学生向けScalaハンズオンテキスト part2 - パラメータ化された型 - 存在型 - 数値とモノイド - 多相と型クラス cf. part1: https://www.slideshare.net/opttechnologies/scala-handson-textbook-83475085

5. Java
Scala

7. C++ swap
Scala length sum

8. length

9. length

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

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

14. val list = List(42, 99, 666)

15. length

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

18. List[Double] List[Int]

19. length
List[Double] => Double

20. length

21. Double Int
String Any

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

25. length
[A]

27. 1

28. new

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

30. length
length
[A]

31. length

32. length

33. val listDouble = List(1.0, 3.14, 2.71)
length[Double](listDouble) // 3
val listInt = List(1, 2, 3, 4, 5)
length[Int](listInt) // 5
val listString = List("hoge", "fuga", "piyo")
length[String](listString) // 3

35. Scala

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

38. length

39. length

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

41. A A

43. 2
2

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

46. 3

47. List[A] List[B]

48. length

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

51. length
List [_]

52. A
A

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

54. A <: B A B

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

57. 8-1

58. sum

59. sum

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

61. Double List

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

63. List(42, 99, 666)

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

65. length

66. Double
Int

67. length
sum Int Double

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

69. ……

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

74. sum Scala

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

76. A A
A

77. case Nil => 0

78. A
Int

79. Scala

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

82. +
A +

83. sum
sum

84. 2
0

87. 2

88. <:

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

90. T +
+

91. Addable

92. Scala Int
Double

93. Scala
Numeric

94. Haskell
Scala

95. implicit

98. Scala

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

100. 2 implicit
[A]

101. A
ev evidence

103. Ruby

104. Numeric

105. 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 negate(x: T): T
def fromInt(x: Int): T
def toInt(x: T): Int
def toLong(x: T): Long
def toFloat(x: T): Float
def toDouble(x: T): Double
def zero = fromInt(0)
def one = fromInt(1)
def abs(x: T): T = if (lt(x, zero)) negate(x) else x

106. sum

107. 0

108. plus zero
Numeric

109. sum Numeric
Scala
Numeric

110. Numeric

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

112. sum

113. ev

114. ev

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

116. sum

117. 9-1

118. 9-2
sum

119. def sum[A: Numeric](list: List[A]): A =
list match {
case Nil => implicitly[Numeric[A]].zero
case head :: tail => implicitly[Numeric[A]].plus(head, sum(ta
}

120. [A: Numeric]
context bound

122. implicitly

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

124. 9-3
Scala