Set up a benchmark for our trampolines.

bench/src/main/scala/cats/bench/FoldBench.scala

@@ -12,11 +12,13 @@ class FoldBench {
   val chars: List[String] = ('a' to 'z').map(_.toString).toList
 
   /** Benchmark fold of Foldable[List] */
-  @Benchmark def fold(): String =
+  @Benchmark
+  def fold(): String =
     Foldable[List].fold(chars)
 
   /** Benchmark fold using traverse with Const */
-  @Benchmark def traverseConst(): String =
+  @Benchmark
+  def traverseConst(): String =
     Traverse[List].traverse[Const[String, ?], String, String](chars)(Const(_)).getConst
 
 }

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

@@ -0,0 +1,45 @@
+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 {
+
+  val N = 15
+
+  @Benchmark
+  def eval(): Int = evalFib(N).value
+
+  def evalFib(n: Int): Eval[Int] =
+    if (n < 2) Eval.now(n) else for {
+      x <- Eval.defer(evalFib(n - 1))
+      y <- Eval.defer(evalFib(n - 2))
+    } yield x + y
+
+
+  @Benchmark
+  def trampoline(): Int = trampolineFib(N).run
+
+  def trampolineFib(n: Int): Trampoline[Int] =
+    if (n < 2) Trampoline.done(n) else for {
+      x <- Trampoline.suspend(trampolineFib(n - 1))
+      y <- Trampoline.suspend(trampolineFib(n - 2))
+    } yield x + y
+
+  // TailRec[A] only has .flatMap in 2.11.
+
+  // @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
+}

build.sbt

@@ -1,7 +1,6 @@
 import com.typesafe.sbt.pgp.PgpKeys.publishSigned
 import com.typesafe.sbt.SbtSite.SiteKeys._
 import com.typesafe.sbt.SbtGhPages.GhPagesKeys._
-import pl.project13.scala.sbt.SbtJmh._
 import sbtunidoc.Plugin.UnidocKeys._
 import ReleaseTransformations._
 import ScoverageSbtPlugin._
@@ -189,8 +188,8 @@ lazy val bench = project.dependsOn(macrosJVM, coreJVM, freeJVM, lawsJVM)
   .settings(moduleName := "cats-bench")
   .settings(catsSettings)
   .settings(noPublishSettings)
-  .settings(jmhSettings)
   .settings(commonJvmSettings)
+  .enablePlugins(JmhPlugin)
 
 // cats-js is JS-only
 lazy val js = project

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

@@ -82,6 +82,12 @@ sealed abstract class Eval[A] { self =>
               val run = f
             }
         }
+      case c: Eval.Call[A] =>
+        new Eval.Compute[B] {
+          type Start = A
+          val start = c.thunk
+          val run = f
+        }
       case _ =>
         new Eval.Compute[B] {
           type Start = A
@@ -194,7 +200,7 @@ object Eval extends EvalInstances {
    * which produces an Eval[A] value. Like .flatMap, it is stack-safe.
    */
   def defer[A](a: => Eval[A]): Eval[A] =
-    Eval.Unit.flatMap(_ => a)
+    new Eval.Call[A](a _) {}
 
   /**
    * Static Eval instances for some common values.
@@ -208,6 +214,18 @@ object Eval extends EvalInstances {
   val Zero: Eval[Int] = Now(0)
   val One: Eval[Int] = Now(1)
 
+  /**
+   * Call is a type of Eval[A] that is used to defer computations
+   * which produce Eval[A].
+   *
+   * Users should not instantiate Call instances themselves. Instead,
+   * they will be automatically created when needed.
+   */
+  sealed abstract class Call[A](val thunk: () => Eval[A]) extends Eval[A] {
+    def memoize: Eval[A] = new Later(() => thunk().value)
+    def value: A = thunk().value
+  }
+
   /**
    * Compute is a type of Eval[A] that is used to chain computations
    * involving .map and .flatMap. Along with Eval#flatMap it

project/plugins.sbt

@@ -9,7 +9,7 @@ addSbtPlugin("com.jsuereth"      % "sbt-pgp"                % "1.0.0")
 addSbtPlugin("com.typesafe.sbt"  % "sbt-ghpages"            % "0.5.3")
 addSbtPlugin("com.typesafe.sbt"  % "sbt-site"               % "0.8.1")
 addSbtPlugin("org.tpolecat"      % "tut-plugin"             % "0.4.0")
-addSbtPlugin("pl.project13.scala"% "sbt-jmh"                % "0.1.10")
+addSbtPlugin("pl.project13.scala"% "sbt-jmh"                % "0.2.3")
 addSbtPlugin("org.scalastyle"    %% "scalastyle-sbt-plugin" % "0.6.0")
 addSbtPlugin("org.scoverage"     % "sbt-scoverage"          % "1.2.0")
 addSbtPlugin("com.typesafe.sbt"  % "sbt-git"                % "0.8.4")