = true
  4. Assignment in Scala always return Unit type.

    var line = ""
    // line = readLine() will return () Unit value.
    // Then () always not equals to "" in while loop.
    // This is not the sam as Java.
    while(line = readLine() != "")
    	println("Read: " + line)
  5. for expression in collection, generator file <- files generate new val named file in each iteration. Scala compiler infer file's type to File because files is Array[File].

    val fileHere = (new java.io.File('.')).listFiles
    
    for (file <- filesHere)
    	println(file)
  6. Create range by 1 to 4, or exclude upperbound by 1 until 4.

    1 to 4
    // scala.collection.immutable.Range.Inclusive = Range(1, 2, 3, 4)
    
    1 until 4
    // scala.collection.immutable.Range = Range(1, 2, 3)
  7. for filtering:

    val filesHere = (new java.io.File(".")).listFiles
    
    // add filtering if condition
    for (file <- filesHere if file.getName.endsWith(".scala"))
    	println(file)

    Above is equivalent to:

    for (file <- filesHere)
    	if (file.getName.endsWith(".scala"))
    		println(file)
  8. You can include more filters, just keep adding if clauses.

    for (file <- filesHere
    	if file.isFile
    	if file.getName.endsWith(".scala")
    ) println(file)
  9. If you add multiple <- clauses, you will create nested iterations.

    for (
    	file <- filesHere
    	if file.getName.endsWith(".scala");
    	line <- fileLines(file)
    	if line.trim.matches(pattern)
    ) println(file +": "+ line.trim)
  10. Mid-stream variable bindings. Above example code repeatedly compute line.trim in two places. To avoid nontrivial computation, you can introduce mid-stream variable by = assignment to reduce the reduntant computation.

    for {
      file <- filesHere
      if file.getName.endsWith(".scala")
      line <- fileLines(file)
      trimmed = line.trim
      if trimmed.matches(pattern)
    } println(file +": "+ trimmed)
  11. We can return new collection by yield. The return collection's type depends on its input collection type ex: files in below example. The implicit conversion will occur.

    ####alvinalexander.com#scala-for-loop-yield-examples-yield-tutorial

    // Syntax: for clauses yield body
    
    val new_files =
    for {file <- files if file.indexOf(".scala") != -1}
    	yield { file + " file" }
    // Array[String] = Array(a.txt file, b.txt file)

    Such as 0 to 9 is a Range object and its collection type is Vector from its IndexedSeq traits.

    ####docs.scala-lang.org#vectors

  12. throw, try and catch exceptions. Exception object can initialize with message string to replace default message. Use finally to ensure some executions before termination.

    // throw exception
    if x != 10
    	throw new IllegalArgumentException("R must be even.")
    
    import java.io.FileReader
    import java.io.FileNotFoundException
    import java.io.IOException
    
    // catch exception
    try {
    	val x = new FileReader("input.txt")
    } catch {
    	case ex: FileNotFoundException =>
    		println(ex + " File Not Found")
    	case ex: IOException
    }
    
    // add finally
    
    val x = new FileReader("input.txt")
    try {
    	// Use file
    } catch {
    	case ex: FileNotFoundException =>
    		println(ex + " File Not Found")
    	case ex: IOException =>
    		println(ex + " IO error")
    } finally {
    	// val x must declare outside of try-catch block
    	x.close()
    	println("File closed")
    }
  13. Not like Java, Scala returns value for try-catch block, As in Java, if a finally clause (1) includes an explicit return statement, (2) or throws an exception, that return value or exception will overrule any previous one that originated in the try block or one of its catch clauses.

    // return 2, explicitly return occurs in finally block
    def f(): Int = try { return 1 } finally { return 2 }
    
    // return 1
    // because exception or throw does not occur.
    def f(): Int = try { 1 } finally { 2 }
    
    // using catch to return new URL
    import java.net.URL
    import java.net.MalformedURLException
    def urlFor(path: String) =
    	try {
    		new URL(path)
    	} catch {
    		case e: MalformedURLException =>
    			new URL("http://www.scala-lang.org")
    	}
  14. Scala use match...case which is the same as switch...case in Ruby. Default case (wildcard case) marked as _.

    val firstArg = "default"
    
    firstArg match {
    	case "salt" => println("pepper")
    	case "chips" => println("salsa")
    	case "eggs" => println("bacon")
    	case _ => println("huh?")
    }

    Scala match...case also result values.

    val firstArg = "default"
    
    // result values returned to val res.
    val res = firstArg match {
    case "salt" =>
    	println("pepper")
    	"pepper"
    case "chips" =>
    	println("salsa")
    	"salsa"
    case "eggs" =>
    	println("bacon")
    	"bacon"
    case _ =>
    	println("huh?")
    	"huh?"
    }
    
    result
    // String = "huh?"
  15. Scala does not support break and continue. Alternative way to approach this by if...else with explicitly return, or put a boolean variable in pre-condition.

    int i = 0;                // This is Java
    boolean foundIt = false;
    while (i < args.length) {
      if (args[i].startsWith("-")) {
        i = i + 1;
        continue;
      }
      if (args[i].endsWith(".scala")) {
        foundIt = true;
        break;
      }
      i = i + 1;
    }
    
    var i = 0;
    var foundIt = false;
    while(i < args.length && !foundIt) {
      if (!args[i].startsWith("-") &&
      args[i].endsWith(".scala")) {
        foundIt = true
      }else{
        i = i + 1;
      }
    }
  16. We can try to get rid of the vars in above code by approaching a recursive function.

    def searchForm(i: Int): Int = {
    	if (i > args.length) -1
    	else if (args[i].startsWith("-")) searchForm(i + 1)
    	else if (args[i].endsWith(".scala")) i
    	else searchForm(i + 1)
    }
    
    val i = searchForm(0)
  17. If you really needs the break, it is in Scala's standard library. Which is allowed to break the enclosing breakable block.

    import scala.util.control.Breaks._
    import java.io._
    
    val in = new BufferedReader(new InputStreamReader(System.in))
    
    breakable {
      while (true) {
        println("? ")
        if (in.readLine() == "") break
      }
    }

    The Breaks class implements break by throwing an exception that is caught by an enclosing application of the breakable method. Therefore, the call to break does not need to be in the same method as the call to breakable.

  18. Scala compiler will not actually emit a new recursive function to increase stack frame. Scala will try to optimize recursion if it is a tail recursion. Scala jump back to the begining of function in same stack frame.

    // Tail call mth(x-1) + 1 will hold the stack frame
    // to compute result,
    // or for conditional check.
    // So this is not tail recurison
    def mth(x: Int): Int = {
    	if (x == 0) println("END")
    	else mth(x - 1) + 1
    }
    
    // Tail call mth(x-1) can reuse same stack frame.
    def mth(x: Int): Int = {
    	if (x == 0) println("END")
    	else mth(x - 1)
    }
    1. The trait for tail recursion is that no control must return to caller stack frame to decide return value before returning it. In other words, the last call in a function must be a function call instead of any other statement.

      // have arithmatic operation,
      // function call is not in tail position.
      def a(data) = a(data) + 1
      
      // have to check z,
      // function call is not in tail position.
      def b(data) = {
      	val z = x(data)
      	return z == 0 ? 1 : z
      }

      Variable assignment as last call will not be tail recursion, because it needs to turn the control back to caller for assignment.

