ackerman

ttfi-core/src/main/scala/TTFI.scala

@@ -347,7 +347,7 @@ object TTFI {
       object OpenRecursion {
         import scala.annotation.tailrec
 
-        // not tail-recursive
+        // not @tailrec 
         def fix[A, B](f: (A => B) => (A => B)): A => B = {
           f((x: A) => fix(f)(x))
         }
@@ -368,6 +368,17 @@ object TTFI {
           }
         }
 
+        // http://rosettacode.org/wiki/Y_combinator#Scala
+        object Fix2 {
+          def apply[A, B](f: (A => B) => (A => B)) = {
+            case class W(wf: W => A => B) {
+              def apply(w: W) = wf(w)
+            }
+            val g: W => A => B = w => f(w(w))(_)
+            g(W(g))
+          }
+        }
+
         // {{{ FixCurried
 
         object FixCurried {

ttfi-core/src/test/scala/FixSpecs.scala

@@ -0,0 +1,21 @@
+import TTFI._
+
+import org.specs2._
+class Fix_Spec extends Specification {
+  def is = s2"""
+	Fix should not blow the stack $stackSafe
+"""
+
+  def stackSafe = {
+
+    // en.wikipedia.org/wiki/Ackermann_function
+    def acktabs(ack: ((Int, Int)) => Int)(in: ((Int, Int))): Int = {
+      val (m, n) = in
+      if (m <= 0) n + 1
+      else if (n <= 0) ack((m - 1, 1))
+      else ack((m - 1, ack((m, n - 1))))
+    }
+
+    Final.TreeSem.OpenRecursion.fix(acktabs)((5, 5)) ==== 0
+  }
+}