all tasks

Task/Y-combinator/D/y-combinator.d

@@ -0,0 +1,31 @@
+import std.stdio, std.traits, std.algorithm, std.range;
+
+auto Y(F)(F f) {
+  alias D = void delegate();
+  alias Ret = ReturnType!(ParameterTypeTuple!F);
+  alias Args = ParameterTypeTuple!(ParameterTypeTuple!F);
+
+  return ((Ret delegate(Args) delegate(D) x) =>
+    x(cast(D)x))(
+      (D y) =>
+        f((Args args) =>
+          (cast(Ret delegate(Args))(cast(D delegate(D))y)(y))(args)
+        )
+    );
+}
+
+void main() { // Demo code --------------------
+  auto factorial = Y((int delegate(int) self) =>
+    (int n) => 0 == n ? 1 : n * self(n - 1)
+  );
+
+  auto ackermann = Y((ulong delegate(ulong, ulong) self) =>
+    (ulong m, ulong n) {
+      if (m == 0) return n + 1;
+      if (n == 0) return self(m - 1, 1);
+      return self(m - 1, self(m, n - 1));
+    });
+
+  writeln("factorial: ", map!factorial(iota(10)));
+  writeln("ackermann(3, 5): ", ackermann(3, 5));
+}