      // val z = boom(x-1) is not tail recursion
      def boom(x: Int): Unit = {
      	if (x == 0) throw new Exception("Bang!")
      	else val z = boom(x-1)
      }

      Start scala interpreter with tail recursion optimization, the callstack did not jump back.

      scala> boom(5)
      java.lang.Exception: Bang!
      at .boom(<console>:11)
      at .boom(<console>:12)
      at .boom(<console>:12)
      at .boom(<console>:12)
      at .boom(<console>:12)
      at .boom(<console>:12)
      ... 33 elided
  19. Unlike javascript and ruby, conditional statement or loops will create new local scope. Curly braces create new scope. Variables cannot redefine in same scope.

    val a = 2
    // Redefine val a, this won't compile
    val a = 2

    Java does not allow inner scope with same-named variables, but Scala does. An inner variable is said shadow the outer variable.

    var a = 2;
    {
    	var a = 3
    	println(a)
    	// 3
    }
    println(a)
    // 2
  20. Interpreter allows you to redefine same variable. Conceptually, the interpreter creates a new nested scope for each new val or var statement you type in.

    // In Scala interpreter
    scala> val a = 2
    // 2
    scala> val a = 3
    // 3
    scala> println(a)
    
    // Above is equivalent to
    val a = 2;
    {
    	val a = 3;
    	{
    		println(a)
    	}
    }

    By the way, semi-colon should placed in var definition, otherwise the statement won't end because 4.6.2's rule.


##Chp.8 Functions and Closures

  1. Scala allow inner function because function can be first class. So the class will not be polluted by tons of private methods. Inner function also able to access variables of its outer function scope.

    object Lines {
    	def processFile(filename: String, width: Int) {
    		// local function processLine
    		def	processLine(line: String) {
    			if (line.length > width)
    				println(filename + ": " + line)
    		}
    		val source = Source.fromFile(filename)
    		for (line <- source.getLines())
    			processLine(line)
    	}
    }
  2. Function literal exist in source, whereas function value exist in object at runtime. A function literal is compiled into a class that when instantiated at runtime is a function value.

    // function literal
    (x: Int) => x + 1
    
    // function values are objects, so
    // you can store them in variables.
    
    var increase = (x: Int) => x + 1

    Every function value is an instance of some class that extends one of serveral FunctionN traits. N starts from 0. Each FunctionN trait has apply method used to invoke the function by parentheses.

    ####kwangyulseo.com#functions-are-objects-in-scala

  3. Function literal. Last statment in function body will be result value.

    // Syntax:
    // (params...) => { //code } or single line statement.
    
    val sum = (x: Int, y: Int) => {
    	println("x: " + x)
    	println("y: " + y)
    	x + y
    }
    
    sum(5, 6)
    // 5
    // 6
    // Int = 11
  4. Short forms of function literals: By target typing, if Scala can infer the type of parameters, then leave the type casting off. Also, we can reduce the parentheses if that function literal only have single parameter.

    val ary = Arra(1, 2, 3)
    
    ary.foreach((x: Int) => println(x))
    
    // short forms
    ary.foreach((x) => println(x))
    ary.foreach(x => println(x))
  5. Use _ underscore as placeholder for a single parameter, or parameter list in function literal. If the parameters are all consist of underscores, then you dont have to specify parameter list.

    someNumbers.foreach(_ > 0)
    // same as
    someNumbers.foreach(x => x > 0)

    If you use multiple underscores as multiple parameters, wrapped them with its type because compiler may not able to infer it. Each underscore maps to the parameter by the order of parameter list.

    // not enough info to infer two _'s type
    val f = _ + _
    
    val f = (_: Int) + (_: Int)
    
    f(5, 6)
    // Int = 11
  6. Because val/var assignment is running at runtime, so you must provide required function params to invoke that function as function values. Or use _ to represent parameter list as partial applied function. Partial applied function will defer the invokation until _ paramters fully supplied.

    def s() { println("No args is fine.") }
    
    val d = s
    
    def sum(x: Int, y: Int) = { x + y }
    
    // failed, fail to pass params.
    val k = sum
    
    // use partial applied function
    val z = sum _

    Or just prepresent as some parameters:

    def sum(x: Int, y: Int) = { x + y }
    
    val t = sum(5, _)
    
    // supplied _ params
    t(6)
    // Int = 11

    In above code, Scala compiler generate a new function class whose apply method takes one parameter. once t(6) is called, then it called apply to invoke sum with fully supplied parameters.

  7. If you are writing a partial applied function which leaves off all parameters by _ and the types can be inferred, then you can leave off the _ too.

    someNumbers.foreach(println _)
    
    // shorthand
    someNumbers.foreach(println)
    
    val x = sum
    // leads error because compiler cannot infer sum's arguments
    
    val x = sum _
  8. Javascript can directly assign function object to variables(actually Javascript hoist undefined value in parse phase, and assign to function object later in running phase).

    Scala has functions(function literals, partial applied function, or FunctionN traits) and methods(def). Methods is parsed at compile phase, functions is executed at runtime. That's why it needs partial applied function to wrap methods with wildcard underscore to convert a method to a function in runtime.

    ####jim-mcbeath.blogspot.tw#scala-functions-vs-methods.html

    ####stackoverflow.com#difference-between-method-and-function-in-scala

    The trailing underscore sum _ is because Scala is more related to Java imperative language, where a method that's not applied to all its arguments is considered an error.

  9. Closure in Scala is a function literal, function values(object), or partial applied function at runtime.

    A closure call outside variable is called open term, that variable is free variable and not binding to that closure.

    A colsure without any external variable is called closed term. the closure's local variables are called bind variable.

    val lit = (x: Int) => x + more
    // compiler complain: not found: value more
    
    // declare free variable
    val more = 5
    
    val lit = (x: Int) => x + more
    // lit: Int => Int = <function1>
    
    // summation
    var sum = 0
    val sumNumbers = List(1,2,3,4,5,6,7)
    
    sumNumbers(sum += _)
  10. We can also defined a method which returns a function value with open term.

    def makeIncrease(x: Int) = (y: Int) => x + y
    
    val inc5 = makeIncrease(5)
    
    val inc999 = makeIncrease(999)
    
    inc5(5)
    // 10
    
    inc999(1)
    // 1000

    When you create inc5, a closure is created and capture free variable x's value as 5. When you apply those closures to arguments, the result depends on how the free variable was defined when the closure was created.

    You can capture any variable you like: val, var, or parameters.

  11. Repeated parameters: append * to parameter type to declare that may have one or more repeated parmaters type. That repeated parameters must be listed in the last of paramter list.

