Alter TailRec to have map and flatMap

As described in the "Stackless Scala with Free Monads" paper

scala> import scala.util.control.TailCalls._
import scala.util.control.TailCalls._

scala> :paste
// Entering paste mode (ctrl-D to finish)

def isEven(xs: List[Int]): TailRec[Boolean] =
  if (xs.isEmpty) done(true) else tailcall(isOdd(xs.tail))

def isOdd(xs: List[Int]): TailRec[Boolean] =
  if (xs.isEmpty) done(false) else tailcall(isEven(xs.tail))

// Exiting paste mode, now interpreting.

isEven: (xs: List[Int])util.control.TailCalls.TailRec[Boolean]
isOdd: (xs: List[Int])util.control.TailCalls.TailRec[Boolean]

scala> isEven((1 to 100000).toList).result
res0: Boolean = true

scala> def fib(n: Int): TailRec[Int] =
     |   if (n < 2) done(n) else for {
     |     x <- tailcall(fib(n - 1))
     |     y <- tailcall(fib(n - 2))
     |   } yield (x + y)
fib: (n: Int)util.control.TailCalls.TailRec[Int]

scala> fib(40).result
res1: Int = 102334155

1 files changed, 13 insertions, 0 deletions

diff --git a/test/files/run/tailcalls.scala b/test/files/run/tailcalls.scala
index 1d4124e138..e5d8891cc7 100644
--- a/test/files/run/tailcalls.scala
+++ b/test/files/run/tailcalls.scala
@@ -391,7 +391,20 @@ object Test {
   def isOdd(xs: List[Int]): TailRec[Boolean] =
     if (xs.isEmpty) done(false) else tailcall(isEven(xs.tail))
 
+  def fib(n: Int): TailRec[Int] =
+    if (n < 2) done(n) else for {
+      x <- tailcall(fib(n - 1))
+      y <- tailcall(fib(n - 2))
+    } yield (x + y)
+
+  def rec(n: Int): TailRec[Int] =
+    if (n == 1) done(n) else for {
+      x <- tailcall(rec(n - 1))
+    } yield x
+
   assert(isEven((1 to 100000).toList).result)
+  //assert(fib(40).result == 102334155) // Commented out, as it takes a long time
+  assert(rec(100000).result == 1)
 
 }