@odbc1986: Homework2.5 - stack safety for FreeMagma apply and reduceAll

src/main/scala/fpspeedrun/Magma.scala

@@ -0,0 +1,25 @@
+package fpspeedrun
+
+trait Magma[T] {
+  def combine(x: T, y: T): T
+}
+
+sealed trait FreeMagma[T]
+case class Leaf[T](x: T) extends FreeMagma[T]
+case class Branch[T](x: FreeMagma[T], y: FreeMagma[T]) extends FreeMagma[T]
+
+object FreeMagma {
+
+  def apply[T](x: T, y: T*): FreeMagma[T] = y match {
+    case Seq()  => Leaf(x)
+    case Seq(a) => Branch(Leaf(x), Leaf(a))
+    case ys     =>
+      val list   = (x +: ys).reverse
+      val first +: second +: rest = list
+
+      rest.foldLeft(Branch(Leaf(second), Leaf(first)))((acc, e) => Branch(Leaf(e), acc))
+  }
+
+  implicit def freeMagmaMagma[T]: Magma[FreeMagma[T]] = (x: FreeMagma[T], y: FreeMagma[T]) => Branch(x, y)
+
+}

src/main/scala/fpspeedrun/syntax/package.scala

@@ -1,6 +1,8 @@
 package fpspeedrun.syntax
 import fpspeedrun._
 
+import scala.annotation.tailrec
+
 object eq extends Eq.ToEqOps
 
 object ord extends Ord.ToOrdOps
@@ -45,6 +47,23 @@ object semigroup extends Semigroup.ToSemigroupOps {
     def first: First[T] = First(x)
     def last: Last[T] = Last(x)
   }
+
+  implicit class FreeMagmaOps[T](val x: FreeMagma[T]) extends AnyVal {
+    @tailrec final def flat(x: List[FreeMagma[T]]): List[Leaf[T]] = {
+      val list: List[FreeMagma[T]] = x.flatMap {
+        case l@Leaf(_) => List(l)
+        case Branch(l, r) => List(l, r)
+      }
+
+      if (x.size == list.size) x.map(_.asInstanceOf[Leaf[T]])
+      else flat(list)
+    }
+
+    def reduceAll(implicit sg: Semigroup[T]): T = x match {
+      case Leaf(a) => a
+      case b@Branch(_, _) => flat(List(b)) map { case Leaf(l) => l } reduce sg.combine
+    }
+  }
 }
 
 object monoid extends Monoid.ToMonoidOps