    ####Caveat: You cannot directly write a varargs function literal, so repeated paramters is impossible supply to a function literal. Only method can have that feature. You can first define a method then convert to partial applied function as a function literal as a workaround.

    def echo(strs: String*) =
    	for (str <- strs) println(str)
    
    echo("gi", "joe")
    // gi
    // joe
  12. If you want to pass collection's each element to repeated parameters method, declare type : _* to tell compiler iterate each element as a parameter instead of just passing that collection. Though the repeated parameter(strs in example) still represent as a collection in internal function scope.

    def echo(strs: String*) = for(str <- strs) println(str)
    
    val list = List("John", "Cena", "is", "awesome")
    
    echo(list: _*)

    You can supply type for any object in scala.

    list: List[String]
    1: Int
    "X": String
    'C': Char
  13. We can pass named arguments by argName = value instead of passing them by its order in method definition.

    def namedArgs(x: Int, h: String, y: Char) = println(x + h + y)
    
    namedArgs(x = 5, y = '!', h = "hello")
    // 5hello!
  14. Also, you can supply default parameter value by argName: Type = defaultValue.

    def printTime(out: java.io.PrintStream = Console.out) = {
    	out.println("time = " +
    		System.currentTimeMillis())
    }
  15. Usually programming language does not support tail recursion optimization lead recursion slow because it does not jump back to the begining of function stack frame. Scala comiler default enables tail recursion optimization. So time complexity of below imperative approach is almost the same as functional one by simply jump back to the begining of the function.

    Check 7.18.1 for more info.

    // With tail recursion,
    // jump back to the begining of function
    def approX(guess: Double): Double =
    	if (isGoodEnough(guess)) guess
    	else approx(improve(guess))
    
    // loop just jump back to the begining of loop
    def approXLoop(guess: Double): Double = {
    	var i = guess
    	while(!isGoodEnough(i)){
    		i = improve(i)
    	}
    	i
    }

    If you want Scala compiler don't optimize for tail recursion, start scala interpreter or scalac with -g:notailcalls.

  16. Scala's tail recursion optimization is limited by Java JVM. Scala only optimizes directly recursive calls back to the same func- tion making the call.

    1. If the recursion is indirect, then no optimization is possible.

      def isEven(x: Int): Boolean =
      	if (x == 0) true else isOdd(x - 1)

    def isOdd(x: Int): Boolean = if (x == 0) false else isEven(x - 1) ```

    1. If final call goes to a function value, then it won't get tail-recursion optimization.

      val funValue = nestedFun _
      def nestedFun(x: Int) {
        if (x != 0) { println(x); funValue(x - 1) }
      }
  17. Extra example for placeholder and partial applied function:

    def mth(fnc: (String, String) => String) = {
    	// partial applied function
    	// Here placeholder can represent as
    	// a param list, or a parameter.
    	fnc(_: String, _:String)
    }
    
    // placeholder syntax _ + _
    // to shorthand the function literal.
    val paf = mth(_ + _)
    
    // Above is the same as
    val paf = mth((a: String, b:String) = > a + b)
    
    paf
    // (String, String) => String = <function2>
    // partial applied function2
    
    paf("Hello", " World!")
    // Hello World!

    Repeated paramters:

    def mth(fnc: (String) => Unit, strs: String*){
    	for (str <- strs) fnc(str)
    }
    
    val x = mth(println _, Array("Hello", "World", "!!!"): _*)
    // Hello
    // World
    // !!!

##Chp.9 Control Abstraction

  1. function literal reduction:

    def filesMatching(query: String,
    	matcher: (String, String) => Boolean) = {
    	for (file <- filesHere; if matcher(file.getName, query))
    		yield file
    }
    
    def filesEnding(query: String) =
    	filesMatching(query, _.endsWith(_))

    In above case, _.endsWith(_) is not a closure because it does not capture any free variables. Only two bound variables represented by underscore.

    private def filesMatching(matcher: String => Boolean) =
      for (file <- filesHere; if matcher(file.getName))
        yield file
    
    def filesEnding(query: String) =
      filesMatching(_.endsWith(query))

    In above code, _.endsWith(query) with one free variable query, and one bounded variable represent by underscore. This is a closure and Scala supports it so we can reduce previous example to above code.

  2. Scala's exists API is a control abstraction, it is a special-purpose looping.

    def containsOdd(nums: List[Int]) = nums.exists(_ % 2 == 1)
    
    // or self-implementation
    def containsOdd(nums: List[Int]): Boolean = {
    	var exists = false
    	for (num <- nums)
    		if (num % 2 == 1) exists = true
    	exists
    }
  3. Curry: A curried function is applied to multiple argument lists, instead of just one. The similar way to achieve this is return a function literals or transform method to function object.

    // Curried method
    def sum(x: Int)(y: Int) = x + y
    
    sum(5)(6)
    // 11
    
    // Similar way, but not the same as above code
    def sum(x: Int) = (y: Int) => x + y
    
    sum(6)(5)
    // 11

    We can set placeholder to either one parameter list. There has slightly difference when we assign the last parameter.

    // curried function
    def sum(x: Int)(y: Int) = x + y
    
    val x = sum(_: Int)(6)
    
    x(5)
    // 11
    
    val x = sum(6)_
    // val x = sum(6) _
    
    x(5)
    // 11
    
    // function literal version
    def sum(x: Int) = (y: Int) => x + y
    
    val x = sum(6)_
    // error: _ must follow method; cannot follow Int => Int
    
    val x = sum(6)(_)
    // or val x = sum(6)
  4. In any method invocation in Scala in which you’re passing in exactly one argument, you can opt to use curly braces to surround the argument instead of parentheses.

  5. The purpose of above ability to substitute curly braces for parentheses for passing in one argument is to enable client programmers to write function literals between curly braces. This can make a method call feel more like a control abstraction.

    def withPrintWriter(file: File, op: PrintWriter => Unit) {
      val writer = new PrintWriter(file)
      try {
        op(writer)
      } finally {
        writer.close()
      }
    }
    
    // client code
    withPrintWriter(new File("date.txt"),
      writer => writer.println(new java.util.Date))
    
    // Replace above code by curly braces approach
    
    // use curried function to workaround single parameter   restriction
    def withPrintWriter(file: File)(op: PrintWriter => Unit) {
      val writer = new PrintWriter(file)
      try {
        op(writer)
      } finally {
        writer.close()
      }
    }
    
    // Then client developer write as
    val file = new File("date.txt")
    withPrintWriter(file) {
      writer => writer.println(new java.util.Date)
    }
  6. By-name parameters: if a function literal parameter takes no parameter, then we can shorthand to by-name parameter.

    var assertionsEnabled = true
    def myAssert(predicate: () => Boolean) =
    	if (assertionsEnabled && !predicate())
    		throw new AssertionError
    
    // must call with ()
    myAssert(() => 5 > 3)
    
    myAssert(5 > 3)
    // Won’t work, because missing () =>
    
    // by-name parameter - predicate: => Boolean
    def byNameAssert(predicate: => Boolean) =
    if (assertionsEnabled && !predicate)
      throw new AssertionError
    
    byNameAssert(5 > 3)

    We can just pass 5 > 3 as boolean result, the problem is, that is not lazy evaluation so may cause unpredictable error before enter the function scope.

    // predicate is not function value,
    // will not put off evaluation until runtime.
    def boolAssert(predicate: Boolean) =
      if (assertionsEnabled && !predicate)
        throw new AssertionError
    
    var assertionsEnabled = false
    boolAssert(x / 0 == 0)
    // java.lang.ArithmeticException: / by zero
    
    byNameAssert(x / 0 == 0)
    // prints nothing,
    // assertionsEnabled && !predicate
    // short circuit because assertionsEnabled is false

##Chp.10 Composition and Inheritance

  1. Terminology distinguishes between declarations and definitions. Class Element declares the abstract method contents, but currently defines no concrete methods.

