Templates as first-class citizens in C++11

fn_universal.cpp

// Templates as first-class citizens in C++11
// Example code from http://vitiy.info/templates-as-first-class-citizens-in-cpp11/
// Written by Victor Laskin ([email protected])


#include <iostream>
#include <algorithm>
#include <functional>
#include <vector>
#include <tuple>
#include <memory>
#include <sstream>
using namespace std;

#define LOG cout
#define NL endl

// apply tuple to function...
    
    namespace fn_detail {
        
        template<int ...>
        struct int_sequence {};
        
        template<int N, int ...S>
        struct gen_int_sequence : gen_int_sequence<N-1, N-1, S...> {};
        
        template<int ...S>
        struct gen_int_sequence<0, S...> {
            typedef int_sequence<S...> type;
        };
        
        template <typename F, typename... Args, int... S>
        inline auto fn_tuple_apply(int_sequence<S...>, const F& f, const std::tuple<Args...>& params) -> decltype(                                                                                                                 f((std::get<S>(params))...)
        )
        {
            return f((std::get<S>(params))...);
        }
        
        
    }
    
    template <typename F, typename... Args> //f(std::declval<Args>()...)
    inline auto fn_tuple_apply(const F& f, const std::tuple<Args...>& params) -> decltype( f(std::declval<Args>()...) )
    {
        return fn_detail::fn_tuple_apply(typename fn_detail::gen_int_sequence<sizeof...(Args)>::type(), f, params);
    }
    
    // end of apply tuple
    
    
    // universal function / extended function wrapper !
    template<typename F, typename TPackBefore = std::tuple<>, typename TPackAfter = std::tuple<>>
    class fn_universal  {
    private:
        F f;                            ///< main functor
        TPackAfter after;               ///< curryed arguments
        TPackBefore before;             ///< curryed arguments
    public:
        
        fn_universal(F && f) : f(std::forward<F>(f)), after(std::tuple<>()), before(std::tuple<>()) {}
        
        fn_universal(const F & f, const TPackBefore & before, const TPackAfter & after) : f(f), after(after), before(before) {}
        
        
      
        template <typename... Args>
        auto operator()(Args... args) const -> decltype(
            fn_tuple_apply(f, std::tuple_cat(before, make_tuple(args...), after))
        ) {
            // execute via tuple
            return fn_tuple_apply(f, std::tuple_cat(before, make_tuple(std::forward<Args>(args)...), after));
        }
        
        
        // curry
        
        template <typename T>
        auto curry(T && param) const -> decltype(fn_universal<F,decltype(std::tuple_cat(before, std::make_tuple(param))),TPackAfter>(f, std::tuple_cat(before, std::make_tuple(param)), after))
        {
            return fn_universal<F,decltype(std::tuple_cat(before, std::make_tuple(param))),TPackAfter>(f, std::tuple_cat(before, std::make_tuple(std::forward<T>(param))), after);
        }
        
        template <typename T>
        auto curry_right(T && param) const -> decltype(fn_universal<F, TPackBefore, decltype(std::tuple_cat(after, std::make_tuple(param)))>(f, before, std::tuple_cat(after, std::make_tuple(param))))
        {
            return fn_universal<F, TPackBefore, decltype(std::tuple_cat(after, std::make_tuple(param)))>(f, before, std::tuple_cat(after, std::make_tuple(std::forward<T>(param))));
        }
    
    };
    
    template <typename F>
    auto fn_to_universal(F && f) -> fn_universal<F>
    {
        return fn_universal<F>(std::forward<F>(f));
    }
    
    
    
    // left curry by << operator
    template<typename UF, typename Arg>
    auto operator<<(const UF & f, Arg && arg) -> decltype(f.template curry<Arg>(std::forward<Arg>(arg)))
    {
        return f.template curry<Arg>(std::forward<Arg>(arg));
    }
    
    // right curry by >> operator
    template<typename UF, typename Arg>
    auto operator>>(const UF & f, Arg && arg) -> decltype(f.template curry_right<Arg>(std::forward<Arg>(arg)))
    {
        return f.template curry_right<Arg>(std::forward<Arg>(arg));
    }
    
