backported #3041, added tailrec instance for StacksafeMonad and Defer (#3332)

Author: gagandeepkalra

bench/src/main/scala/cats/bench/TrampolineBench.scala

@@ -1,11 +1,12 @@
 package cats.bench
 
 import org.openjdk.jmh.annotations.{Benchmark, Scope, State}
-
 import cats._
 import cats.implicits._
 import cats.free.Trampoline
 
+import scala.util.control.TailCalls
+
 @State(Scope.Benchmark)
 class TrampolineBench {
 
@@ -30,14 +31,12 @@ class TrampolineBench {
       y <- Trampoline.defer(trampolineFib(n - 2))
     } yield x + y
 
-  // TailRec[A] only has .flatMap in 2.11.
+   @Benchmark
+   def stdlib(): Int = stdlibFib(N).result
 
-  // @Benchmark
-  // def stdlib(): Int = stdlibFib(N).result
-  //
-  // def stdlibFib(n: Int): TailCalls.TailRec[Int] =
-  //   if (n < 2) TailCalls.done(n) else for {
-  //     x <- TailCalls.tailcall(stdlibFib(n - 1))
-  //     y <- TailCalls.tailcall(stdlibFib(n - 2))
-  //   } yield x + y
+   def stdlibFib(n: Int): TailCalls.TailRec[Int] =
+     if (n < 2) TailCalls.done(n) else for {
+       x <- TailCalls.tailcall(stdlibFib(n - 1))
+       y <- TailCalls.tailcall(stdlibFib(n - 2))
+     } yield x + y
 }

core/src/main/scala/cats/Eval.scala

@@ -378,6 +378,7 @@ sealed abstract private[cats] class EvalInstances extends EvalInstances0 {
       def flatMap[A, B](fa: Eval[A])(f: A => Eval[B]): Eval[B] = fa.flatMap(f)
       def extract[A](la: Eval[A]): A = la.value
       def coflatMap[A, B](fa: Eval[A])(f: Eval[A] => B): Eval[B] = Later(f(fa))
+      override def unit: Eval[Unit] = Eval.Unit
     }
 
   implicit val catsDeferForEval: Defer[Eval] =

core/src/main/scala/cats/instances/all.scala

@@ -69,4 +69,5 @@ trait AllInstancesBinCompat7
     with VectorInstancesBinCompat1
     with EitherInstancesBinCompat0
     with StreamInstancesBinCompat1
+    with TailRecInstances
     with SortedSetInstancesBinCompat2

core/src/main/scala/cats/instances/package.scala

@@ -43,6 +43,7 @@ package object instances {
       with SortedSetInstancesBinCompat2
   object stream extends StreamInstances with StreamInstancesBinCompat0 with StreamInstancesBinCompat1
   object string extends StringInstances
+  object tailRec extends TailRecInstances
   object try_ extends TryInstances
   object tuple extends TupleInstances with Tuple2InstancesBinCompat0
   object unit extends UnitInstances

core/src/main/scala/cats/instances/tailrec.scala

@@ -0,0 +1,26 @@
+package cats.instances
+
+import cats.{Defer, StackSafeMonad}
+import scala.util.control.TailCalls.{done, tailcall, TailRec}
+
+trait TailRecInstances {
+  implicit def catsInstancesForTailRec: StackSafeMonad[TailRec] with Defer[TailRec] =
+    TailRecInstances.catsInstancesForTailRec
+}
+
+private object TailRecInstances {
+  val catsInstancesForTailRec: StackSafeMonad[TailRec] with Defer[TailRec] =
+    new StackSafeMonad[TailRec] with Defer[TailRec] {
+      def defer[A](fa: => TailRec[A]): TailRec[A] = tailcall(fa)
+
+      def pure[A](a: A): TailRec[A] = done(a)
+
+      override def map[A, B](fa: TailRec[A])(f: A => B): TailRec[B] =
+        fa.map(f)
+
+      def flatMap[A, B](fa: TailRec[A])(f: A => TailRec[B]): TailRec[B] =
+        fa.flatMap(f)
+
+      override val unit: TailRec[Unit] = done(())
+    }
+}

tests/src/test/scala/cats/tests/TailRecSuite.scala

@@ -0,0 +1,29 @@
+package cats.tests
+
+import cats.{Defer, Eq, Monad}
+import cats.laws.discipline.{DeferTests, MonadTests, SerializableTests}
+import org.scalacheck.Arbitrary.arbitrary
+import org.scalacheck.{Arbitrary, Cogen, Gen}
+
+import scala.util.control.TailCalls.{done, tailcall, TailRec}
+
+class TailRecSuite extends CatsSuite {
+
+  implicit def tailRecArb[A: Arbitrary: Cogen]: Arbitrary[TailRec[A]] =
+    Arbitrary(
+      Gen.frequency(
+        (3, arbitrary[A].map(done)),
+        (1, Gen.lzy(arbitrary[(A, A => TailRec[A])].map { case (a, fn)          => tailcall(fn(a)) })),
+        (1, Gen.lzy(arbitrary[(TailRec[A], A => TailRec[A])].map { case (a, fn) => a.flatMap(fn) }))
+      )
+    )
+
+  implicit def eqTailRec[A: Eq]: Eq[TailRec[A]] =
+    Eq.by[TailRec[A], A](_.result)
+
+  checkAll("TailRec[Int]", MonadTests[TailRec].monad[Int, Int, Int])
+  checkAll("Monad[TailRec]", SerializableTests.serializable(Monad[TailRec]))
+
+  checkAll("TailRec[Int]", DeferTests[TailRec].defer[Int])
+  checkAll("Defer[TailRec]", SerializableTests.serializable(Defer[TailRec]))
+}