  2. Unlike Java, no abstract modifier is necessary (or allowed) on method declarations. Methods that do have an implementation are called concrete.

    abstract class Element {
    	// abstract method
    	def contents: Array[String]
    }

    An abstract class cannot instantiate and is subclassed by other class which should supply medthod definition.

  3. Defining parameterless methods: If methods defined with empty parentheses, we called it empty-paren methods. And declare it without parameter list.

    abstrac class Element {
    	def contents(): Array[String]
    
    	// empty-paren methods
    	def width: Int = contents.length
    	def height: Int = if (height == 0) 0 else contents(0).length
    }

    This convention supports the uniform access principle, it says that client code should not be affected by a decision to implement an attribute as a field or method.

    By above empty-paren methods, you can also declare them as variables.

    // fields, instead of member methods
    val width: Int = contents.length
    val height: Int = if (height == 0) 0 else contents(0).length

    The two pairs of definitions are completely equivalent from a client’s point of view. The only difference is that field accesses might be slightly faster than method invocations, because the field values are pre-computed when the class is initialized, instead of being computed on each method call.

  4. It is recommended to still write the empty parentheses when the invoked method represents more than a property of its receiver object.

    For instance, empty parentheses are appropriate if the method performs I/O, or writes reassignable variables (vars), or reads vars other than the receiver’s fields, either directly or indirectly by using mutable objects.

    "hello".length  // no () because no side-effect
    println()       // better to not drop the (), I/O
  5. Exetending class has several effects:

    1. It makes subclass inherit all non-private members from super class.
    2. It makes the type of subclass to a subtype of superclass.
    3. If you leave out extends in a class, Scala extends scala.AnyRef which is the same as java.lang.Object in java for you.
    class ArrayElement(conts: Array[String]) extends Element {
    	def contents: Array[String] = conts
    }
  6. Subtyping means that a value of the subclass can be used wherever a value of the superclass is required.

    val e: Element = new ArrayElement(Array("hello"))
  7. You could change the implementation in subclass from a method to a field without having to modify the abstract method definition of contents in superclass.

    abstract class Element {
    	def contents: Array[String]
    	def height: Int = contents.length
    	def width: Int = if (height == 0) 0 else contents(0).length
    }
    
    // change from method to field
    class ArrayElement(conts: Array[String]) extends Element {
    	val contents: Array[String] = conts
    }

    Therefore Scala forbid to define a field and method with the same name in the same class, whereas it is allowed in Java.

    class WontCompile {
    	private var f = 0 // Won’t compile, because a field
    	def f = 1         // and method have the same name
    }

    Generally, Scala has just two namespaces for definitions in place of Java’s four. Java’s four namespaces are fields, methods, types, and packages. By contrast, Scala’s two namespaces are:

    • values (fields, methods, packages, and singleton objects)
    • types (class and trait names)

    By above approach, Scala is able to override a val to method, which Java cannot.

  8. We can shorthand field definition by parametric fields.

    class ArrayElement(val contents: Array[String]) extends Element
    // shorthand of below code
    
    class ArrayElement(x123: Array[String]) extends Element {
    	val contents: Array[String] = x123
    }

    That field could be either val or var. It is possible to add modifiers such as private, protected, or override to these parametric fields, just as you can do for any other class member.

    class Cat { val dangerous = false }
    // shorthand of next code
    class Tiger(
      override val dangerous: Boolean,
      private var age: Int) extends Cat
    
    
    class Tiger(param1: Boolean, param2: Int) extends Cat {
      override val dangerous = param1
      private var age = param2
    }
  9. We can invoke superclass constructors:

    class ArrayElement(x123: Array[String]) extends Element {
    	val contents: Array[String] = x123
    }
    
    class LineElement(s: String) extends ArrayElement(Array(s)) {
    	override def width = s.length
    	override def height = 1
    }
    // now has an Array[String] field/method content.
  10. Scala requires override modifier for all members that override a concrete member in a parent class. Which means the abstract member overriden can safely ignore this modifier.

  11. Scala also have dynamic binding feature, its overriden methods depends on what instantiated instance during runtime, not the type of the variables.

    abstract class Element {
      def demo() { println("Element's implementation invoked") }
      }
    class ArrayElement extends Element {
    	override def demo() { ... }
      }
    
    class LineElement extends ArrayElement {
    	override def demo() { ... }
    }
    
    // UniformElement inherits Element’s demo
       class UniformElement extends Element
    
    
    val ae = new ArrayElement
    ae.demo
    // override message
    
    // dynamic binding to UniformElement,
    // which does not override Element's demo method.
    val ue = new UniformElement
    ue.demo
    // Element's implementation invoked

    You cannot call a variable or method which is not declared in its class or inherited/mixed from other class/trait. Fields and methods, which are defined in superclass and overriden in subclass, are dynamic binding to instance's method.

    class Z {}
    class B extends Z { val x = 5 }
    
    val b: Z = new B
    b.x
    //error: value x is not a member of Z
    
    class A extends B {}
    
    val a: B = new A
    a.x
    // 5
    // value x is a member of B

    Methods and variables, which are singleton, private and final, are resolved using static binding. Because private and final methods are not allowed overriden, and singleton method only provide staic methods to its type, not the instance.

    class A {
      def g = println("A mth g")
      private val x = "A private x"
    }
    
    class B extends A {
      def mth = println("B normal mth")
      private def pmth = println("B private mth")
      val b = "value b"
      private val pb = "private value b"
      final val fb = "final value b"
    }
    
    // b is a Type A instance,
    // accessable vars/methods must be defined in class A
    val b: A = new B
    
    b.pmth
    // error: value pmth is not a member of A
    
    b.x
    // error: value x in class A cannot be accessed in A
    // private value x not explode to outside
    
    b.mth
    // error: value mth is not a member of A
    
    // b is now a Type B instance
    val b: B = new B
    
    // get inherited method g from super class A
    b.g
    // A mth g
    
    b.mth
    // B normal mth

    Java's dynamic binding vs static binding:

    ####javarevisited.blogspot#what-is-static-and-dynamic-binding-in

  12. Add final modifier to restrict that members(methods/fields) are overriden.

    class B {
    	final val x = 5
    	final var y = 6
    }
    
    class G(override var y: Int = 6) extends B
    // error: overriding variable y in class B of type Int;
    // variable z cannot override final member
    
    class G(override val x: Int = 6) extends B
    // error: overriding value x in class B of type Int(5);
    // value x cannot override final member
  13. Using inheritance to consider the strong related is-a relationship. Otherwise pick composition instead.

  14. Create a factory object by companion singelton object.

    // import singleton's factory method elem
    import Element.elem
    abstract class Element {
    	// composition Array object
    	def contents: Array[String]
    
    	def above(that: Element): Element =
      elem(this.contents ++ that.contents)
    
    def beside(that: Element): Element =
      elem(
        for (
          (line1, line2) <- this.contents zip that.contents
        ) yield line1 + line2
      )
    
    }
    // Array(1, 2, 3) zip Array("a", "b")
    // evaluate to Array((1, "a"), (2, "b"))
    
    // another scala file
    object Element {
    	def elem(contents: Array[String]): Element =
    		new ArrayElement(contents)
    	def elem(chr: Char, width: Int, height: Int): Element =
    		new UniformElement(chr, width, height)
    	def elem(line: String): Element =
    		new LineElement(line)
    }

##Chp.11 Scala’s Hierarchy

  1. Scala hierarchy: scala.Any has two subclass scala.AnyRef and scala.AnyVal. scala.Nothing is subtype of every builtin class.