    
    
    // OBJECT from TEMPLATE function!
    // Template as first-class citizen
    
    
    #define make_citizen(X) class tfn_##X { public: template <typename... Args> auto operator()(Args&&... args) const ->decltype(X(std::forward<Args>(args)...))  { return X(std::forward<Args>(args)...); } }
    
    
    
    // tuple concatenation via << operator
    template<typename... OldArgs, typename NewArg>
    tuple<OldArgs...,NewArg> operator<<(const tuple<OldArgs...> & t, const NewArg& arg)
    {
        return std::tuple_cat(t, std::make_tuple(arg));
    }
    
  
    
    template<typename T, class F>
    auto operator|(T&& param, const F& f) -> decltype(f(param))  // typename std::result_of<F(T)>::type
    {
        return f(std::forward<T>(param));
    }
    
    
    
    
    template <typename... Args>
    void print(Args... args)
    {
        (void)(int[]){((LOG << args), 0)...}; LOG << NL;
    }
    
    make_citizen(print);
    
    // Return count of elements as templated operator
    template <typename T>
    int count(const T& container)
    {
        return container.size();
    }
    
    make_citizen(count);
    
    
    
    /// Universal operators
    
    // MAP
    template <typename T, typename... TArgs, template <typename...>class C, typename F>
    auto fn_map(const C<T,TArgs...>& container, const F& f) -> C<decltype(f(std::declval<T>()))>
    {
        using resultType = decltype(f(std::declval<T>()));
        C<resultType> result;
        for (const auto& item : container)
            result.push_back(f(item));
        return result;
    }
    
    // REDUCE (FOLD)
    template <typename TResult, typename T, typename... TArgs, template <typename...>class C, typename F>
    TResult fn_reduce(const C<T,TArgs...>& container, const TResult& startValue, const F& f)
    {
        TResult result = startValue;
        for (const auto& item : container)
            result = f(result, item);
        return result;
    }
    
    // FILTER
    template <typename T, typename... TArgs, template <typename...>class C, typename F>
    C<T,TArgs...> fn_filter(const C<T,TArgs...>& container, const F& f)
    {
        C<T,TArgs...> result;
        for (const auto& item : container)
            if (f(item))
                result.push_back(item);
        return result;
    }

    
 
    #define make_universal(NAME, F) make_citizen(F); const auto NAME = fn_to_universal(tfn_##F());
    
    make_universal(fmap, fn_map);
    make_universal(reduce, fn_reduce);
    make_universal(filter, fn_filter);
    
    
    template <typename T, typename... Args>
    T sum_impl(T arg, Args... args)
    {
        T result = arg;
        [&result](...){}((result += args, 0)...);
        return result;
    }
    
    make_universal(sum, sum_impl);
    
    template <typename T, typename TName>
    bool isNameEqualImpl(const T& obj, const TName& name)
    {
        return (obj->name == name);
    }
    
    make_universal(isName, isNameEqualImpl);
    
    template <typename T, typename TId>
    bool isIdEqualImpl(const T& obj, const TId& id)
    {
        return (obj->id == id);
    }
    
    make_universal(isId, isIdEqualImpl);
    
    template <typename T>
    bool isNotNullImpl(const T& t)
    {
        return (t != nullptr);
    }
    
    make_universal(isNotNull, isNotNullImpl);
    
    
    template <typename F, typename TKey, typename T, typename... TArgs, template <typename...>class C>
    T findOneImpl(const C<T,TArgs...>& container, const F& f, const TKey& key)
    {
        for (const auto& item : container)
            if (f(item, key))
                return item;
        return nullptr;
    }
    
    make_universal(ffind, findOneImpl);
    
    
    
    
    
