Step 7: Create type class Flattener

src/main/scala/training/Program.scala

@@ -8,13 +8,16 @@ object Program extends App {
   case class Account(id: String, balance: Int)
   case class Statement(isRich: Boolean, accounts: Int)
 
-  def getBalanceBank1: Maybe[Account] = Yes(Account("a1", 100))
+  def getBank1Credentials: Maybe[String] = Yes("MyUser_MyPassword")
+
+  def getBalanceBank1(credentials: String): Maybe[Account] = Yes(Account("a1", 100))
 
   def getBalanceBank2: Maybe[Int] = Yes(80)
 
   val moneyInPocket: Int = 20
 
-  val b1: Maybe[Int] = getBalanceBank1.map(_.balance)
+  val c: Maybe[String] = getBank1Credentials
+  val b1: Maybe[Int] = getBank1Credentials.flatMap(cred => getBalanceBank1(cred).map(_.balance))
   val b2: Maybe[Int] = getBalanceBank2
   val p: Maybe[Int] = moneyInPocket.pure
 

src/main/scala/training/implicits.scala

@@ -16,6 +16,8 @@ object implicits {
 
   implicit def maybeLifter: Lifter[Maybe] = new MaybeLifter
 
+  implicit def maybeFlattener: Flattener[Maybe] = new MaybeFlattener
+
   //Syntax
 
   implicit class CombinatorSyntax[A](self: Maybe[A]) {
@@ -30,4 +32,8 @@ object implicits {
     def pure[F[_]](implicit ev: Lifter[F]): F[A] = ev.pure(self)
   }
 
+  implicit class FlattenerSyntax[A](self: Maybe[A])(implicit ev: Flattener[Maybe]){
+    def flatMap[B](f: A => Maybe[B]): Maybe[B] = ev.flatMap(self, f)
+  }
+
 }

src/main/scala/training/instances.scala

@@ -38,5 +38,12 @@ object instances {
     override def pure[A](a: A): Maybe[A] = Maybe(a)
   }
 
+  class MaybeFlattener extends Flattener[Maybe] {
+    override def flatMap[A, B](fa: Maybe[A], f: A => Maybe[B]): Maybe[B] = fa match {
+      case Yes(a) => f(a)
+      case No => No
+    }
+  }
+
 
 }

src/main/scala/training/typeclasses.scala

@@ -25,5 +25,9 @@ object typeclasses {
     def pure[A](a: A): F[A]
   }
 
+  trait Flattener[F[_]]{
+    def flatMap[A, B](fa: F[A], f: A => F[B]): F[B]
+  }
+
 
 }