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스칼라 튜토리얼, scala의 sub types에 대해 알아보자 Sub Type Polymorphism (Concept) Let's assume that we want this: object tom { val name = "Tom" val home = "02-880-1234" } object bob { val name = "Bob" val home = "02-123-1234" } def greeting(r: ???) = "Hi " + r.name + ", How are you?" greeting(tom) greeting(bob) // Note that we have tom: {val name: String; val home: String} bob: {val name: String; val mobile: ..

스칼라 튜토리얼, scala의 parametric polymophism에 대해 알아보자 Parametric Polymorphism: Functions Problem def id1(x: Int): Int = x def id2(x: Double): Double = x Can we avoid DRY(Do not Reapeat Yourself)? Parametric Polymorphism(a.k.a. For-all Types) def id[A](x: A): A = x // type for definition val f = (x:Int) => x // type for value The type of id is [A](x: A)A. id is a parametric expression. Only definition..

스칼라 튜토리얼, scala datatypes part2 Pattern Matching A way to use algebraic datatypes Syntax: e match { case C1(...) => e1 ... case Cn(...) => en } 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())) def length(xs: IList) : Int = xs match { case INil() => 0 case ICons(x, tl) => 1 + length(t..

스칼라 튜토리얼, scala의 datatypes에 대해 알아보자 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) Tuples Intro : (1, 2, 3) : (Int, Int, Int) (1, "a") : (Int, String) ..

스칼라 튜토리얼, scala에서 currying에 대해 알아보자 Currying Why we use currying? Let's look at the code, simplified by currying. def sum(f: Int=>Int, a: Int, b: Int): Int = if (a Int): (Int, Int) => Int = { def sumF(a: Int, b: Int): Int = if (a Int)(a: Int, b: Int): Int = if (a T can be turned into one of type T1 => (T2 => (... Tn =>T) This is called 'currying' named after Haskell Brooks Curry. The opposite di..

스칼라 튜토리얼, scala의 closure에 대해 알아보자 Parameterized expression vs. values Functions defined using “def” are not values but parameterized expressions. • Anonymous functions are values. • But, parameterized expressions are implicitly converted to values. • Explicit conversion: f _ • Anonymous functions can be seen as syntactic sugar: (x:T)=>e is equivalent to { def \_\_noname(x:T) = e; \_\_noname \_ }..

스칼라 튜토리얼, scala의 tail recursion을 알아봅시다 Recursion needs care Let's look at typical summation fuction. def sum(n: Int) : Int = if (n x*x*x, n) // or def sumLinear(n: Int) = sum((x)=>x, n) def sumSquare(n: Int) = sum((x)=>x*x, n) def sumCubes(n: Int) = sum((x)=>x*x*x, n) Excercise def sum(f: Int => Int, a: Int, b: Int): Int = if (a Int, a: Int, b: Int): Int = if (a Int, inival: Int, f: Int => Int, ..

스칼라 튜토리얼, scala에서 lazy val에 대해 알아보자 Blocks and Name Scoping Blocks in scala Block is an expression. It also allow nested name binding. Block allows arbitraty order of "def"s, but not "val"s(think about why). Scope of names Block example : val t = 0 def f(x: Int) = t + g(x) def g(x: Int) = x * x val x = f(5) val r = { val t = 10 val s = f(5) t + s } val y = t + r A definition inside a block is on..

스칼라에서 call by value와 call by name의 차이를 알아보자 There is two strategy to evaluate values. Call-by-value Evaluate the arguments first, then apply the function to them Call-by-name Just apply the function to its arguments, without evaluating them Let's see the example. def square (x: Int) = x * x [cbv] square(1+1) ~ square(2) ~ 2 * 2 ~ 4 [cbn] square(1+1) ~ (1+1) * (1+1) ~ 2 * (1+1) ~ 2 * 2 ~ 4 So wha..

파이썬으로 만든 간단한 크롤러 참고 자료 : 웹 크롤러 만들기, Implicit vs. Explict wait 비교하기 유투브에서 음악 다운로드하는 작업 자동화 하기 친구가 유투브를 통해 음악을 변환해서 다운로드 받는 것을 보고, 이를 간편하게 해주고자 파이썬 selenium module, chromedriver을 이용해 실행키(Win+R)를 통해 쉽게 음악을 다운로드하는 코드를 작성해 보았습니다. 아, 참고로 mp3juices.cc 라는 사이트로부터 음악을 변환해서 다운을 했는데, 여기가 제일 깔끔한 것 같아요(제 기준) Prerequsite : Python3 (Latest version is recommemded) Installing 3rd party modules (pyperclip, seleniu..