Typecollections.cpp: Pattern Matching of Variadic Templates for Type Collections in C++

.gitignore

@@ -31,4 +31,5 @@
 *.exe
 *.out
 *.app
-
+/peano
+/typecollections

README.md

@@ -3,3 +3,4 @@
 A repository of samples in C++
 
 - peano.cpp: Pattern Matching and Type Arithmetic in C++
+- typecollections.cpp : Pattern Matching of Variadic Templates for Type Collections in C++

typecollections.cpp

@@ -0,0 +1,318 @@
+// Pattern Matching of Variadic Templates for Type Collections in C++
+// - see peano.cpp for an introduction to type arithmetic
+// - check a functional programming language like Haskell for
+//   the declaration and usage of recursive lists
+
+// compile and test: "c++ typecollections.cpp -o typecollections && ./typecollections"
+
+//
+// typelist declaration
+//
+
+template<class ...elems>
+struct typelist {
+};
+
+//
+// typelist_index: get the (first) index of a type
+//
+
+// empty declaration: compilation error if the type is not found
+template<class type, class list>
+struct typelist_index {
+};
+
+// end of the recursion, the type match the head
+template<class type, class ...tail>
+struct typelist_index<type, typelist<type, tail...>> {
+    enum : unsigned { value = 0 };
+};
+
+// recursive declaration
+template<class type, class head, class ...tail>
+struct typelist_index<type, typelist<head, tail...>> {
+    enum : unsigned { value = typelist_index<type, typelist<tail...>>::value + 1u };
+};
+
+//
+// typelist_get: get the type at the given index
+//
+
+// empty declaration: compilation error if the index is out of bound
+template<unsigned index, class list>
+struct typelist_get {
+};
+
+// end of the recursion, the type match
+template<class _type, class ...tail>
+struct typelist_get<0u, typelist<_type, tail...>> {
+    using type = _type;
+};
+
+// recursive declaration
+template<unsigned index, class head, class ...tail>
+struct typelist_get<index, typelist<head, tail...>> {
+    using type = typename typelist_get<index - 1, typelist<tail...>>::type;
+};
+
+//
+// typelist_insert: insert an element at the beginning of a list
+//
+
+// declaration
+template<class elem, class list>
+struct typelist_insert {
+};
+
+// insert the new head, actually not recursive
+template<class head, class ...tail>
+struct typelist_insert<head, typelist<tail...>> {
+    using type = typelist<head, tail...>;
+};
+
+//
+// typedict_entry declaration
+//
+
+// typedict_entry declaration
+template<auto key, class type>
+struct typedict_entry {
+};
+
+//
+// typedict_contains: check if the typelist contains the key
+//
+
+// declaration and fallback if the key is missing
+template<auto key, class list>
+struct typedict_contains {
+	enum : bool { value = false };
+};
+
+// end of the recursion when the key is found
+template<auto key, class type, class ...tail>
+struct typedict_contains<key, typelist<typedict_entry<key, type>, tail...>> {
+	enum : bool { value = true };
+};
+
+// recursive declaration
+template<auto key, class head, class ...tail>
+struct typedict_contains<key, typelist<head, tail...>> {
+    enum : bool { value = typedict_contains<key, typelist<tail...>>::value };
+};
+
+//
+// typedict_index: get the (first) index of a key
+//
+
+// empty declaration: compilation error if the key is missing
+template<auto key, class list>
+struct typedict_index {
+};
+
+// end of the recursion, the key match
+template<auto key, class type, class ...tail>
+struct typedict_index<key, typelist<typedict_entry<key, type>, tail...>> {
+	enum : unsigned { value = 0 };
+};
+
+// recursive declaration
+template<auto key, class head, class ...tail>
+struct typedict_index<key, typelist<head, tail...>> {
+    enum : unsigned { value = typedict_index<key, typelist<tail...>>::value + 1u };
+};
+
+//
+// typedict_get: get the type associated with a key
+//
+
+// empty declaration: compilation error if the key is missing
+template<auto key, class list>
+struct typedict_get {
+};
+
+// end of the recursion: the key match a typedict_entry in the list
+template<auto key, class _type, class ...tail>
+struct typedict_get<key, typelist<typedict_entry<key, _type>, tail...>> {
+    using type = _type;
+};
+
+// recursive declaration
+template<auto key, class head, class ...tail>
+struct typedict_get<key, typelist<head, tail...>> {
+    using type = typename typedict_get<key, typelist<tail...>>::type;
+};
+
+//
+// typetree_flatten: flatten a tree
+//
+
+// empty declaration: end of the recursion if the typelist is empty
+template<class list>
+struct typetree_flatten {
+	using type = typelist<>;
+};
+
+// recursive declaration when the first element is a list
+template<class ...elems, class ...tail>