    Those classes must have 7 methods.

    final def ==(that: Any): Boolean

final def !=(that: Any): Boolean def equals(that: Any): Boolean def ##: Int def hashCode: Int def toString: String ```

  1. Nine value classes: Double, Int, Float, Long, Byte, Short, Char, Unit, and Boolean.

  2. You cannot create instance of these classes because they are all defined to both abstract and final.

  3. Unit has single instance value which is written ().

  4. All value classes are subtype of scala.Any but does not subclass to each other. Instead they are implicit conversions between different value class type. Ex: scala.Int is widened to scala.Long when required.

  5. min, max, and until are defined in scala.RichInt class but not in scala.Int class. However, we can invoke those methods in Int object which implicitly converts to scala.RichInt.

  6. scala.AnyRef in fact is just an alias of java.lang.Object. You can use them interchangeably in Scala program on Java platform.

  7. Boxing in Java, is a int wrapped by Integer class. For value types in Scala, == is the natural equality(not about reference id). For reference types other than Java's boxed numeric types, == is an alias of equals method. Though you can override it, such as == in Scala has override to the natural eqality.

    "asdf" == "asdf"
    // true, don't have to use equals method
  8. scala.AnyRef defines a eq method which cannot be overriden and use the reference equality.

    "asdf" eq "asdf"
    // true
    // because string literal will only have one copy in memory
    // That's for optimization in Java.
    
    val x = new String("asdf")
    val y = new String("asdf")
    
    x eq y
    // false, two different references.
  9. class scala.Null is a bottom type of any scala.AnyRef classes. It is not compatible to value classes.

  10. scala.Nothing is a type that has no value. One use of Nothing is that it signals abnormal termination. So don't have return value for it.

    def error(msg: String): Nothing =
    	throw new RuntimeException(msg)

    Nothing is a bottom type of AnyRef and AnyVal classes. It can be used like this:

    def divide(x: Int, y: Int): Int =
    	if (y != 0) x / y
    	else error("can't divide by zero")

##Chp. 12 Traits

  1. Trait can mixing either by extends or with keyword.

    trait Power {
    	def	square(value: Int) = {
    		value * value
    	}
    }
    
    class Values(val value: Int = 5) extends Power {}
    
    val traitType: Power = new Values()
    // traitType: Power = Values@2d69fc5
  2. Trait also have its type, in above example, type is Power and is a subtype of scala.AnyRef because it does not explicitly extend any class.

  3. You can do anything in a trait definition that you can do in a class definition, and the syntax looks exactly the same, except 2 differences:

    1. A trait cannot have any “class” parameters.
    2. In classes, super calls are statically bound, in traits, they are dynamically bound.

    If you write super.toString in a class, you know exactly which method implementation will be invoked. When you write the same thing in a trait, however, the method implementation to invoke for the super call is undefined when you define the trait.

  4. Trait cannot have class parameters, but can supply with type parameters.

    // Scala Ordered trait
    class Rational(n: Int, d: Int) extends Ordered[Rational] {
    	// ...
    	def compare(that: Rational) =
    		(this.numer * that.denom) - (that.numer * this.denom)
    }
  5. Traits with abstract override and extends modifiers:

    class Rational(n: Int, d: Int) {}
    
    abstract class IntQueue {
    	def get(): Int
    	def put(x: Int)
    }
    
    import scala.collection.mutable.ArrayBuffer
    class BasicIntQueue extends IntQueue {
    	private val buf = new ArrayBuffer[Int]
    	def get() = buf.remove(0)
    	def put(x: Int) { buf += x }
    }
    
    trait Doubling extends IntQueue {
    	abstract override def put(x: Int){ super.put(2 * x) }
    }

    The Doubling trait:

    1. It declares a superclass, IntQueue. This declaration means that the trait can only be mixed into a class that also extends IntQueue. Thus, you can mix Doubling into BasicIntQueue, but not into Rational.

    2. The trait has a super call on a method declared abstract. Such calls are illegal for normal classes, because they will certainly fail at run time.

      Since super calls in a trait are dynamically bound, the super call in trait Doubling will work so long as the trait is mixed in after another trait or class that gives a concrete definition to the method.

      This arrangement is to tell the compiler you are doing this on purpose, you must mark such methods as abstract override. This combination of modifiers is only allowed for members of traits, not classes, and it means that the trait must be mixed into some class that has a concrete definition of the method in question.

      class MyQueue extends BasicIntQueue with Doubling
      
      val queue = new MyQueue
      
      queue.put(10)
      
      queue.get
      // 20
  6. We can also mixed in trait when initialize a instance.

    // BasicIntQueue class and Doubling trait are defined.
    val queue = new BasicIntQueue with Doubling
    
    queue.put(10)
    
    queue.get
    // 20
  7. Traits as stackable modifications:

    The order of mixins is significant. Roughly speaking, traits further to the right take effect first then to left.

    trait Incrementing extends IntQueue {
    	abstract override def put(x: Int) { super.put(x + 1) }
    }
    
    trait Filtering extends IntQueue {
    	abstract override def put(x: Int) {
    		if (x >= 0) super.put(x)
    	}
    }
    
    al queue = (new BasicIntQueue
                   with Incrementing with Filtering)
    
    queue.put(-1); queue.put(0); queue.put(1)
    queue.get()
    // Int = 1
    
    queue.get()
    // Int = 2
  8. In traditional multiple inheritance, the super cannot call as stackable modifications. If the rule is the last supeclass(Filtering) wins, then its super call will not stack to another super class(Incrementing).

  9. In trait, the method called is determined by a linearization of the classes and traits that are mixed into a class.

    In any linearization, a class is always linearized before all of its superclasses and mixed in traits. That method must be all override until extends class/trait.

    class Animal

trait Furry extends Animal trait HasLegs extends Animal trait FourLegged extends HasLegs class Cat extends Animal with Furry with FourLegged ```

Linearization(order of method call):

| Type      | Linearization |
|:----------|:------------- |
| Animal    | Animal, AnyRef, Any |
| Furry     | Furry, Animal, AnyRef, Any |
|FourLegged | FourLegged, HasLegs, Animal, AnyRef, Any |
|HasLegs    | HasLegs, Animal, AnyRef, Any |
|Cat        | Cat, FourLegged, HasLegs, Furry, Animal, AnyRef, Any |

Example:

```scala
class Animal {
def put(str: String) = { println("Animal"); println(str) }
}

trait Furry extends Animal {
	abstract override def put(str: String) = {
		super.put("Furry")
		println(str)
	}
}

trait HasLegs extends Animal {
	abstract override def put(str: String) = {
	super.put("HasLegs")
	println(str)
}
}

trait FourLegged extends HasLegs {
	abstract override def put(str: String) = {
		super.put("FourLegged")
		println(str)
	}
}

class Cat extends Animal with Furry with FourLegged {
	override def put(str: String) = {
		super.put("Cat")
		println(str)
	}
}

val c = new Cat
c.put("End")

// Animal
// Furry
// HasLegs
// FourLegged
// Cat
// End
```

The order is, **start from subclass through traits by right to left mixed-in order, then finally enter to superclass and its' inheritance chain**.
  1. To trait or not to trait?

