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+#ifndef _CPPROMISE_HPP
+#define _CPPROMISE_HPP
+
+/**
+ * MIT License
+ * Copyright (c) 2018 Ted Yin <tederminant@gmail.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#include <stack>
+#include <vector>
+#include <memory>
+#include <functional>
+#include <type_traits>
+
+#if __cplusplus >= 201703L
+#ifdef __has_include
+#   if __has_include(<any>)
+#       include <any>
+#       ifdef __cpp_lib_any
+#           define _CPPROMISE_STD_ANY
+#       endif
+#   endif
+#endif
+#endif
+
+#ifndef _CPPROMISE_STD_ANY
+#include <boost/any.hpp>
+#endif
+
+/**
+ * Implement type-safe Promise primitives similar to the ones specified by
+ * Javascript Promise/A+.
+ */
+namespace promise {
+#ifdef _CPPROMISE_STD_ANY
+    using pm_any_t = std::any;
+    template<typename T>
+    constexpr auto any_cast = static_cast<T(*)(const std::any&)>(std::any_cast<T>);
+    using bad_any_cast = std::bad_any_cast;
+#else
+#   warning "using boost::any"
+#   pragma message "using boost::any"
+    using pm_any_t = boost::any;
+    template<typename T>
+    constexpr auto any_cast = static_cast<T(*)(const boost::any&)>(boost::any_cast<T>);
+    using bad_any_cast = boost::bad_any_cast;
+#endif
+    using callback_t = std::function<void()>;
+    using values_t = std::vector<pm_any_t>;
+
+    /* match lambdas */
+    template<typename T>
+    struct function_traits:
+        public function_traits<decltype(&T::operator())> {};
+ 
+    template<typename ReturnType>
+    struct function_traits<ReturnType()> {
+        using ret_type = ReturnType;
+        using arg_type = void;
+        using empty_arg = void;
+    };
+   
+    /* match plain functions */
+    template<typename ReturnType, typename ArgType>
+    struct function_traits<ReturnType(ArgType)> {
+        using ret_type = ReturnType;
+        using arg_type = ArgType;
+        using non_empty_arg = void;
+    };
+ 
+    /* match function pointers */
+    template<typename ReturnType, typename... ArgType>
+    struct function_traits<ReturnType(*)(ArgType...)>:
+        public function_traits<ReturnType(ArgType...)> {};
+
+    /* match const member functions */
+    template<typename ClassType, typename ReturnType, typename... ArgType>
+    struct function_traits<ReturnType(ClassType::*)(ArgType...) const>:
+        public function_traits<ReturnType(ArgType...)> {};
+
+    /* match member functions */
+    template<typename ClassType, typename ReturnType, typename... ArgType>
+    struct function_traits<ReturnType(ClassType::*)(ArgType...)>:
+        public function_traits<ReturnType(ArgType...)> {};
+
+    template<typename Func, typename ReturnType>
+    using enable_if_return = typename std::enable_if<
+            std::is_same<typename function_traits<Func>::ret_type,
+                         ReturnType>::value>;
+
+    template<typename Func, typename ReturnType>
+    using disable_if_return = typename std::enable_if<
+            !std::is_same<typename function_traits<Func>::ret_type,
+                         ReturnType>::value>;
+
+    template<typename Func, typename ArgType>
+    using enable_if_arg = typename std::enable_if<
+            std::is_same<typename function_traits<Func>::arg_type,
+                         ArgType>::value>;
+
+    template<typename Func, typename ArgType>
+    using disable_if_arg = typename std::enable_if<
+            !std::is_same<typename function_traits<Func>::arg_type,
+                         ArgType>::value>;
+
+    class Promise;
+    //class promise_t: public std::shared_ptr<Promise> {
+    class promise_t {
+        Promise *pm;
+        size_t *ref_cnt;
+        public:
+        friend Promise;
+        template<typename PList> friend promise_t all(const PList &promise_list);
+        template<typename PList> friend promise_t race(const PList &promise_list);
+
+        inline promise_t();
+        inline ~promise_t();
+        template<typename Func> inline promise_t(Func callback);
+
+        void swap(promise_t &other) {
+            std::swap(pm, other.pm);
+            std::swap(ref_cnt, other.ref_cnt);
+        }
+
+        promise_t &operator=(const promise_t &other) {
+            if (this != &other)
+            {
+                promise_t tmp(other);
+                tmp.swap(*this);
+            }
+            return *this;
+        }
+
+        promise_t &operator=(promise_t &&other) {
+            if (this != &other)
+            {
+                promise_t tmp(std::move(other));
