Kotlin step 2: Create typeclass Combinator

Author: JorgeCastilloPrz

kotlin/src/main/kotlin/com/fortyseven/fpfundamentals/Contents.kt

@@ -1,17 +1,55 @@
 package com.fortyseven.fpfundamentals
 
+import arrow.extension
 import arrow.higherkind
+import com.fortyseven.fpfundamentals.maybe.combinator.combinator
 
 // Data types
 
 @higherkind
 sealed class Maybe<out A> : MaybeOf<A> {
+  companion object {
+  }
+
   data class Present<A>(val a: A) : Maybe<A>()
   object Absent : Maybe<Nothing>()
 
-  inline fun <B> fold(ifAbsent: () -> B, ifSome: (A) -> B): B = when (this) {
+  inline fun <B> fold(ifAbsent: () -> B, ifPresent: (A) -> B): B = when (this) {
     is Absent -> ifAbsent()
-    is Present<A> -> ifSome(a)
+    is Present<A> -> ifPresent(a)
+  }
+}
+
+// Typeclasses
+
+interface Combinator<A> {
+
+  fun combine(x: A, y: A): A
+}
+
+// Instances
+
+@extension
+interface IntCombinator : Combinator<Int> {
+  override fun combine(x: Int, y: Int): Int = x + y
+}
+
+fun Int.Companion.combinator(): Combinator<Int> =
+  object : IntCombinator {}
+
+@extension
+interface MaybeCombinator<A> : Combinator<Maybe<A>> {
+
+  fun CA(): Combinator<A>
+
+  override fun combine(x: Maybe<A>, y: Maybe<A>): Maybe<A> {
+    return x.fold(
+      { y },
+      { xx ->
+        y.fold(
+          { x },
+          { yy -> Maybe.Present(CA().combine(xx, yy)) })
+      })
   }
 }
 
@@ -24,7 +62,8 @@ class Program {
   fun getBalanceBank2(): Maybe<Int> = Maybe.Present(80)
 
   val balance: Maybe<Int>
-    get() = getBalanceBank1() + getBalanceBank2() // It won't compile
+    get() = Maybe.combinator(Int.combinator())
+      .combine(getBalanceBank1(), getBalanceBank2())
 }
 
 fun main(args: Array<String>) {