    
    // -------------------- chain of functors --------------------->
    
    // The chain of functors ... is actualy just a tuple of functors
    template <typename... FNs>
    class fn_chain {
    private:
        const std::tuple<FNs...> functions;
        
        template <std::size_t I, typename Arg>
        inline typename std::enable_if<I == sizeof...(FNs) - 1, decltype(std::get<I>(functions)(std::declval<Arg>())) >::type
        call(Arg arg) const
        {
            return std::get<I>(functions)(std::forward<Arg>(arg));
        }
        
       
        template <std::size_t N, std::size_t I, typename Arg>
        struct final_type : final_type<N-1, I+1, decltype(std::get<I>(functions)(std::declval<Arg>())) > {};
        
        template <std::size_t I, typename Arg>
        struct final_type<0, I, Arg> {
            using type = decltype(std::get<I>(functions)(std::declval<Arg>()));
        };
        
        template <std::size_t I, typename Arg>
        inline typename std::enable_if<I < sizeof...(FNs) - 1, typename final_type<sizeof...(FNs) - 1 - I, I, Arg>::type >::type
        call(Arg arg) const
        {
            return this->call<I+1>(std::get<I>(functions)(std::forward<Arg>(arg)));
        }

       
        
    public:
        fn_chain() : functions(std::tuple<>()) {}
        fn_chain(std::tuple<FNs...> functions) : functions(functions) {}
        
        // add function into chain
        template< typename F >
        inline auto add(const F& f) const -> fn_chain<FNs..., F>
        {
            return fn_chain<FNs..., F>(std::tuple_cat(functions, std::make_tuple(f)));
        }
        
        
        // call whole functional chain
        template <typename Arg>
        inline auto operator()(Arg arg) const -> decltype(this->call<0,Arg>(arg))
        
        {
            return call<0>(std::forward<Arg>(arg));
        }
        
    };
    
    // pipe function into functional chain via | operator
 
    
    template<typename... FNs, typename F>
    inline auto operator|(fn_chain<FNs...> && chain, F&& f) -> decltype(chain.add(f))
    {
        return chain.add(std::forward<F>(f));
    }
    
    
    // ------------------------------- Monads ---------------------------------->
    
    
    
    enum class maybe_state { normal, empty };
    
    template <typename T>
    typename std::enable_if< std::is_object<decltype(T()) >::value, T>::type
    set_empty() { return T(); }
    
    template<> int set_empty<int>() { return 0; }
    template<> string set_empty<string>() { return ""; }
    
    template<typename T>
    class maybe {
    private:
        const maybe_state state;
        const T x;
        
        template <typename R>
        maybe<R> fromValue(R&& result) const
        {
            return maybe<R>(std::forward<R>(result));
        }
        
        template <typename R>
        maybe<std::shared_ptr<R>> fromValue(std::shared_ptr<R>&& result) const
        {
            if (result == nullptr)
                return maybe<std::shared_ptr<R>>();
            else
                return maybe<std::shared_ptr<R>>(std::forward<std::shared_ptr<R>>(result));
        }
        
       
    public:
        // monadic return
        maybe(T&& x) : x(std::forward<T>(x)), state(maybe_state::normal) {}
        maybe() : x(set_empty<T>()), state(maybe_state::empty) {}
        
        // monadic bind
        template <typename F>
        auto operator()(F f) const -> maybe<decltype(f(std::declval<T>()))>
        {
            using ResultType = decltype(f(std::declval<T>()));
            if (state == maybe_state::empty)
                return maybe<ResultType>();
            return fromValue(f(x));
        }
         
        // extract value
        T getOr(T&& anotherValue) const { return (state == maybe_state::empty) ? anotherValue : x; };
    };
    
    template<typename T, typename F>
    inline auto operator|(maybe<T> && monad, F&& f) -> decltype(monad(f))
    {
        return monad(std::forward<F>(f));
    }
    
    
    template<typename T, typename TDefault>
    inline T operator||(maybe<T> && monad, TDefault&& t)
    {
        return monad.getOr(std::forward<TDefault>(t));
    }
    
    template <typename T>
    maybe<T> just(T&& t)
    {
        return maybe<T>(std::forward<T>(t));
    }
    