+                tmp.swap(*this);
+            }
+            return *this;
+        }
+
+        promise_t(const promise_t &other):
+            pm(other.pm),
+            ref_cnt(other.ref_cnt) {
+            ++*ref_cnt;
+        }
+
+        promise_t(promise_t &&other):
+            pm(other.pm),
+            ref_cnt(other.ref_cnt) {
+            other.pm = nullptr;
+        }
+
+        Promise *operator->() const {
+            return pm;
+        }
+
+        template<typename T> inline void resolve(T result) const;
+        template<typename T> inline void reject(T reason) const;
+        inline void resolve() const;
+        inline void reject() const;
+
+        template<typename FuncFulfilled>
+        inline promise_t then(FuncFulfilled on_fulfilled) const;
+
+        template<typename FuncFulfilled, typename FuncRejected>
+        inline promise_t then(FuncFulfilled on_fulfilled,
+                            FuncRejected on_rejected) const;
+
+        template<typename FuncRejected>
+        inline promise_t fail(FuncRejected on_rejected) const;
+    };
+
+#define PROMISE_ERR_INVALID_STATE do {throw std::runtime_error("invalid promise state");} while (0)
+#define PROMISE_ERR_MISMATCH_TYPE do {throw std::runtime_error("mismatching promise value types");} while (0)
+    
+    class Promise {
+        template<typename PList> friend promise_t all(const PList &promise_list);
+        template<typename PList> friend promise_t race(const PList &promise_list);
+        std::vector<callback_t> fulfilled_callbacks;
+        std::vector<callback_t> rejected_callbacks;
+#ifdef CPPROMISE_USE_STACK_FREE
+        std::vector<Promise *> fulfilled_pms;
+        std::vector<Promise *> rejected_pms;
+#endif
+        enum class State {
+            Pending,
+#ifdef CPPROMISE_USE_STACK_FREE
+            PreFulfilled,
+            PreRejected,
+#endif
+            Fulfilled,
+            Rejected,
+        } state;
+        pm_any_t result;
+        pm_any_t reason;
+
+        void add_on_fulfilled(callback_t &&cb) {
+            fulfilled_callbacks.push_back(std::move(cb));
+        }
+
+        void add_on_rejected(callback_t &&cb) {
+            rejected_callbacks.push_back(std::move(cb));
+        }
+
+        template<typename Func,
+            typename function_traits<Func>::non_empty_arg * = nullptr>
+        static constexpr auto cps_transform(
+                Func f, const pm_any_t &result, const promise_t &npm) {
+            return [&result, npm, f = std::forward<Func>(f)]() mutable {
+#ifndef CPPROMISE_USE_STACK_FREE
+                f(result)->then(
+                    [npm] (pm_any_t result) {npm->resolve(result);},
+                    [npm] (pm_any_t reason) {npm->reject(reason);});
+#else
+                promise_t rpm{f(result)};
+                rpm->then(
+                    [rpm, npm] (pm_any_t result) {
+                        npm->_resolve(result);
+                    },
+                    [rpm, npm] (pm_any_t reason) {
+                        npm->_reject(reason);
+                    });
+                rpm->_dep_resolve(npm);
+                rpm->_dep_reject(npm);
+#endif
+            };
+        }
+
+        template<typename Func,
+            typename function_traits<Func>::empty_arg * = nullptr>
+        static constexpr auto cps_transform(
+                Func f, const pm_any_t &, const promise_t &npm) {
+            return [npm, f = std::forward<Func>(f)]() mutable {
+#ifndef CPPROMISE_USE_STACK_FREE
+                f()->then(
+                    [npm] (pm_any_t result) {npm->resolve(result);},
+                    [npm] (pm_any_t reason) {npm->reject(reason);});
+#else
+                promise_t rpm{f()};
+                rpm->then(
+                    [rpm, npm] (pm_any_t result) {
+                        npm->_resolve(result);
+                    },
+                    [rpm, npm] (pm_any_t reason) {
+                        npm->_reject(reason);
+                    });
+                rpm->_dep_resolve(npm);
+                rpm->_dep_reject(npm);
+#endif
+            };
+        }
+
+        template<typename Func,
+            typename enable_if_return<Func, promise_t>::type * = nullptr>
+        constexpr auto gen_on_fulfilled(Func on_fulfilled, const promise_t &npm) {
+            return cps_transform(std::forward<Func>(on_fulfilled), this->result, npm);
+        }
+
+        template<typename Func,
+            typename enable_if_return<Func, promise_t>::type * = nullptr>
