C++ is a Dynamic, Pure, Functional Programming Language

functional.cpp

#include <iostream>

template <typename T>
struct Print {
    static void print() {
        std::cout << T::val;
    }
};

template <int N>
struct Int {
    typedef int type;
    static const type val = N;
};

//int main() {
//    Print<Int<5> >::print();
//}

template <typename A, typename B>
struct Plus {
    typedef typename A::type type;
    static const type val = A::val + B::val;
};
//int main() {
//    Print<Plus<Int<5>, Int<3> > >::print();
//}


struct Nil {
    typedef Nil self;
};
template <typename Head, typename Tail>
struct Cons {
    typedef Cons<Head, Tail> self;
};

template <typename List>
struct ListSum : public Int<0> {};

template <typename Head, typename Tail>
struct ListSum<Cons<Head, Tail> > : public
    Plus<Head, ListSum<typename Tail::self> > {};

typedef Cons<Int<1>, Nil> l1;
typedef Cons<Int<2>, l1> l2;
typedef Cons<Int<3>, l2> l3;

//int main() {
// Print<ListSum<l3> >::print(); // will print 6
//}

template <typename Func, typename Arg>
struct Apply_ {};

template <typename Func, typename Arg>
struct Apply : public
    Apply_<typename Func::self, Arg> {};

struct Inc {
    typedef Inc self;
};

template <typename Arg>
struct Apply_<Inc, Arg> : public Plus<Arg, Int<1> > {};

template <typename List, typename Func>
struct Map; // Forward definitino of Map

template <typename List, typename Func>
struct Map_ : public List {};

template <typename Head, typename Tail, typename Func>
struct Map_<Cons<Head, Tail>, Func> : public
    Cons<Apply<Func, Head>, Map<Tail, Func> > {};

template <typename List, typename Func>
struct Map : public Map_<List, Func>::self {};

template <typename Val>
struct Inf : public Cons<Val, Inf<Val> > {};

int main() {
    Print<ListSum<Map<l3, Inc>::self> >::print();
}

This gist was created as support material for this post: https://cloudalion.org/2016/09/08/c-is-a-dynamic-pure-functional-programming-language/