Implemented `Filter` in terms of `Fold`

* as its name implies this _function_ only returns elements which evaluate to _true_ when passed to a given _function_
** this marks the moment where _TypeAsValue_ supports something _ConstList_ does not, i.e. primary goal is achieved
*** namely returning different types depending on the actual _values_ of a _Cons_ structure
* added appropriate test case

1 files changed, 8 insertions, 0 deletions

diff --git a/test.cc b/test.cc
index 655aed3..35126fb 100644
--- a/test.cc
+++ b/test.cc
@@ -14,6 +14,9 @@ class TypeAsValueTest : public ::testing::Test { };
 template <typename Element>
 using quadruple = tav::Multiply<tav::Int<4>, Element>;
 
+template <typename Element>
+using odd = tav::Boolean<(Element::value % 2 == 1)>;
+
 TEST_F(TypeAsValueTest, BasicMath) {
 	// (+ 1 2)
 	EXPECT_EQ(3,  ( tav::Add<tav::Int<1>, tav::Int<2>>::value ));
@@ -99,6 +102,11 @@ TEST_F(TypeAsValueTest, ListMap) {
 	EXPECT_TRUE(( std::is_same<tav::List<tav::Int<4>, tav::Int<8>, tav::Int<12>>::type, tav::Map<quadruple, tav::List<tav::Int<1>, tav::Int<2>, tav::Int<3>>::type>::type>::value ));
 }
 
+TEST_F(TypeAsValueTest, ListFilter) {
+	// (filter odd (list 1 2 3))
+	EXPECT_TRUE(( std::is_same<tav::List<tav::Int<1>, tav::Int<3>>::type, tav::Filter<odd, tav::List<tav::Int<1>, tav::Int<2>, tav::Int<3>>::type>::type>::value ));
+}
+
 TEST_F(TypeAsValueTest, ListReverse) {
 	// (reverse (list 1 2 3))
 	EXPECT_TRUE(( std::is_same<tav::List<tav::Int<3>, tav::Int<2>, tav::Int<1>>::type, tav::Reverse<tav::List<tav::Int<1>, tav::Int<2>, tav::Int<3>>::type>::type>::value ));