+        constexpr auto gen_on_rejected(Func on_rejected, const promise_t &npm) {
+            return cps_transform(std::forward<Func>(on_rejected), this->reason, npm);
+        }
+
+
+        template<typename Func,
+            typename enable_if_return<Func, void>::type * = nullptr,
+            typename function_traits<Func>::non_empty_arg * = nullptr>
+        constexpr auto gen_on_fulfilled(Func on_fulfilled, const promise_t &npm) {
+            return [this, npm,
+                    on_fulfilled = std::forward<Func>(on_fulfilled)]() mutable {
+                on_fulfilled(result);
+                npm->_resolve();
+            };
+        }
+
+        template<typename Func,
+            typename enable_if_return<Func, void>::type * = nullptr,
+            typename function_traits<Func>::empty_arg * = nullptr>
+        constexpr auto gen_on_fulfilled(Func on_fulfilled, const promise_t &npm) {
+            return [on_fulfilled = std::forward<Func>(on_fulfilled), npm]() mutable {
+                on_fulfilled();
+                npm->_resolve();
+            };
+        }
+
+        template<typename Func,
+            typename enable_if_return<Func, void>::type * = nullptr,
+            typename function_traits<Func>::non_empty_arg * = nullptr>
+        constexpr auto gen_on_rejected(Func on_rejected, const promise_t &npm) {
+            return [this, npm,
+                    on_rejected = std::forward<Func>(on_rejected)]() mutable {
+                on_rejected(reason);
+                npm->_reject();
+            };
+        }
+
+        template<typename Func,
+            typename enable_if_return<Func, void>::type * = nullptr,
+            typename function_traits<Func>::empty_arg * = nullptr>
+        constexpr auto gen_on_rejected(Func on_rejected, const promise_t &npm) {
+            return [npm,
+                    on_rejected = std::forward<Func>(on_rejected)]() mutable {
+                on_rejected();
+                npm->_reject();
+            };
+        }
+
+        template<typename Func,
+            typename enable_if_return<Func, pm_any_t>::type * = nullptr,
+            typename function_traits<Func>::non_empty_arg * = nullptr>
+        constexpr auto gen_on_fulfilled(Func on_fulfilled, const promise_t &npm) {
+            return [this, npm,
+                    on_fulfilled = std::forward<Func>(on_fulfilled)]() mutable {
+                npm->_resolve(on_fulfilled(result));
+            };
+        }
+
+        template<typename Func,
+            typename enable_if_return<Func, pm_any_t>::type * = nullptr,
+            typename function_traits<Func>::empty_arg * = nullptr>
+        constexpr auto gen_on_fulfilled(Func on_fulfilled, const promise_t &npm) {
+            return [npm, on_fulfilled = std::forward<Func>(on_fulfilled)]() mutable {
+                npm->_resolve(on_fulfilled());
+            };
+        }
+
+        template<typename Func,
+            typename enable_if_return<Func, pm_any_t>::type * = nullptr,
+            typename function_traits<Func>::non_empty_arg * = nullptr>
+        constexpr auto gen_on_rejected(Func on_rejected, const promise_t &npm) {
+            return [this, npm, on_rejected = std::forward<Func>(on_rejected)]() mutable {
+                npm->_reject(on_rejected(reason));
+            };
+        }
+
+        template<typename Func,
+            typename enable_if_return<Func, pm_any_t>::type * = nullptr,
+            typename function_traits<Func>::empty_arg * = nullptr>
+        constexpr auto gen_on_rejected(Func on_rejected, const promise_t &npm) {
+            return [npm, on_rejected = std::forward<Func>(on_rejected)]() mutable {
+                npm->_reject(on_rejected());
+            };
+        }
+
+#ifdef CPPROMISE_USE_STACK_FREE
+        void _trigger() {
+            std::stack<std::tuple<
+                std::vector<Promise *>::const_iterator,
+                std::vector<Promise *> *,
+                Promise *>> s;
+            auto push_frame = [&s](Promise *pm) {
+                if (pm->state == State::PreFulfilled)
+                {
+                    pm->state = State::Fulfilled;
+                    for (auto &cb: pm->fulfilled_callbacks) cb();
+                    s.push(std::make_tuple(pm->fulfilled_pms.begin(),
+                                          &pm->fulfilled_pms,
+                                          pm));
+                }
+                else if (pm->state == State::PreRejected)
+                {