    
    
    string xmlWrapImpl(string name, string item)
    {
        return "<" + name + ">" + item + "</" + name + ">";
    }
    
    make_universal(xmlWrap, xmlWrapImpl);
    
    
       /// Simple immutable data
    class UserData {
    public:
        const int id;
        const string name;
        const int parent;
        UserData(int id, string name, int parent) : id(id), name(name), parent(parent) {}
    };
    
    /// Shared pointer to immutable data
    using User = std::shared_ptr<UserData>;
    
    template <class T, class... P>
    inline auto make(P&&... args) -> T {
        return std::make_shared<typename T::element_type>(std::forward<P>(args)...);
    }

int main() {

   // let's test citizenship...
        tfn_print print;
        print(5);
        print("hello");
        
        // let's call function by providing tuple
        
        auto f = [](int x, int y, int z) { return x + y - z; };
        auto params = make_tuple(1,2,3);
        auto res = fn_tuple_apply(f, params);
        print(res);
        
        
        // combine tuple using overloaded operators and apply function
        auto list = make_tuple(1,4);
        auto res2 = fn_tuple_apply(f, list << 4);
        print(res2);
        
        // ok, now i want piped templates
        tfn_count count;
        vector<string> slist = {"one", "two", "three"};
        slist | count | print;
        
        
        // piping....
        auto ucount = fn_to_universal(count);
        auto uprint = fn_to_universal(print);
        slist | ucount | print;
        
        // currying....
        auto uf = fn_to_universal(f);
        auto uf1 = uf << 1;
        auto uf2 = uf1 << 2 << 5;
        uf2() | print;
        
        1 | (uf << 4 << 6) | print; // 4+6-1 = 9
        
        3 | (uf >> 6 >> 7) | print; // 3+6-7 = 2
        
       
        // count sum length of all strings in the list
        
        slist | (fmap >> count) | (reduce >> 0 >> sum) | (uprint << "Total: " >> " chars");
        
        slist | (reduce >> string("") >> sum) | (uprint << "All: ");
        
        vector<int>{1,2,3} | (reduce >> 0 >> sum) | (uprint << "Sum: ");
        
        // Some real objects...
        vector<User> users {make<User>(1, "John", 0), make<User>(2, "Bob", 1), make<User>(3, "Max", 1)};
        
        users | (filter >> (isName >> "Bob")) | ucount | uprint;
        
        vector<int>{1,2,6} | (fmap >> (ffind << users << isId)) | (filter >> isNotNull) | ucount | uprint;
        
          // Produce XML
        users | fmap >> [](User u){ return u->name; } | fmap >> (xmlWrap << "name") | reduce >> string("") >> sum | xmlWrap << "users" | print;
        
        // Use functional chain:
        auto f1 = [](int x){ return x+3; };
        auto f2 = [](int x){ return x*2; };
        auto f3 = [](int x) { return (double)x / 2.0; };
        auto f4 = [](double x) { std::stringstream ss; ss << x; return ss.str(); };
        auto f5 = [](string s) { return "Result: " + s; };
        auto testChain = fn_chain<>() | f1 | f2 | f3 | f4 | f5;
        testChain(3) | print;
        
        auto countUsers = fn_chain<>() | (fmap >> (ffind << users << isId)) | (filter >> isNotNull) | ucount;
        vector<int>{1,2,6} | countUsers | (uprint << "count of users: ");
        
        
        // Ok, pipe through monadic execution!
        
        maybe<int>(2) | (ffind << users << isId) | [](User u){ return u->name; } | [](string s){ LOG << s << NL; return s; };
        
        (maybe<int>(6) | (ffind << users << isId) | [](User u){ return u->name; }).getOr("Not found") | (uprint << "Found user: ");
        
        just(vector<int>{1,2,6}) | countUsers | [&](int count){ count | (uprint << "Count: "); return count; };
        
        (just(vector<int>{1,2,6}) | countUsers || -1) | (uprint << "Count:");
        
  

	return 0;
}

woow !

very useful!