学生向けScalaハンズオンテキスト part2

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Java
Scala

C++ swap
Scala length sum

length

length

$def length(list: List[Double]): Double = list match { case Nil => 0 case _ :: tail => 1 + length(tail) }$

val list = List(1.0, 3.14, 2.71)
length(list) // 3

val list = List(42, 99, 666)

length

error: type mismatch;
found : List[Int]
required: List[Double]
length(list)

List[Double] List[Int]

length
List[Double] => Double

length

Double Int
String Any

$def length[A](list: List[A]): Double = list match { case Nil => 0 case _ :: tail => 1 + length(tail) }$

length
[A]

new

def func[A, B, C](arga: A, argb: B): C = ...

length
length
[A]

length

length

val listDouble = List(1.0, 3.14, 2.71)
length[Double](listDouble) // 3
val listInt = List(1, 2, 3, 4, 5)
length[Int](listI...

Scala

val listDouble = List(1.0, 3.14, 2.71)
length(listDouble) // 3
val listInt = List(1, 2, 3, 4, 5)
length(listInt) // 5

length

length

$def length[A](list: List[A]): Double = list match { case Nil => 0 case _ :: tail => 1 + length(tail) }$

A A

2
2

def twoList[A, B, C](list1: List[A], list2: List[B], func: List[?
(func(list1), func(list2))

List[A] List[B]

length

$def length(list: List[_]): Double = list match { case Nil => 0 case _ :: tail => 1 + length(tail) }$

length
List [_]

A
A

def func(list: List[_ <: AnyVal]) = ...

A <: B A B

twoList(List(1,2,3,4,5), List("hoge","fuga","piyo"), length) // (

8-1

sum

sum

def sum(list: List[Double]): Double =
list match {
case Nil => 0
case head :: tail => head + sum(tail)
}

Double List

val list = List(1.0, 3.14, 2.71)
sum(list) // 6.85

List(42, 99, 666)

val list = List(42, 99, 666)
sum(list)
// error: type mismatch;
// found : List[Int]
// required: List[Double]
// sum(list)

length

Double
Int

length
sum Int Double

def sum[A](list: List[A]): A =
list match {
case Nil => 0
case head :: tail => head + sum(tail)
}

type mismatch;
found : Int(0)
required: A
case Nil => 0
^
type mismatch;
found : List[A]
required: List[String]
case head ...

sum Scala

def sum[A](list: List[A]): A =

A A
A

case Nil => 0

A
Int

Scala

case head :: tail => head + sum(tail)

+
A +

sum
sum

2
0

trait Addable {
def +(other: Addable): Addable
}
def add[T <: Addable](a: T, b: T) = a + b

T +
+

Addable

Scala Int
Double

Scala
Numeric

Haskell
Scala

implicit

Scala

def sum[A](list: List[A])(implicit ev: Numeric[A]): A =

2 implicit
[A]

A
ev evidence

Ruby

Numeric

trait Numeric[T] extends Ordering[T] {
def plus(x: T, y: T): T
def minus(x: T, y: T): T
def times(x: T, y: T): T
def negat...

sum

plus zero
Numeric

sum Numeric
Scala
Numeric

Numeric

def sum[A](list: List[A])(implicit ev: Numeric[A]): A =
list match {
case Nil => ev.zero
case head :: tail => ev.plus(head...

sum

val listDouble = List(1.0, 3.14, 2.71)
sum(listDouble) // 6.85
val listInt = List(1, 2, 3, 4, 5)
sum(listInt) // 15

sum

9-1

9-2
sum

def sum[A: Numeric](list: List[A]): A =
list match {
case Nil => implicitly[Numeric[A]].zero
case head :: tail => implicit...

[A: Numeric]
context bound

implicitly

def implicitly[T](implicit e: T) = e

9-3
Scala