    1. If the behavior will not be reused, then make it a concrete class. It is not reusable behavior after all.

    2. If it might be reused in multiple, unrelated classes, make it a trait. Only traits can be mixed into different parts of the class hierarchy.

  2. If you want to inherit from it in Java code, use an abstract class. Since traits with code do not have a close Java analog.

    As one exception, **a Scala trait with only `abstract` members translates directly to a Java `interface`**, so you should feel free to define such traits even if you expect Java code to inherit from it.
    
    1. If you plan to distribute it in compiled form, and you expect outside groups to write classes inheriting from it, you might lean towards using an abstract class.
    2. If efficiency is very important, lean towards using a class. Most Java runtimes make a virtual method invocation of a class member a faster operation than an interface method invocation. Traits get compiled to interfaces and therefore may pay a slight performance overhead.
    3. If you still do not know, after considering the above, then start by making it as a trait. You can always change it later, and in general using a trait keeps more options open.

##Chp.13 Packages and Imports

Put curly braces to create a scope.

package bobsrockets.com

Several patterns:

// 1.

package bobsrockets {
  package navigation {
    // In package bobsrockets.navigation
    class Navigator
    package tests {
      // In package bobsrockets.navigation.tests
      class NavigatorSuite
    }
} }

Concise access to classes and packages:

// 2.

package bobsrockets {
  package navigation {
    class Navigator {
      // No need to say bobsrockets.navigation.StarMap
      val map = new StarMap
    }
    class StarMap
  }
  class Ship {
    // No need to say bobsrockets.navigation.Navigator
    val nav = new navigation.Navigator
  }
  package fleets {
    class Fleet {
      // No need to say bobsrockets.Ship
      def addShip() { new Ship }
    }
  }
}

Symbols in enclosing packages not automatically available:

// 3.

package bobsrockets {
  class Ship
}

package bobsrockets.fleets {
  class Fleet {
    // Doesn’t compile! Ship is not in scope.
    def addShip() { new Ship }
  }
}

Accessing hidden package names via _root_:

// 4.

// In file launch.scala
package launch {
  class Booster3
}
 // In file bobsrockets.scala
package bobsrockets {
  package navigation {
    package launch {
      class Booster1
    }
    class MissionControl {
      val booster1 = new launch.Booster1
      val booster2 = new bobsrockets.launch.Booster2
      val booster3 = new _root_.launch.Booster3
    }
  }
  package launch {
    class Booster2
  }
}
  1. A class can be accessed from within its own package without needing a prefix.

  2. A package itself can be accessed from its containing package without needing a prefix.

  3. When using the curly-braces packaging syntax, all names accessible in scopes outside the packaging are also available inside it ex: def addShip() { new Ship } in code snippet 2.

  4. This kind of access is only available if you explicitly nest the packagings. If you stick to one package per file, then —like in Java— the only names available will be the ones defined in the current package. Ex: code snippet 3.

  5. If nesting packages with braces shifts your code uncomfortably to the right, you can also use multiple package clauses without the braces:

    package bobsrockets
    package fleets
    class Fleet {
      // Doesn’t compile! Ship is not in scope.
      def addShip() { new Ship }
    }
  6. Scala provides a package named _root_ that is outside any package a user can write. Put another way, every top-level package you can write is treated as a member of package _root_. So _root_.launch.Booster3 designates the outermost booster class.


13.3 Imports

// easy access to Fruit
import bobsdelights.Fruit
// easy access to all members of bobsdelights
import bobsdelights._
// easy access to all members of Fruits
import bobsdelights.Fruits._

Imports in Scala can appear anywhere, not just at the beginning of a compilation unit. Also, they can refer to arbitrary values.

def showFruit(fruit: Fruit) {
    import fruit._
    println(name +"s are "+ color)
}

The subsequent println statement can refer to name and color directly. These two references are equivalent to fruit.name and fruit.color. In Scala, imports:

May apeer anywhere. May refer to objects (singleton or regular) in addition to packages.
Let you rename and hide some of the imported members.

Another way Scala’s imports are flexible is that they can import packages themselves, not just their non-package members:

import java.util.regex
class AStarB {
    // Accesses java.util.regex.Pattern
    val pat = regex.Pattern.compile("a*b")
}

Imports in Scala can also rename or hide members:

import Fruits.{Apple, Orange}
// This imports just members Apple and Orange from object Fruits.

import Fruits.{Apple => McIntosh, Orange}
// This imports the two members Apple and Orange from object Fruits.
// and rename Apple object to McIntosh

import Fruits.{Apple => McIntosh, _}

import Fruits.{Pear => _, _}
// This imports all members of Fruits except Pear.

<original-name> => _ excludes <original-name> from the names that are imported.


13.4 Implicit Imports

import java.lang._ // everything in the java.lang package
import scala._     // everything in the scala package
import Predef._    // everything in the Predef object

Because scala is imported implicitly, you can write List instead of scala.List, for instance.

An important rule - Later imports overshadow earlier ones. So if scala and java package both have StringBuilder then StringBuilder will refer to Scala.StringBuilder instead.


13.5 Access Modifier

Members of packages, classes, or objects can be labeled with the access modifiers private and protected.

A member labeled private is visible only inside the class or object that contains the member definition. In Scala, this rule applies also for inner classes. This treatment is more consistent, but differs from Java.

class Outer {
    class Inner {
        private def f() { println("f") }
        class InnerMost {
            f() // OK }
        }
    }
    (new Inner).f() // error: f is not accessible
}

Access to protected members is also a bit more restrictive than in Java. In Scala, a protected member is only accessible from subclasses of the class in which the member is defined in same package. In Java such accesses are also possible from other classes in the same package.

package p {
    class Super {
        protected def f() { println("f") }
    }
    class Sub extends Super {
        f()
    }
    class Other {
        (new Super).f()  // error: f is not accessible
    }
}

http://stackoverflow.com/questions/2632247/scala-giving-me-illegal-start-of-definition

Every member not labeled private or protected is public. There is no explicit modifier for public members.

A modifier of the form private[X] or protected[X] means that access is private or protected up to X, where X designates some enclosing package, class or singleton object.

package bobsrockets

package navigation {
    private[bobsrockets] class Navigator {
        protected[navigation] def useStarChart() {}
        class LegOfJourney {
            private[Navigator] val distance = 100
        }
        private[this] var speed = 200
    }
}
package launch {
    import navigation._
    object Vehicle {
        private[launch] val guide = new Navigator
    }
}
modifier scope
no access modifier public access
private[bobsrockets] access within outer package
protected[navigation] same as package visibility in Java
private[Navigator] same as private in Java
private[LegOfJourney] same as private in Scala
private[this] access only from same object

Visibility and Companion Objects

In Scala there are no static members; instead you can have a companion object that contains members that exist only once.

