Add FlatMapRec, MonadRec.

core/src/main/scala/cats/FlatMapRec.scala

@@ -0,0 +1,24 @@
+package cats
+
+import simulacrum.typeclass
+
+import cats.data.Xor
+
+/**
+ * Version of [[cats.FlatMap]] capable of stack-safe recursive `flatMap`s.
+ *
+ * Based on Phil Freeman's
+ * [[http://functorial.com/stack-safety-for-free/index.pdf Stack Safety for Free]].
+ */
+@typeclass trait FlatMapRec[F[_]] extends FlatMap[F] {
+
+  /**
+   * Keeps calling `f` until a `[[cats.data.Xor.Right Right]][B]` is returned.
+   *
+   * Implementations of this method must use constant stack space.
+   *
+   * `f` must use constant stack space. (It is OK to use a constant number of
+   * `map`s and `flatMap`s inside `f`.)
+   */
+  def tailRecM[A, B](a: A)(f: A => F[A Xor B]): F[B]
+}

core/src/main/scala/cats/MonadRec.scala

@@ -0,0 +1,5 @@
+package cats
+
+import simulacrum.typeclass
+
+@typeclass trait MonadRec[F[_]] extends Monad[F] with FlatMapRec[F]