53 // User-defined Datatypes

    54 //========================

    55 

    56 

    57 abstract class Colour

    58 case object Red extends Colour 

    59 case object Green extends Colour 

    60 case object Blue extends Colour

    61 

    62 def fav_colour(c: Colour) : Boolean = c match {

    63   case Red   => false

    64   case Green => true

    65   case Blue  => false 

    66 }

    67 

    68 fav_colour(Green)

    69 

    70 

    71 // ... a tiny bit more useful: Roman Numerals

    72 

    73 abstract class RomanDigit 

    74 case object I extends RomanDigit 

    75 case object V extends RomanDigit 

    76 case object X extends RomanDigit 

    77 case object L extends RomanDigit 

    78 case object C extends RomanDigit 

    79 case object D extends RomanDigit 

    80 case object M extends RomanDigit 

    81 

    82 type RomanNumeral = List[RomanDigit] 

    83 

    84 List(X,I)

    85 

    86 /*

    87 I -> 1

    88 II -> 2

    89 III  -> 3

    90 IV -> 4

    91 V -> 5

    92 VI -> 6

    93 VII -> 7

    94 VIII -> 8

    95 IX -> 9

    96 X -> X

    97 */

    98 

    99 def RomanNumeral2Int(rs: RomanNumeral): Int = rs match { 

   100   case Nil => 0

   101   case M::r    => 1000 + RomanNumeral2Int(r)  

   102   case C::M::r => 900 + RomanNumeral2Int(r)

   103   case D::r    => 500 + RomanNumeral2Int(r)

   104   case C::D::r => 400 + RomanNumeral2Int(r)

   105   case C::r    => 100 + RomanNumeral2Int(r)

   106   case X::C::r => 90 + RomanNumeral2Int(r)

   107   case L::r    => 50 + RomanNumeral2Int(r)

   108   case X::L::r => 40 + RomanNumeral2Int(r)

   109   case X::r    => 10 + RomanNumeral2Int(r)

   110   case I::X::r => 9 + RomanNumeral2Int(r)

   111   case V::r    => 5 + RomanNumeral2Int(r)

   112   case I::V::r => 4 + RomanNumeral2Int(r)

   113   case I::r    => 1 + RomanNumeral2Int(r)

   114 }

   115 

   116 RomanNumeral2Int(List(I,V))             // 4

   117 RomanNumeral2Int(List(I,I,I,I))         // 4 (invalid Roman number)

   118 RomanNumeral2Int(List(V,I))             // 6

   119 RomanNumeral2Int(List(I,X))             // 9

   120 RomanNumeral2Int(List(M,C,M,L,X,X,I,X)) // 1979

   121 RomanNumeral2Int(List(M,M,X,V,I,I))     // 2017

   122 

   123 

   124 // another example

   125 //=================

   126 

   127 // Once upon a time, in a complete fictional 

   128 // country there were Persons...

   129 

   130 

   131 abstract class Person

   132 case object King extends Person

   133 case class Peer(deg: String, terr: String, succ: Int) extends Person

   134 case class Knight(name: String) extends Person

   135 case class Peasant(name: String) extends Person

   136 

   137 

   138 def title(p: Person): String = p match {

   139   case King => "His Majesty the King"

   140   case Peer(deg, terr, _) => s"The ${deg} of ${terr}"

   141   case Knight(name) => s"Sir ${name}"

   142   case Peasant(name) => name

   143 }

   144 

   145 def superior(p1: Person, p2: Person): Boolean = (p1, p2) match {

   146   case (King, _) => true

   147   case (Peer(_,_,_), Knight(_)) => true

   148   case (Peer(_,_,_), Peasant(_)) => true

   149   case (Peer(_,_,_), Clown) => true

   150   case (Knight(_), Peasant(_)) => true

   151   case (Knight(_), Clown) => true

   152   case (Clown, Peasant(_)) => true

   153   case _ => false

   154 }

   155 

   156 val people = List(Knight("David"), 

   157                   Peer("Duke", "Norfolk", 84), 

   158                   Peasant("Christian"), 

   159                   King, 

   160                   Clown)

   161 

   162 println(people.sortWith(superior).mkString("\n"))

   163 

   164 

   165 // String interpolations as patterns

   166 

   167 val date = "2000-01-01"

   168 val s"$year-$month-$day" = date

   169 

   170 def parse_date(date: String) = date match {

   171   case s"$year-$month-$day" => Some((year.toInt, month.toInt, day.toInt))

   172   case s"$day/$month/$year" => Some((year.toInt, month.toInt, day.toInt))

   173   case _ => None

   174 } 

   175 

   176 

   177 

   181 

   182