Complete ch07

Author: Abhijit Sarkar

.scalafmt.conf

@@ -6,5 +6,6 @@ assumeStandardLibraryStripMargin = true
 # https://github.com/scalameta/scalameta/issues/4090
 project.excludePaths = [
     "glob:**/ch04/src/**.scala",
-    "glob:**/ch06/src/Cat.scala"
+    "glob:**/ch06/src/Cat.scala",
+    "glob:**/ch07/src/*.scala"
   ]

README.md

@@ -14,6 +14,7 @@ The older code is available in branches.
 4. [Contextual Abstraction](ch04)
 5. [Reified Interpreters](ch05)
 6. [Using Cats](ch06)
+7. [Monoids and Semigroups](ch07)
 
 ## Running tests
 ```

ch07/src/Lib.scala

@@ -0,0 +1,51 @@
+package ch07
+
+/*
+We can use Semigroups and Monoids by importing two things: the type classes themselves, 
+and the semigroup syntax to give us the |+| operator.
+*/
+import cats.{Monoid as CatsMonoid}
+import cats.syntax.semigroup.catsSyntaxSemigroup
+
+object Lib:
+
+  /*
+  7.3.3.1 Exercise: Adding All The Things
+  The cutting edge SuperAdder v3.5a-32 is the world's first choice for adding together numbers.
+  The main function in the program has signature def add(items: List[Int]): Int.
+  In a tragic accident this code is deleted! Rewrite the method and save the day!
+
+  SuperAdder's market share continues to grow, and now there is demand for additional functionality.
+  People now want to add List[Option[Int]]. Change add so this is possible. The SuperAdder code base
+  is of the highest quality, so make sure there is no code duplication!
+   */
+  def add[A: CatsMonoid as m](items: List[A]): A =
+    items.foldLeft(m.empty)(_ |+| _)
+
+//   import cats.instances.int.catsKernelStdGroupForInt
+//   add(List(1, 2, 3))
+
+//   add(List(Some(1), None, Some(2), None, Some(3)))
+
+// Doesn't compile: No given instance of type cats.kernel.Monoid[Some[Int]] was found.
+// The inferred type of the list is List[Some[Int]], Cats Monoid is invariant, so,
+// Monoid[Option[A]] is not applicable.
+//   add(List(Some(1), Some(2), Some(3)))
+
+  /*
+  SuperAdder is entering the POS (point-of-sale, not the other POS) market.
+  Now we want to add up Orders.
+
+  We need to release this code really soon so we can’t make any modifications to add.
+  Make it so!
+   */
+  case class Order(totalCost: Double, quantity: Double)
+
+  given Monoid[Order]:
+    def combine(o1: Order, o2: Order) =
+      Order(
+        o1.totalCost + o2.totalCost,
+        o1.quantity + o2.quantity
+      )
+
+    def empty = Order(0, 0)

ch07/src/Monoid.scala

@@ -0,0 +1,44 @@
+package ch07
+
+trait Monoid[A] extends Semigroup[A]:
+  def empty: A
+
+object Monoid:
+  def apply[A : Monoid as m]: Monoid[A] = m
+
+  /*
+  7.2.0.1 Exercise: The Truth About Monoids
+  Consider Boolean. How many monoids can you define for this type?
+   */
+  given booleanAndMonoid: Monoid[Boolean]:
+    def combine(a: Boolean, b: Boolean) = a && b
+    def empty                           = true
+
+  given booleanOrMonoid: Monoid[Boolean]:
+    def combine(a: Boolean, b: Boolean) = a || b
+    def empty                           = false
+
+  given booleanXorMonoid: Monoid[Boolean]:
+    def combine(a: Boolean, b: Boolean) =
+      a != b
+    def empty = false
+
+  // The negation of XOR
+  given booleanXnorMonoid: Monoid[Boolean]:
+    def combine(a: Boolean, b: Boolean) =
+      (!a || b) && (a || !b)
+
+    def empty = true
+
+  /*
+  7.2.0.2 Exercise: All Set for Monoids
+  What monoids and semigroups are there for sets?
+  */
+  given setUnionMonoid: [A] => Monoid[Set[A]]:
+    def combine(a: Set[A], b: Set[A]) = a.union(b)
+    def empty                         = Set.empty[A]
+
+  given symDiffMonoid: [A] => Monoid[Set[A]]:
+    def combine(a: Set[A], b: Set[A]): Set[A] =
+      (a.diff(b)).union(b.diff(a))
+    def empty: Set[A] = Set.empty

ch07/src/Semigroup.scala

@@ -0,0 +1,10 @@
+package ch07
+
+trait Semigroup[A]:
+  def combine(x: A, y: A): A
+
+object Semigroup:
+  // No identity element, can't form a Monoid.
+  given setIntersectionSemigroup: [A] => Semigroup[Set[A]]:
+    def combine(a: Set[A], b: Set[A]) =
+      a.intersect(b)

ch07/test/src/LibSpec.scala

@@ -0,0 +1,22 @@
+package ch07
+import org.scalatest.funspec.AnyFunSpec
+import org.scalatest.matchers.should.Matchers.shouldBe
+import ch07.Lib.add
+
+class LibSpec extends AnyFunSpec:
+  describe("Monoid"):
+    it("should add integers"):
+      val ints = List(1, 2, 3)
+      add(ints) `shouldBe` 6
+
+    it("should add strings"):
+      val strings = List("Hi ", "there")
+      add(strings) `shouldBe` "Hi there"
+
+    it("should add sets"):
+      val sets = List(Set("A", "B"), Set("B", "C"))
+      add(sets) `shouldBe` Set("A", "B", "C")
+
+    it("should add options"):
+      val opts = List(Option(22), Option(20))
+      add(opts) `shouldBe` Option(42)