in
Scala Fukuoka 2019
https://goo.gl/iKyEKs

•

Scala

Scala ?

?

•Scala
•
•
Scala ?

•
•
•
•

•
•
•
•

f(x) = x + 1
Scala
• = ( )
•
Which
is
better?
def f(x: Int) {x + 1}
def f(x: Int) = x + 1
def f(x: Int) = return x + 1
※ : 2

( )
• (mutable)
•
•

?
int sum = 0;
for (int i = 0; i < array.length; i++) {
sum += array[i];
}
return sum;

https://goo.gl/iKyEKs

• 1 n
•
int f(int n) {
int total = 0;
for (int i = 1; i <= n; i++) {
total += i;
}
return total;
}

f(1) = 1
f(2) = 1 + 2
f(3) = 1 + 2 + 3
...
f(n) = 1 + 2 + 3 + ... + n

f(1) = 1
f(2) = f(1) + 2
f(3) = f(2) + 3
...
f(n) = f(n - 1) + n
f(1) = 1
f(n) = f(n - 1) + n

def f(n: Int): Int =
if (n == 1) 1
else f(n - 1) + n
f(1) = 1
f(n) = f(n - 1) + n

f(0) = 1
f(1) = 1
f(2) = 2 * 1
f(3) = 3 * 2 * 1
...
f(n) = n * ... * 3 * 2 * 1

f(0) = 1
f(1) = 1 * f(0)
f(2) = 2 * f(1)
f(3) = 3 * f(2)
...
f(n) = n * f(n - 1)

f(0) = 1
f(n) = n * f(n - 1)
def f(n: Int): Int =
if (n == 0) 1
else n * f(n - 1)

n
?
1, 1, 2, 3, 5, 8, 13, …

f(0) = 0
f(1) = 1
f(2) = 0 + 1
f(3) = 1 + 1
f(4) = 1 + 2
f(5) = 2 + 3
...

f(0) = 0
f(1) = 1
f(2) = f(0) + f(1)
f(3) = f(1) + f(2)
f(4) = f(2) + f(3)
f(5) = f(3) + f(4)
...
f(n) = f(n - 2) + f(n - 1)

f(0) = 0
f(1) = 1
f(n) = f(n - 2) + f(n - 1)
def f(n: Int): Int =
if (n == 0) 0
else if (n == 1) 1
else f(n - 2) + f(n - 1)

sum
def sum(ints: List[Int]): Int

• Nil
•head tail
head :: tail
3
Nil
Nil::
3 :: Nil2 ::
2 :: 3 :: Nil1 ::
•
•

Nil
Nil::8
8 :: Nil::2
2 :: 8 :: Nil::1
1 :: 2 :: 8 :: Nil::5
5 :: 1 :: 2 :: 8 :: Nil

sum(5 :: 1 :: 2 :: 8 :: Nil)
sum( )
= + sum( )
sum( ) = + sum( )
sum( ) = + sum( )
sum( ) = + sum( )
sum( ) = 0Nil
8 :: Nil
2 :: 8 :: Nil
5 :: 1 :: 2 :: 8 :: Nil
Nil8
8 :: Nil2
2 :: 8 :: Nil1
5 1 :: 2 :: 8 :: Nil
1 :: 2 :: 8 :: Nil

sum( ) = + sum( )
sum( ) = + sum( )
sum( ) = + sum( )
sum( ) = 0Nil
8 :: Nil
2 :: 8 :: Nil
Nil8
8 :: Nil22 :: 8 :: Nil
1
head
head tail
head tail
head tail head :: tail
1 :: 2 :: 8 :: Nil
tail

sum(Nil) = 0
sum(head :: tail) = head + sum(tail)
def sum(list: List[Int]): Int =
if (list.isEmpty) 0
else list.head + sum(list.tail)
def sum(list: List[Int]): Int = list match {
case Nil => 0
case head :: tail => head + sum(tail)
}

product
def product(ints: List[Int]): Int

max
def max(ints: List[Int]): Int

reverse
def reverse(ints: List[Int]): List[Int]

length
def length(ints: List[Int]): Int

sum
def sum(tree: Tree): Int

•Node Empty Tree
•Node Tree
•Empty Tree
8
6 10
4 7 E 12
E E E E E E

Tree
Node
Tree Tree
8
6 10
4 7 E 12
E E E E E E

8
6 10
4 7 E 12
E E E E E E
sum

8
6 10
4 7 E 12
E E E E E E
sum sum
+

sum(tree)
tree
8
6 10
4 7 E 12
E E E E E E

sum(tree) = node.value
+ sum(leftTree) + sum(rightTree)
leftTree rightTree
+
node.value
8
6 10
4 7 E 12
E E E E E E

sum(empty) = 0
sum(tree) = node.value + sum(left) + sum(right)
trait Tree
case class Node(value: Int,
left: Tree,
right: Tree) extends Tree
case object Empty extends Tree
def sum(tree: Tree): Int = tree match {
case Empty => 0
case n: Node =>
n.value + sum(n.left) + sum(n.right)
}

max
def max(tree: Tree): Int

Node
size
def size(tree: Tree): Int

Node
depth
def depth(tree: Tree): Int

def sum(list: List[Int]): Int = list match {
case Nil => 0
case head :: tail => head + sum(tail)
}
def sum(list: List[Int]): Int = {
def loop(acc: Int, l: List[Int]): Int = l match {
case Nil => acc
case head :: tail => loop(acc + head, tail)
}
loop(0, list)
}

• Functional Programming Principles in Scala
Scala Martin Odersky
https://www.coursera.org/course/progfun
•
OCaml
http://www.amazon.co.jp/dp/4781911609

• Scala & ―
Scalaz
2
http://www.amazon.co.jp/dp/4844337769