add generics example (compatible with go1.18 and above)

https://go.dev/doc/tutorial/generics

expert/generics.go

@@ -0,0 +1,58 @@
+package main
+
+import (
+	"fmt"
+)
+
+type Number interface {
+	int64 | float64
+}
+
+func SumInts(m ...int64) int64 {
+	var s int64
+	for _, v := range m {
+		s += v
+	}
+	return s
+}
+
+func SumFloats(m ...float64) float64 {
+	var s float64
+	for _, v := range m {
+		s += v
+	}
+	return s
+}
+
+func SumIntsOrFloats[V int64 | float64](m ...V) V {
+	var s V
+	for _, v := range m {
+		s += v
+	}
+	return s
+}
+
+func SumNumbers[V Number](m ...V) V {
+	var s V
+	for _, v := range m {
+		s += v
+	}
+	return s
+}
+
+func main() {
+	ints := []int64{31, 37, 41, 43, 47, 53, 59}
+	floats := []float64{31.17, 37.2, 41.9, 43.002, 47, 53, 59}
+
+	fmt.Printf("Non-Generic Sums: %v and %v\n",
+		SumInts(ints...),
+		SumFloats(floats...))
+
+	fmt.Printf("Generic Sums: %v and %v\n",
+		SumIntsOrFloats(ints...),
+		SumIntsOrFloats(floats...))
+
+	fmt.Printf("Generic Sums: %v and %v\n",
+		SumNumbers(ints...),
+		SumNumbers(floats...))
+}