fix composition is not inheretance

composition/main.go

@@ -0,0 +1,44 @@
+package main
+
+import (
+	"fmt"
+	"path/filepath"
+)
+
+type Pair struct {
+	Path string
+	Hash string
+}
+
+func (p Pair) String() string {
+	return fmt.Sprintf("Hash of %s is %s", p.Path, p.Hash)
+}
+
+type PairWithLength struct {
+	Pair
+	Length int
+}
+
+func (p PairWithLength) String() string {
+	return fmt.Sprintf("Hash of %s is %s; length %d", p.Path, p.Hash, p.Length)
+}
+
+func (p Pair) Filename() string {
+	return filepath.Base(p.Path)
+}
+
+type Filenamer interface {
+	Filename() string
+}
+
+func main() {
+	p := Pair{"/urs", "0xfdfe"}
+	pl := PairWithLength{Pair{"/usr/bin", "0xdead"}, 13643532}
+
+	var fn Filenamer = PairWithLength{Pair{"/usr/bin", "0xdead"}, 13643532}
+
+	fmt.Println(p)
+	fmt.Println(fn)
+	fmt.Println(pl.Path)
+	fmt.Println(p.Filename())
+}