feat:线下课：高阶函数

src/main/scala/chapter2/demo4/DemoMethod3.scala

@@ -0,0 +1,77 @@
+package chapter2.demo4
+
+object DemoMethod3 {
+  def main(args: Array[String]): Unit = {
+
+    // b叫做默认参数，不传值的时候，b就会等于1
+    def minus(a: Int, b: Int = 1): Int = {
+      a - b
+    }
+
+    def minus2(a: Int = 10, b: Int): Int = {
+      a - b
+    }
+
+    println(minus(10, 5))
+    // 命名参数（python），顺序可以和定义时不一致
+    println(minus(b = 10, a = 5))
+
+    println(minus(5))
+    // 报错，非命名参数调用时，参数列表和函数定义时的参数顺序默认是一致的，这里缺少了b的传递
+//     println(minus2(_,5))
+
+
+    trait Comparator[A] {
+      def compare(x: A, y: A): Int
+    }
+
+    object Comparator {
+      implicit object IntComparator extends Comparator[Int] {
+        def compare(x: Int, y: Int): Int = Integer.compare(x, y)
+      }
+
+      implicit object StringComparator extends Comparator[String] {
+        def compare(x: String, y: String): Int = x.compareTo(y)
+      }
+    }
+
+    def max[A](x: A, y: A)(implicit comparator: Comparator[A]): A =
+      if (comparator.compare(x, y) >= 0) x
+      else y
+
+    println(max(10, 6)) // 10
+    println(max("hello", "world")) // world
+    // does not compile:
+    //    println(max(false, true))
+    //     error: could not find implicit value for parameter comparator: Comparator[Boolean]
+  }
+
+  // https://docs.scala-lang.org.cn/tour/implicit-parameters.html
+  trait Comparator[A] {
+    def compare(x: A, y: A): Int
+  }
+
+  object Comparator {
+    implicit object IntComparator extends Comparator[Int] {
+      def compare(x: Int, y: Int): Int = Integer.compare(x, y)
+    }
+
+    implicit object StringComparator extends Comparator[String] {
+      def compare(x: String, y: String): Int = x.compareTo(y)
+    }
+    //      implicit object BooleanComparator extends Comparator[Boolean] {
+    //        def compare(x: Boolean, y: Boolean): Int = x.compareTo(y)
+    //      }
+  }
+
+  def max[A](x: A, y: A)(implicit comparator: Comparator[A]): A =
+    if (comparator.compare(x, y) >= 0) x
+    else y
+
+  println(max(10, 6)) // 10
+  println(max("hello", "world")) // world
+  // does not compile:
+  //        println(max(false, true))
+  //     error: could not find implicit value for parameter comparator: Comparator[Boolean]
+
+}