Step 2: Create type class Combinator

src/main/scala/training/Program.scala

@@ -1,6 +1,7 @@
 package training
 
 import training.datatypes._
+import training.implicits._
 
 object Program extends App {
 
@@ -13,7 +14,7 @@ object Program extends App {
     val b1: Maybe[Int] = getBalanceBank1
     val b2: Maybe[Int] = getBalanceBank2
 
-    b1 + b2 //It will fail
+    maybeCombinator.combine(b1, b2)
   }
 
 

src/main/scala/training/implicits.scala

@@ -1,6 +1,13 @@
 package training
 
+import training.datatypes._
+import training.instances._
+import training.typeclasses._
+
 object implicits {
 
+  implicit def intCombinator: Combinator[Int] = new IntCombinator
+
+  implicit def maybeCombinator[A](implicit CA: Combinator[A]): Combinator[Maybe[A]] = new MaybeCombinator[A]
 
 }

src/main/scala/training/instances.scala

@@ -1,6 +1,24 @@
 package training
 
+import training.datatypes._
+import training.typeclasses._
+
 object instances {
 
+  class IntCombinator extends Combinator[Int] {
+    override def combine(x: Int, y: Int): Int = x + y
+  }
+
+  class MaybeCombinator[A](implicit CA: Combinator[A]) extends Combinator[Maybe[A]] {
+
+    override def combine(x: Maybe[A], y: Maybe[A]): Maybe[A] = (x, y) match {
+      case (No, No) => No
+      case (No, Yes(yy)) => y
+      case (Yes(xx), No) => x
+      case (Yes(xx), Yes(yy)) => Yes(CA.combine(xx, yy))
+    }
+
+  }
+
 
 }

src/main/scala/training/typeclasses.scala

@@ -2,5 +2,9 @@ package training
 
 object typeclasses {
 
+  trait Combinator[A] {
+    def combine(x: A, y: A): A
+  }
+
 
 }