From c257d314ef0e570cad7acc965405dc784767c1e3 Mon Sep 17 00:00:00 2001
From: odbc <odbc@yandex.ru>
Date: Tue, 4 Sep 2018 02:26:02 +0300
Subject: [PATCH] HomeWork 3: Lazy Foldable

---
 src/main/scala/fpspeedrun/Foldable.scala | 52 ++++++++++++++++++++++++
 1 file changed, 52 insertions(+)
 create mode 100644 src/main/scala/fpspeedrun/Foldable.scala

diff --git a/src/main/scala/fpspeedrun/Foldable.scala b/src/main/scala/fpspeedrun/Foldable.scala
new file mode 100644
index 0000000..080e3b4
--- /dev/null
+++ b/src/main/scala/fpspeedrun/Foldable.scala
@@ -0,0 +1,52 @@
+package fpspeedrun
+
+import cats.Eval
+import simulacrum.typeclass
+
+@typeclass
+trait Foldable[F[_]] {
+  type LazyMonoid[A] = Monoid[Eval[A]]
+
+  def foldMapLazy[A, B: LazyMonoid](fa: F[A])(f: A => Eval[B]): Eval[B]
+
+  def foldRightLazy[A, B](fa: F[A], lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B] = {
+
+    type EvalEndo = Eval[B] => Eval[B]
+
+    implicit def evalEndoMonoid: LazyMonoid[EvalEndo] = new Monoid[Eval[EvalEndo]] {
+      override def empty: Eval[EvalEndo] = Eval.later(identity[Eval[B]])
+
+      override def combine(lx: Eval[EvalEndo], ly: Eval[EvalEndo]): Eval[EvalEndo] =
+        Eval.later { e: Eval[B] => (for { x <- lx; y <- ly } yield x compose y).flatMap(_(e)) }
+    }
+
+    val folder: A => Eval[EvalEndo] = (a: A) => Eval.later { e: Eval[B] => f(a, e) }
+
+    foldMapLazy[A, EvalEndo](fa)(folder).flatMap(_(lb))
+  }
+
+  def foldLeft[A, B](fa: F[A], b: B)(f: (B, A) => B): B = {
+
+    type Endo = B => B
+
+    implicit def endoMonoid: Monoid[Eval[B => B]] = new Monoid[Eval[B => B]] {
+      override def empty: Eval[B => B] = Eval.later(identity[B])
+
+      override def combine(lx: Eval[B => B], ly: Eval[B => B]): Eval[B => B] =
+        Eval.later { e: B => (for { x <- lx; y <- ly } yield x compose y).value(e) }
+    }
+
+    foldMapLazy[A, B => B](fa)({ a: A => Eval.later((b: B) => f(b, a)) }).value(b)
+  }
+}
+
+object Foldable {
+
+  implicit def streamFoldable: Foldable[Stream] = new Foldable[Stream] {
+    override def foldMapLazy[A, B: LazyMonoid](fa: Stream[A])(f: A => Eval[B]): Eval[B] =
+      Eval.defer {
+        implicitly[LazyMonoid[B]].combine(f(fa.head), foldMapLazy(fa.tail)(f))
+      }
+  }
+
+}
\ No newline at end of file