+struct typetree_flatten<typelist<typelist<elems...>, tail...>> {
+    using type = typename typetree_flatten<typelist<elems..., tail...>>::type;
+};
+
+// recursive declaration
+template<class head, class ...tail>
+struct typetree_flatten<typelist<head, tail...>> {
+    using type = typename typelist_insert<head, typename typetree_flatten<typelist<tail...>>::type>::type;
+};
+
+//
+// now let's test!
+//
+
+#include <cstdio>
+
+void test_typelist() {
+    using testlist = typelist<bool, int, float>;
+
+    // output: index of bool = 0
+    printf("index of bool = %u\n",
+        typelist_index<bool, testlist>::value);
+
+    // output: index of int = 1
+    printf("index of int = %u\n",
+        typelist_index<int, testlist>::value);
+
+    // output: index of float = 2
+    printf("index of float = %u\n",
+        typelist_index<float, testlist>::value);
+
+    // compilation error: type not found
+    // printf("index of double = %u\n",
+    //     typelist_index<double, testlist>::value);
+
+    auto const bool_value = typelist_get<0u, testlist>::type(true);
+    auto const int_value = typelist_get<1u, testlist>::type(42);
+    auto const float_value = typelist_get<2u, testlist>::type(3.14159f);
+
+    // compilation error: cast not allowed
+    // auto const badtype_value = typelist_get<2u, testlist>::type("string");
+
+    // compilation error: index out of range
+    // auto const outofrange_value = typelist_get<3u, testlist>::type();
+
+    // output: bool = true, int = 42, float = 3.141590
+    printf("bool = %s, int = %d, float = %f\n",
+        bool_value ? "true" : "false", int_value, float_value);
+}
+
+void test_typedict() {
+    enum {
+        key1,
+        key2,
+        key3,
+        key4
+    };
+
+    using testdict = typelist<
+        typedict_entry<key1, bool>,
+        typedict_entry<key2, int>,
+        typedict_entry<key3, float>
+    >;
+
+    // output: contains key1 = true
+    printf("contains key1 = %s\n",
+        typedict_contains<key1, testdict>::value ? "true" : "false");
+
+    // output: contains key4 = false
+    printf("contains key4 = %s\n",
+        typedict_contains<key4, testdict>::value ? "true" : "false");
+
+    // output: index of key1 = 0
+    printf("index of key1 = %u\n",
+        typedict_index<key1, testdict>::value);
+
+    // output: index of key2 = 1
+    printf("index of key2 = %u\n",
+        typedict_index<key2, testdict>::value);
+
+    // output: index of key3 = 2
+    printf("index of key3 = %u\n",
+        typedict_index<key3, testdict>::value);
+
+    // compilation error: entry not found
+    // printf("index of key4 = %u\n",
+    //     typedict_index<key4, testdict>::value);
+
+    auto const bool_value = typedict_get<key1, testdict>::type(true);
+    auto const int_value = typedict_get<key2, testdict>::type(42);
+    auto const float_value = typedict_get<key3, testdict>::type(3.14159f);
+
+    // compilation error: cast not allowed
+    // auto const badtype_value = typedict_get<key3, testdict>::type("string");
+
+    // compilation error: entry not found
+    // auto const outofrange_value = typedict_get<key4, testdict>::type();
+
+    // output: bool = true, int = 42, float = 3.141590
+    printf("bool = %s, int = %d, float = %f\n",
+        bool_value ? "true" : "false", int_value, float_value);
+}
+
+void test_typetree() {
+    enum {
+        key1,
+        key2,
+        key3,
+        key4
+    };
+
+    using testtree = typelist<
+        typelist<
+            typedict_entry<key1, bool>
+        >,
+        typelist<
+            typelist<
+                typedict_entry<key2, int>
+            >
+        >,
+        typelist<
+            typelist<
+                typelist<
+                    typedict_entry<key3, float>
+                >,
+                typedict_entry<key4, double>
+            >
+        >
+    >;
+
+    using flattened = typetree_flatten<testtree>::type;
+
+    // output: index of key1 = 0
+    printf("index of key1 = %u\n",
+        typedict_index<key1, flattened>::value);
+
+    // output: index of key2 = 1
+    printf("index of key2 = %u\n",
+        typedict_index<key2, flattened>::value);
+
+    // output: index of key3 = 2
+    printf("index of key3 = %u\n",
+        typedict_index<key3, flattened>::value);
+
+    // output: index of key4 = 3
+    printf("index of key4 = %u\n",
+        typedict_index<key4, flattened>::value);
+}
+
+int main () {
+    printf("\n> typelist test:\n");
+    test_typelist();
+
+    printf("\n> typedict test:\n");
+    test_typedict();
+
+    printf("\n> typetree test:\n");
+    test_typetree();
+
+    return 0;
+}