Interface example embellished, use slice instead of array.

Makefile

@@ -32,6 +32,7 @@ dirs = 	testppack \
 	interface_03 \
 	interface_04 \
 	interface_04a \
+	interface_04b \
 	list \
 	list_struct \
 	loghello \

interface_04b/interface_04b.go

@@ -0,0 +1,92 @@
+/*
+A more realistic demonstration of interfaces, with explicit (?) declaration.
+*/
+package main
+
+import (
+	"fmt" //
+)
+
+type Gap interface {
+	// Get data
+	Get() (s string, e error)
+	// Put data
+	Put(s string) (e error)
+}
+
+// Will implement Gap
+type ImpOne struct {
+	Gap // Implement Gap
+	ios string
+}
+
+// Implement Gap.Get
+func (o *ImpOne) Get() (s string, e error) {
+	return o.ios, nil
+}
+
+// Implement Gap.Put
+func (o *ImpOne) Put(s string) (e error) {
+	o.ios = s
+	return nil
+}
+
+// Will also implement Gap
+type ImpTwo struct {
+	Gap // Implement Gap
+	its string
+}
+
+// Implement Gap.Get
+func (o *ImpTwo) Get() (s string, e error) {
+	return o.its + o.its, nil
+}
+
+// Implement Gap.Put
+func (o *ImpTwo) Put(s string) (e error) {
+	o.its = s + s
+	return nil
+}
+
+func main() {
+	fmt.Println("Start...")
+
+	// ImpOne pointer
+	dOne := &ImpOne{} // Parmater initialization not allowed like this
+	fmt.Println("dOne.ios:", dOne.ios)
+
+	// ImpTwo pointer
+	dTwo := &ImpTwo{} // Parmater initialization not allowed like this
+	fmt.Println("dTwo.its:", dTwo.its)
+
+	// ImpOne leaves data alone
+	_ = dOne.Put("1time")
+	w, _ := dOne.Get()
+	fmt.Println("dOnePutGet", "1time", w)
+
+	// ImpTwo munges the data
+	_ = dTwo.Put("1time")
+	w, _ = dTwo.Get()
+	fmt.Println("dTwoPutGet", "1time", w)
+
+	// An slice of Gaps
+	ga := []Gap{dOne, dTwo}
+
+	// Use array offsets
+	w, _ = ga[0].Get()
+	fmt.Println("ga0Get", w)
+	w, _ = ga[1].Get()
+	fmt.Println("ga1Get", w)
+
+	// Declare a Gap and use offset 0
+	var g Gap = ga[0]
+	w, _ = g.Get()
+	fmt.Println("gga0Get", w)
+
+	// Now use offset 1
+	g = ga[1]
+	w, _ = g.Get()
+	fmt.Println("gga1Get", w)
+
+	fmt.Println("End...")
+}