Scala 6 - Datatypes

스칼라 튜토리얼, scala의 datatypes에 대해 알아보자

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Types in general

• Introdection : How to construct elements

• Elimination : How to use elements of the type

• Primitive types :

Int, Boolean, Double, String...

Intro for Int: ...,-2,-1,0,1,2
Elim for Int: +,-,*,/...

-Function types :

Int => Int, (Int => Int)=>(Int => Int)
Intro: (x:T)=>e
Elim: f(v)

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Tuples

• Intro :

(1, 2, 3) : (Int, Int, Int)
(1, "a") : (Int, String)

• Elim :

(1, "a", 10)._1 = 1
(1, "a", 10)._2 = "a"

Only up to length 22

If you use tuple types, each has its own closure!

def bar : (x:Int) = x + 1
val a = (3+1, foo)
// def a = (4+1, bar) -> Nope!

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Structural types (a.k.a Record Types)

val foo = new { //or, Object foo
    val a = 3
    def b = a+1
    def f(x:Int) = b+x
    def f(x:String) = "hello"+x
}

foo.b
foo.f(3)
foo.f("gil")

def g(x: {val a: Int; def b: Int;
def f(x:Int): Int;
def f(x:String): String}) =
    x.f(3)

g(foo)

Structural types always have closure within it.

def bar(x: Int) = x+1

object foo {
  def g: Int => Int = bar _
}

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Structural Types : Scope and Type Alias

You can use type as typedef in C.

val gn = 0
object foo {
  val a = 3
  def b = a + 1
  def f(x: Int) = b + x + gn
}

foo.f(3)

type Foo = {val a: Int; def b: Int; def f(x:Int):Int}
def g(x: Foo) = {
  val gn = 10
  x.f(3)
}

g(foo)

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Algebraic Datatypes

• Ideas

T = C of T * … * T
  | C of T * … * T
  | …
  | C of T * … * T

• E.g.

Attr = Name of String
    |  Age of Int
    | DOB of Int * Int * Int
    |  Height of Double

• Intro:

Name(“Chulsoo Kim”), Name(“Younghee Lee”), Age(16), 
DOB(2000,3,10), Height(171.5), …

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Recursive ADT

• E.g.

IList = INil
  | ICons of Int * IList

• Intro:

INil(), 
ICons(3,INil()),
ICons(2, ICons(3,INil())),
ICons(1, ICons(2, ICons(3,INil()))),

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Algebraic Datatypes In Scala

• Attr

sealed abstract class Attr
case class Name(name:String) extends Attr
case class Age(name:Int) extends Attr
case class DOB(year: Int, month: Int,
               day: Int) extends Attr
case class Height(height: Double) extends Attr

val a: Name = Name("Chulsoo Kim")
val b: Attr = DOB(2000, 3, 10)

If we look at the DOB, It is good for readability when we indicate 3 variables with its name(year, month, day).

• IList

sealed abstract class IList
case class INil() extends IList
case class ICons(hd: Int, tl: IList) extends IList

val x: IList = ICons(2, ICons(2, INil()))

def gen(n: Int) : IList = {
    if (n <= 0) INil()
    else ICons(n, gen(n-1))
}

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Exercise

Define IOption and BTree algebraic datatypes.

IOption = INone
| ISome of Int

BTree = Leaf
| Node of Int * BTree * BTree

• Example:

sealed abstract class IList
case class INil() extends IList
case class ICons(hd: Int, tl: IList) extends IList

def x : IList = ICons(2, ICons(1, INil()))

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Solution :

sealed abstract class IOption
case class INone() extends IOption
case class ISome(some: Int) extends IOption

sealed abstract class BTree
case class Leaf() extends BTree
case class Node(value: Int, left: BTree, right: BTree) extends BTree

val t = Node(3, Node(2, Leaf(), Leaf()), Leaf())

If we express t as BTree, It looks like this:

        3
      /   \
    2     Leaf
  /   \
Leaf  Leaf