Scala関数型デザイン&プログラミング10章

chapter10.scala

// EXERCISE 10.1
// 整数の加算と乗算、および論理演算子に対するMonoidインスタンスを考え出せ

trait Monoid[A] {
        def op(a1: A, a2: A): A
        def zero: A
}

val intAddition: Monoid[Int] = new Monoid[Int] {
        def op(a1: Int, a2: Int) = a1 + a2
        val zero = 0
}

val intMultiplication: Monoid[Int] = new Monoid[Int] {
        def op(a1: Int, a2: Int) = a1 * a2
        def zero = 1
}

val booleanOr: Monoid[Boolean] = new Monoid[Boolean] {
        def op(a1: Boolean, a2: Boolean) = a1 || a2
        def zero = false
}

val booleanAnd: Monoid[Boolean] = new Monoid[Boolean] {
        def op(a1: Boolean, a2: Boolean) = a1 && a2
        def zero = true
}

println(intMultiplication.op(10, 30))

// EXERCISE 10.3
// 引数および戻り値の型が同じである関数をendo関数と呼ぶことがある
// endo関数のモノイドを記述せよ

def endoMonoid[A]: Monoid[A => A] = new Monoid[A => A] {
        def op(a1: A => A, a2: A => A) = (a) => a2(a1(a))
        def zero = (a) => a
}

println(endoMonoid.op((a: Int) => a * 2, (a: Int) => a + 2)(10))