+                    pm->state = State::Rejected;
+                    for (auto &cb: pm->rejected_callbacks) cb();
+                    s.push(std::make_tuple(pm->rejected_pms.begin(),
+                                          &pm->rejected_pms,
+                                          pm));
+                }
+            };
+            push_frame(this);
+            while (!s.empty())
+            {
+                auto &u = s.top();
+                auto &it = std::get<0>(u);
+                auto vec = std::get<1>(u);
+                auto pm = std::get<2>(u);
+                if (it == vec->end())
+                {
+                    s.pop();
+                    vec->clear();
+                    pm->fulfilled_callbacks.clear();
+                    pm->rejected_callbacks.clear();
+                    continue;
+                }
+                push_frame(*it++);
+            }
+        }
+
+        void trigger_fulfill() {
+            state = State::PreFulfilled;
+            _trigger();
+        }
+
+        void trigger_reject() {
+            state = State::PreRejected;
+            _trigger();
+        }
+
+        void _resolve() {
+            if (state == State::Pending) state = State::PreFulfilled;
+        }
+
+        void _reject() {
+            if (state == State::Pending) state = State::PreRejected;
+        }
+
+        void _dep_resolve(const promise_t &npm) {
+            if (state == State::Pending)
+                fulfilled_pms.push_back(npm.pm);
+            else
+                npm->_trigger();
+        }
+
+        void _dep_reject(const promise_t &npm) {
+            if (state == State::Pending)
+                rejected_pms.push_back(npm.pm);
+            else
+                npm->_trigger();
+        }
+
+        void _resolve(pm_any_t _result) {
+            if (state == State::Pending)
+            {
+                result = _result;
+                state = State::PreFulfilled;
+            }
+        }
+
+        void _reject(pm_any_t _reason) {
+            if (state == State::Pending)
+            {
+                reason = _reason;
+                state = State::PreRejected;
+            }
+        }
+#else
+        void _resolve() { resolve(); }
+        void _reject() { reject(); }
+        void _resolve(pm_any_t result) { resolve(result); }
+        void _reject(pm_any_t reason) { reject(reason); }
+
+        void trigger_fulfill() {
+            state = State::Fulfilled;
+            for (const auto &cb: fulfilled_callbacks) cb();
+            fulfilled_callbacks.clear();
+        }
+
+        void trigger_reject() {
+            state = State::Rejected;
+            for (const auto &cb: rejected_callbacks) cb();
+            rejected_callbacks.clear();
+        }
+#endif
+        public:
+
+        Promise(): state(State::Pending) {}
+        ~Promise() {}
+
+        template<typename FuncFulfilled, typename FuncRejected>
+        promise_t then(FuncFulfilled on_fulfilled,
+                      FuncRejected on_rejected) {
+            switch (state)
+            {
+                case State::Pending:
+                return promise_t([this,
+                                on_fulfilled = std::forward<FuncFulfilled>(on_fulfilled),
+                                on_rejected = std::forward<FuncRejected>(on_rejected)
+                                ](promise_t &npm) {
+                    add_on_fulfilled(gen_on_fulfilled(std::move(on_fulfilled), npm));
+                    add_on_rejected(gen_on_rejected(std::move(on_rejected), npm));
+#ifdef CPPROMISE_USE_STACK_FREE
+                    _dep_resolve(npm);
+                    _dep_reject(npm);
+#endif
+                });
+                case State::Fulfilled:
+                return promise_t([this,
+                                on_fulfilled = std::forward<FuncFulfilled>(on_fulfilled)
+                                ](promise_t &npm) {
+                    gen_on_fulfilled(std::move(on_fulfilled), npm)();
+                });
+                case State::Rejected:
+                return promise_t([this,
+                                on_rejected = std::forward<FuncRejected>(on_rejected)
+                                ](promise_t &npm) {
+                    gen_on_rejected(std::move(on_rejected), npm)();
+                });
+                default: PROMISE_ERR_INVALID_STATE;
+            }
+        }
+
+        template<typename FuncFulfilled>
+        promise_t then(FuncFulfilled &&on_fulfilled) {
+            switch (state)
