learned I didn't need git for external modules and different folders can have different mains

pkg/gadget/program.go

@@ -0,0 +1,33 @@
+package main
+
+type Player interface {
+	Play(string)
+	Stop()
+}
+
+func TryOut(player Player) {
+	player.Play("Test Track")
+	player.Stop()
+	recorder, ok := player.(TapeRecorder)
+	if ok {
+		recorder.Record()
+	}
+}
+
+func playList(device Player, songs []string) {
+	for _, song := range songs {
+		device.Play(song)
+	}
+
+	device.Stop()
+}
+
+func main() {
+	mixTape := []string{"Jessie's Girl", "Whip It", "9 to 5"}
+	var player Player = TapePlayer{}
+	playList(player, mixTape)
+	player = TapeRecorder{}
+	playList(player, mixTape)
+	TryOut(TapeRecorder{})
+	TryOut(TapePlayer{})
+}

pkg/gadget/tape.go

@@ -0,0 +1,31 @@
+package main
+
+import "fmt"
+
+type TapePlayer struct {
+	Batteries string
+}
+
+func (t TapePlayer) Play(song string) {
+	fmt.Println("Playing", song)
+}
+
+func (t TapePlayer) Stop() {
+	fmt.Println("Stopped")
+}
+
+type TapeRecorder struct {
+	Microphone int
+}
+
+func (t TapeRecorder) Play(song string) {
+	fmt.Println("Playing", song)
+}
+
+func (t TapeRecorder) Record() {
+	fmt.Println("Recording")
+}
+
+func (t TapeRecorder) Stop() {
+	fmt.Println("Stopped")
+}