From 47fe21019ad55497188b54ee87f0fbe0aa2a64d9 Mon Sep 17 00:00:00 2001
From: Kevin Lee <kevin.code@kevinlee.io>
Date: Sat, 28 Sep 2019 15:44:21 +1000
Subject: [PATCH] Close #82 - Add OptionSemiGroup and OptionMonoid

---
 src/main/scala/just/fp/Monoid.scala           | 14 ++++++++--
 src/main/scala/just/fp/SemiGroup.scala        | 26 ++++++++++++++++++-
 src/test/scala/just/fp/MonoidSpec.scala       | 12 ++++++++-
 .../just/fp/syntax/SemiGroupSyntaxSpec.scala  |  4 ++-
 4 files changed, 51 insertions(+), 5 deletions(-)

diff --git a/src/main/scala/just/fp/Monoid.scala b/src/main/scala/just/fp/Monoid.scala
index c20b2cf..aca2ad5 100644
--- a/src/main/scala/just/fp/Monoid.scala
+++ b/src/main/scala/just/fp/Monoid.scala
@@ -33,7 +33,7 @@ trait Monoid[A] extends SemiGroup[A] {
   def monoidLaw: MonoidLaw = new MonoidLaw {}
 }
 
-object Monoid {
+object Monoid extends OptionMonoidInstance {
 
   implicit def listMonoid[A]: Monoid[List[A]] = new Monoid[List[A]] with ListSemiGroup[A] {
     override def zero: List[A] = Nil
@@ -75,4 +75,14 @@ object Monoid {
     override def zero: BigDecimal = BigDecimal(0)
   }
 
-}
\ No newline at end of file
+}
+
+private[fp] trait OptionMonoidInstance extends OptionSemiGroupInstance {
+  implicit def optionMonoid[A](implicit F0: SemiGroup[A]): Monoid[Option[A]] =
+    new Monoid[Option[A]] with OptionSemigroup[A] {
+
+      override implicit def F: SemiGroup[A] = F0
+
+      override def zero: Option[A] = None
+    }
+}
diff --git a/src/main/scala/just/fp/SemiGroup.scala b/src/main/scala/just/fp/SemiGroup.scala
index 4e194c7..bf2d842 100644
--- a/src/main/scala/just/fp/SemiGroup.scala
+++ b/src/main/scala/just/fp/SemiGroup.scala
@@ -8,7 +8,7 @@ trait SemiGroup[A] {
   def append(a1: A, a2: => A): A
 }
 
-object SemiGroup {
+object SemiGroup extends OptionSemiGroupInstance {
 
   trait ListSemiGroup[A] extends SemiGroup[List[A]] {
     override def append(a1: List[A], a2: => List[A]): List[A] = a1 ++ a2
@@ -60,3 +60,27 @@ object SemiGroup {
   }
   implicit val bigDecimalSemiGroup: SemiGroup[BigDecimal] = new BigDecimalSemiGroup {}
 }
+
+private[fp] trait OptionSemigroup[A] extends SemiGroup[Option[A]] {
+
+  implicit def F: SemiGroup[A]
+
+  override def append(a1: Option[A], a2: => Option[A]): Option[A] = (a1, a2) match {
+    case (Some(x), Some(y)) =>
+      Some(implicitly[SemiGroup[A]].append(x, y))
+    case (Some(x), None) =>
+      Some(x)
+    case (None, Some(y)) =>
+      Some(y)
+    case (None, None) =>
+      None
+  }
+}
+
+private[fp] trait OptionSemiGroupInstance {
+
+  implicit def optionSemigroup[A](implicit F0: SemiGroup[A]): SemiGroup[Option[A]] =
+    new OptionSemigroup[A] {
+      override implicit def F: SemiGroup[A] = F0
+    }
+}
\ No newline at end of file
diff --git a/src/test/scala/just/fp/MonoidSpec.scala b/src/test/scala/just/fp/MonoidSpec.scala
index fb66509..f9c59ba 100644
--- a/src/test/scala/just/fp/MonoidSpec.scala
+++ b/src/test/scala/just/fp/MonoidSpec.scala
@@ -10,7 +10,8 @@ import hedgehog.runner._
 object MonoidSpec extends Properties {
 
   override def tests: List[Test] = List(
-    property("testListMonoidLaw", ListMonoidLaws.laws)
+    property("testOptionMonoidLaw", OptionMonoidLaws.laws)
+  , property("testListMonoidLaw", ListMonoidLaws.laws)
   , property("testVectorMonoidLaw", VectorMonoidLaws.laws)
   , property("testStringMonoidLaw", StringMonoidLaws.laws)
   , property("testByteMonoidLaw", ByteMonoidLaws.laws)
@@ -40,6 +41,15 @@ object MonoidSpec extends Properties {
       )
   }
 
+  object OptionMonoidLaws {
+    def genOption: Gen[Option[Int]] = Gens.genOption(Gens.genIntFromMinToMax)
+
+    def laws: Property =
+      Specs.monoidLaws.laws[Option[Int]](
+        genOption
+      )
+  }
+
   object StringMonoidLaws {
     def genString: Gen[String] = Gens.genUnicodeString
 
diff --git a/src/test/scala/just/fp/syntax/SemiGroupSyntaxSpec.scala b/src/test/scala/just/fp/syntax/SemiGroupSyntaxSpec.scala
index dd5fad9..3ddb8cc 100644
--- a/src/test/scala/just/fp/syntax/SemiGroupSyntaxSpec.scala
+++ b/src/test/scala/just/fp/syntax/SemiGroupSyntaxSpec.scala
@@ -12,7 +12,8 @@ object SemiGroupSyntaxSpec extends Properties {
   import just.fp._
 
   override def tests: List[Test] = List(
-      property("test List |+| List", testPlus(Gens.genList(Gens.genIntFromMinToMax, 10)))
+      property("test Option |+| Option", testPlus(Gens.genOption(Gens.genIntFromMinToMax)))
+    , property("test List |+| List", testPlus(Gens.genList(Gens.genIntFromMinToMax, 10)))
     , property("test Vector |+| Vector", testPlus(Gens.genVector(Gens.genIntFromMinToMax, 10)))
     , property("test String |+| String", testPlus(Gens.genUnicodeString))
     , property("test Byte |+| Byte", testPlus(Gens.genByteFromMinToMax))
@@ -22,6 +23,7 @@ object SemiGroupSyntaxSpec extends Properties {
     , property("test Long |+| Long", testPlus(Gens.genLongFromMinToMax))
     , property("test BigInt |+| BigInt", testPlus(Gens.genBigInt))
     , property("test BigDecimal |+| BigDecimal", testPlus(Gens.genBigDecimal))
+    , property("test Option.mappend(Option)", testMappend(Gens.genOption(Gens.genIntFromMinToMax)))
     , property("test List.mappend(List)", testMappend(Gens.genList(Gens.genIntFromMinToMax, 10)))
     , property("test Vector.mappend(Vector)", testMappend(Gens.genVector(Gens.genIntFromMinToMax, 10)))
     , property("test String.mappend(String)", testMappend(Gens.genUnicodeString))