add: interfaces assertion and composition (wip)

interfaces/05-composition/types.go

@@ -0,0 +1,23 @@
+package main
+
+// don't separate your interfaces like this.
+// put them in the same file where you need to use them.
+// this is here for clarity
+
+type printer interface {
+	print()
+}
+
+// TODO: NEW
+type summer interface {
+	sum() float64
+}
+
+// TODO: NEW
+// interface embedding
+// When an interface includes multiple methods,
+// choose a name that accurately describes its purpose.
+type item interface {
+	printer
+	summer
+}