From 7574e3836eec13e6f632b811e6bf001f2e3d2a1d Mon Sep 17 00:00:00 2001
From: Adrian Kummerlaender
Date: Sun, 18 Jan 2015 13:58:36 +0100
Subject: Implemented higher order function `fold` * applies a given _function_
 to each _Cons_ starting with a initial value * added appropriate test case

---
 src/list/operation/higher/fold.h | 28 ++++++++++++++++++++++++++++
 test.cc                          |  6 ++++++
 2 files changed, 34 insertions(+)
 create mode 100644 src/list/operation/higher/fold.h

diff --git a/src/list/operation/higher/fold.h b/src/list/operation/higher/fold.h
new file mode 100644
index 0000000..6ad4bd7
--- /dev/null
+++ b/src/list/operation/higher/fold.h
@@ -0,0 +1,28 @@
+#ifndef TYPEASVALUE_SRC_LIST_OPERATION_HIGHER_FOLD_H_
+#define TYPEASVALUE_SRC_LIST_OPERATION_HIGHER_FOLD_H_
+
+namespace tav {
+
+template <
+	template<typename, typename> class Function,
+	typename                           Initial,
+	typename                           Current
+>
+struct Fold {
+	typedef typename Function<
+		Head<Current>,
+		typename Fold<Function, Initial, Tail<Current>>::type
+	>::type type;
+};
+
+template <
+	template<typename, typename> class Function,
+	typename                           Initial
+>
+struct Fold<Function, Initial, void> {
+	typedef Initial type;
+};
+
+}
+
+#endif  // TYPEASVALUE_SRC_LIST_OPERATION_HIGHER_FOLD_H_
diff --git a/test.cc b/test.cc
index f60adbd..e642b3a 100644
--- a/test.cc
+++ b/test.cc
@@ -5,6 +5,7 @@
 #include "conditional/if.h"
 #include "list/cons.h"
 #include "list/list.h"
+#include "list/operation/higher/fold.h"
 
 class TypeAsValueTest : public ::testing::Test { };
 
@@ -83,6 +84,11 @@ TEST_F(TypeAsValueTest, ListConcatenate) {
 	EXPECT_EQ(2, ( tav::Head<tav::Tail<tav::Concatenate<tav::List<tav::Int<1>>::type, tav::List<tav::Int<2>>::type>::type>>::value ));
 }
 
+TEST_F(TypeAsValueTest, ListFold) {
+	// (fold + 0 (list 1 2 3))
+	EXPECT_EQ(6, ( tav::Fold<tav::Add, tav::Int<0>, tav::List<tav::Int<1>, tav::Int<2>, tav::Int<3>>::type>::type::value ));
+}
+
 int main(int argc, char **argv) {
 	testing::InitGoogleTest(&argc, argv);
 
-- 
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