Scala’s access rules privilege companion objects and classes when it comes to private or protected accesses.

class Rocket {
    import Rocket.fuel
    private def canGoHomeAgain = fuel > 20
}

object Rocket {
    private def fuel = 10
    def chooseStrategy(rocket: Rocket) {
      if (rocket.canGoHomeAgain)
        goHome()
      else
        pickAStar()
    }
    def goHome() {}
    def pickAStar() {}
}

By contrast, a protected member in a companion object makes no sense, as singleton objects don’t have any subclasses.


13.6 Package objects

Each package is allowed to have one package object. Any definitions placed in a package object are considered members of the package itself.

// In file bobsdelights/package.scala
package object bobsdelights {
    def showFruit(fruit: Fruit) {
      import fruit._
      println(name +"s are "+ color)
    }
}
// In file PrintMenu.scala
package printmenu
  import bobsdelights.Fruits
  import bobsdelights.showFruit
  object PrintMenu {
    def main(args: Array[String]) {
      for (fruit <- Fruits.menu) {
        showFruit(fruit)
      }
  }
}

Package objects are frequently used to hold package-wide type aliases and implicit conversions.

Package objects are compiled to class files named package.class that are the located in the directory of the package that they augment. It’s useful to keep the same convention for source files.

So you would typically put the source file of the package object bobsdelights into a file named package.scala that resides in the bobsdelights directory.


##Chp.14 Assertions and Unit Testing

SKIPPED


##Chp.15 Case Classes and Pattern Matching

abstract class Expr
case class Var(name: String) extends Expr
case class Number(num: Double) extends Expr
case class UnOp(operator: String, arg: Expr) extends Expr
case class BinOp(operator: String, left: Expr, right: Expr) extends Expr

case class help several things:

  1. It adds a factory method with the name of the class. So you dont have to create object Object("x") instead of new Object("x")

    val v = Var("x")
    // instead of new Var("x")
  2. All arguments in the parameter list of a case class implicitly get a val prefix, so they are maintained as fields:

    val v = Var("x")
    v.name
    // x
  3. The compiler adds natural implementations of methods toString, hashCode, and equals to your class.

  4. The compiler adds a copy method to your class for making modified copies. This method is useful for making a new instance of the class that is the same as another one except that one or two attributes are different.

    val op = BinOp("+", Number(1), v)
    //  op: BinOp = BinOp(+,Number(1.0),Var(x))
    
    op.copy(operator = "-")
    // res4: BinOp = BinOp(-,Number(1.0),Var(x))

Pattern Matching

selector match { alternatives } instead of Java's switch (selector) { alternatives }

Assume we have 3 patterns in out input:

UnOp("-", UnOp("-", e)) => e // Double negation
BinOp("+", e, Number(0)) => e // Adding zero
BinOp("*", e, Number(1)) => e // Multiplying by one

Then we can write a function:

def simplifyTop(expr: Expr): Expr = expr match {
  case UnOp("-", UnOp("-", e))  => e   // Double negation
  case BinOp("+", e, Number(0)) => e   // Adding zero
  case BinOp("*", e, Number(1)) => e   // Multiplying by one
  case _ => expr
}

A match expression is evaluated by trying each of the patterns in the order they are written. The first pattern that matches is selected, and the part following the arrow is selected and executed.

A constant pattern like + or 1 matches values that are equal to the constant with respect to ==.

A variable pattern like e matches every value.

The wildcard pattern (_) also matches every value, but it does not introduce a variable name to refer to that value.

A constructor pattern looks like UnOp("-", e). This pattern matches all values of type UnOp whose first argument matches - and whose second argument matches e.

Scala's match has different behaviour like:

  1. match is an expression in Scala, it always results in a value.
  2. Scala’s alternative expressions never fall through into the next case.
  3. If none of the patterns match, an exception named MatchError is thrown.

A pattern match with an empty default case:

expr match {
    case BinOp(op, left, right) =>
      println(expr +" is a binary operation")
    case _ =>
}

Kinds of Patterns

Wildcard patterns match ant objects:

expr match {
  case BinOp(op, left, right) =>
    println(expr +" is a binary operation")
  case _ =>
}

Constant patterns matches only itself. Any literal may be used as a constant. Also, any val or singleton object can be used as a constant:

def describe(x: Any) = x match {
  case 5 => "five"
  case true => "truth"
  case "hello" => "hi!"
  case Nil => "the empty list"
  case _ => "something else"
}

A variable pattern matches any object, just like a wildcard. Unlike a wildcard, Scala binds the variable to whatever the object is:

expr match {
  case 0 => "zero"
  case somethingElse => "not zero: "+ somethingElse
}

Variable or constant?

Scala uses a simple lexical rule: a simple name starting with a lowercase letter is taken to be a pattern variable; all other references are taken to be constants.

import math.{E, Pi}

val pi = math.Pi

E match {
  case pi => "strange math? Pi = "+ pi
}
// res12: java.lang.String = strange math? Pi = 2.718281828459045

Since pi is a variable pattern, it will match all inputs, and so no cases following it can be reached:

E match {
  case pi => "strange math? Pi = "+ pi
  case _ => "OK"
}
//error: unreachable code
//    case _ => "OK"

First, if the constant is a field of some object, you can prefix it with a qualifier. For instance, pi is a variable pattern, but this.pi or obj.pi are constants even though they start with lowercase letters.

If that does not work (because pi is a local variable, say), you can alternatively enclose the variable name in back ticks:

import math.{E, Pi}

val pi = math.Pi

E match {
  // use pi's value as constant pattern
  case `pi` => "strange math? Pi = "+ pi
  case _ => "OK"
}
// OK

Earlier on, in Section 6.10, you saw that back-tick syntax can also be used to treat a keyword as an ordinary identifier, e.g., writing Thread.`yield`() treats yield as an identifier rather than a keyword.

Constructor patterns supports deep matches, below example is one line long yet checks three levels deep:

expr match {
  case BinOp("+", e, Number(0)) => println("a deep match")
  case _ =>
}

Sequence patterns

Supply with _* to match arbitrary length List:

expr match {
  case List(0, _*) => println("found it")
  case _ =>
}

Tuple patterns
def tupleDemo(expr: Any) =
  expr match {
    case (a, b, c)  =>  println("matched "+ a + b + c)
    case _ =>
  }

Typed patterns

You can use a typed pattern as a convenient replacement for type tests and type casts.

def generalSize(x: Any) = x match {
  case s: String => s.length
  case m: Map[_, _] => m.size
  case _ => -1
}

generalSize("abc")
// Int = 3

generalSize(Map(1 -> 'a', 2 -> 'b'))
// Int = 2

generalSize(math.Pi)
// Int = -1

Note that, even though s and x refer to the same value, the type of x is Any, but the type of s is String. So you could not write x.length, because the type Any does not have a length member.

To test whether an expression expr has type String, say, you write: expr.isInstanceOf[String].

To cast the same expression to type String, you use: expr.asInstanceOf[String].

As you will have noted by now, writing type tests and casts is rather verbose in Scala. That’s intentional, because it is not encouraged practice. You are usually better off using a pattern match with a typed pattern.


Type erasure
def isIntIntMap(x: Any) = x match {
  case m: Map[Int, Int] => true
  case _ => false
}

// warning: there were unchecked warnings; re-run with
//  -unchecked for details

isIntIntMap(Map(1 -> 1))
// Boolean = true

isIntIntMap(Map("abc" -> "abc"))
// Boolean = true

Scala uses the erasure model of generics, just like Java does. This means that no information about type arguments is maintained at runtime.

Consequently, there is no way to determine at runtime whether a given Map object has been created with Int types.

Because isIntIntMap(Map("abc" -> "abc")) may surprise you, so Scala will warn you that unchecked, but erased type.

The only exception to the erasure rule is arrays, because they are handled specially in Java as well as in Scala.

def isStringArray(x: Any) = x match {
  case a: Array[String] => "yes"
  case _ => "no"
}

val as = Array("abc")
//  as: Array[java.lang.String] = Array(abc)

Variable binding

An at sign (@), and then the pattern. This gives you a variable-binding pattern.

expr match {
  case UnOp("abs", e @ UnOp("abs", _)) => e
  case _ =>
}

// A pattern with a variable binding (via the @ sign).

Pattern Guard
def simplifyAdd(e: Expr) = e match {
  case BinOp("+", x, x) => BinOp("*", x, Number(2))
  case _ => e
}
// x is already defined as value x ...

Scala restricts patterns to be linear: a pattern variable may only appear once in a pattern.

But we can re-formulate the match with pattern guard x == y:

def simplifyAdd(e: Expr) = e match {
  case BinOp("+", x, y) if x == y =>
    BinOp("*", x, Number(2))
  case _ => e
}
// simplifyAdd: (e: Expr)Expr

A pattern guard comes after a pattern and starts with an if. The guard can be an arbitrary boolean expression. If a pattern guard is present, the match succeeds only if the guard evaluates to true.

**it is important that the catch-all cases come after the more specific simplification rules. If you wrote them in the other order, then the catch-all case would be run in favor of the more specific rules. In many cases, the compiler will even complain if you try.


###15.5 Sealed Classes

Whenever you write a pattern match, you need to make sure you have covered all of the possible cases. How can you ever feel safe that you covered all the cases?

In fact, you can enlist the help of the Scala compiler in detecting missing combinations of patterns in a match expression. In general, this is impossible in Scala, because new case classes can be defined at any time and in arbitrary compilation units.

The alternative is to make the superclass of your case classes sealed. A sealed class cannot have any new subclasses added except the ones in the same file.

This is very useful for pattern matching, because it means you only need to worry about the subclasses you already know about.

sealed abstract class Expr
case class Var(name: String) extends Expr
case class Number(num: Double) extends Expr
case class UnOp(operator: String, arg: Expr) extends Expr
case class BinOp(operator: String, left: Expr, right: Expr) extends Expr

def describe(e: Expr): String = e match {
  case Number(_) => "a number"
  case Var(_)    => "a variable"
}

//  warning: match is not exhaustive!
//  missing combination           UnOp
//  missing combination          BinOp

Such a warning tells you that there’s a risk your code might produce a MatchError exception because some possible patterns (UnOp, BinOp) are not handled.

Now you may add wildcard pattern but not ideal because you think it should never entered:

def describe(e: Expr): String = e match {
  case Number(_) => "a number"
  case Var(_) => "a variable"
  case _ => throw new RuntimeException // Should not happen
}

A more lightweight alternative is to add an @unchecked annotation to the selector expression of the match:

def describe(e: Expr): String = (e: @unchecked) match {
  case Number(_) => "a number"
  case Var(_)    => "a variable"
}

The @unchecked annotation has a special meaning for pattern matching. If a match’s selector expression carries this annotation, exhaustivity checking for the patterns that follow will be suppressed.


15.6 The Option type

Optional values are produced by some of the standard operations on Scala’s collections, such as Map#get() then you can deal it with pattern matching:

val capitals = Map("France" -> "Paris", "Japan" -> "Tokyo")

def show(x: Option[String]) = x match {
  case Some(s) => s
  case None => "?"
}

Use Option type mitigate NullPointerException may result at runtime. error prone as in Java.


15.7 Patterns everywhere

Any time you define a val or a var, you can use a pattern instead of a simple identifier.

val myTuple = (123, "abc")
//myTuple: (Int, java.lang.String) = (123,abc)

val (number, string) = myTuple
// number: Int = 123
// string: java.lang.String = abc

This helps you to deconstruct it with a pattern:

val exp = new BinOp("*", Number(5), Number(1))
// exp: BinOp = BinOp(*,Number(5.0),Number(1.0))

val BinOp(op, left, right) = exp
//  op: String = *
//  left: Expr = Number(5.0)
//  right: Expr = Number(1.0)

Case sequences as partial functions

A sequence of cases in curly braces can be used anywhere a function literal can be used.

Instead of having a single entry point and list of parameters, a case sequence has multiple entry points, each with their own list of parameters. Each case is an entry point to the function, and the parameters are specified with the pattern. The body of each entry point is the right-hand side of the case.

val withDefault: Option[Int] => Int = {
  case Some(x) => x
  case None => 0
}

withDefault(Some(10))
// Int = 10

withDefault(None)
// Int = 0

NOTE: a sequence of cases gives you a partial function. If you apply such a function on a value it does not support, it will generate a run-time exception.

val second: List[Int] => Int = {
  case x :: y :: _ => y
}

// warning: match is not exhaustive!
//  missing combination  Nil

second(List())
//  scala.MatchError: List()

If you want to check whether a partial function is defined, you must first tell the compiler that you know you are working with partial functions.

The type that only includes partial functions from lists of integers to integers is written PartialFunction[List[Int], Int].

Partial functions have a method isDefinedAt, which can be used to test whether the function is defined at a particular value.

val second: PartialFunction[List[Int], Int] = {
  case x :: y :: _ => y
}

second.isDefinedAt(List(5,6,7))
// Boolean = true

second.isDefinedAt(List())
// Boolean = false

In fact, such an expression gets translated by the Scala compiler to a partial function by translating the patterns twice, once for the implementation of the real function, and once to test whether the function is defined or not.

// For instance, the function literal { case x :: y :: _ => y }

new PartialFunction[List[Int], Int] {
  def apply(xs: List[Int]) = xs match {
    case x :: y :: _ => y
  }
  def isDefinedAt(xs: List[Int]) = xs match {
    case x :: y :: _ => true
    case _ => false
  }
}

This translation takes effect whenever the declared type of a function literal is PartialFunction. If the declared type is just Function1, or is missing, the function literal is instead translated to a complete function.


Patterns in for Expressions
for ((country, city) <- capitals)
  println("The capital of "+ country +" is "+ city)

// The capital of France is Paris
// The capital of Japan is Tokyo

It is equally possible that a pattern might not match a generated value, below example will not show None value.

val results = List(Some("apple"), None, Some("orange"))
// List[Option[java.lang.String]] = List(Some(apple), None, Some(orange))

for (Some(fruit) <- results) println(fruit)

// apple
// orange

##Chp.16 List

@jacy

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@jacy jacy commented Feb 14, 2018

Very helpful to me to quickly refresh my scala memory, looking forward to your updates of the rest chapters.

@singhalrbl

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@singhalrbl singhalrbl commented Apr 10, 2018

very helpful!!!!! Thanx. looking forward to your updates of the rest chapters.

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