+            {
+                case State::Pending:
+                return promise_t([this,
+                                on_fulfilled = std::forward<FuncFulfilled>(on_fulfilled)
+                                ](promise_t &npm) {
+                    add_on_fulfilled(gen_on_fulfilled(std::move(on_fulfilled), npm));
+                    add_on_rejected([this, npm]() {npm->_reject(reason);});
+#ifdef CPPROMISE_USE_STACK_FREE
+                    _dep_resolve(npm);
+                    _dep_reject(npm);
+#endif
+                });
+                case State::Fulfilled:
+                return promise_t([this,
+                                on_fulfilled = std::forward<FuncFulfilled>(on_fulfilled)
+                                ](promise_t &npm) {
+                    gen_on_fulfilled(std::move(on_fulfilled), npm)();
+                });
+                case State::Rejected:
+                return promise_t([this](promise_t &npm) {npm->_reject(reason);});
+                default: PROMISE_ERR_INVALID_STATE;
+            }
+        }
+ 
+        template<typename FuncRejected>
+        promise_t fail(FuncRejected &&on_rejected) {
+            switch (state)
+            {
+                case State::Pending:
+                return promise_t([this,
+                                on_rejected = std::forward<FuncRejected>(on_rejected)
+                                ](promise_t &npm) {
+                    callback_t ret;
+                    add_on_rejected(gen_on_rejected(std::move(on_rejected), npm));
+                    add_on_fulfilled([this, npm]() {npm->_resolve(result);});
+#ifdef CPPROMISE_USE_STACK_FREE
+                    _dep_resolve(npm);
+                    _dep_reject(npm);
+#endif
+                });
+                case State::Fulfilled:
+                return promise_t([this](promise_t &npm) {npm->_resolve(result);});
+                case State::Rejected:
+                return promise_t([this,
+                                on_rejected = std::forward<FuncRejected>(on_rejected)
+                                ](promise_t &npm) {
+                    gen_on_rejected(std::move(on_rejected), npm)();
+                });
+                default: PROMISE_ERR_INVALID_STATE;
+            }
+        }
+  
+        void resolve() {
+            if (state == State::Pending) trigger_fulfill();
+        }
+
+        void reject() {
+            if (state == State::Pending) trigger_reject();
+        }
+
+        void resolve(pm_any_t _result) {
+            if (state == State::Pending)
+            {
+                result = _result;
+                trigger_fulfill();
+            }
+        }
+
+        void reject(pm_any_t _reason) {
+            if (state == State::Pending)
+            {
+                reason = _reason;
+                trigger_reject();
+            }
+        }
+    };
+        
+    template<typename PList> promise_t all(const PList &promise_list) {
+        return promise_t([&promise_list] (promise_t &npm) {
+            auto size = std::make_shared<size_t>(promise_list.size());
+            auto results = std::make_shared<values_t>();
+            if (!*size) PROMISE_ERR_MISMATCH_TYPE;
+            results->resize(*size);
+            size_t idx = 0;
+            for (const auto &pm: promise_list) {
+                pm->then(
+                    [results, size, idx, npm](pm_any_t result) {
+                        (*results)[idx] = result;
+                        if (!--(*size))
+                            npm->_resolve(*results);
+                    },
+                    [npm](pm_any_t reason) {npm->_reject(reason);});
+#ifdef CPPROMISE_USE_STACK_FREE
+                pm->_dep_resolve(npm);
+                pm->_dep_reject(npm);
+#endif
+                idx++;
+            }
+        });
+    }
+
+    template<typename PList> promise_t race(const PList &promise_list) {
+        return promise_t([&promise_list] (promise_t &npm) {
+            for (const auto &pm: promise_list) {
+                pm->then([npm](pm_any_t result) {npm->_resolve(result);},
+                        [npm](pm_any_t reason) {npm->_reject(reason);});
+#ifdef CPPROMISE_USE_STACK_FREE
+                pm->_dep_resolve(npm);
+                pm->_dep_reject(npm);
+#endif
+            }
+        });
+    }
+
+    template<typename Func>
+    inline promise_t::promise_t(Func callback):
+            pm(new Promise()),
+        ref_cnt(new size_t(1)) {
+        callback(*this);
+    }
+
+    inline promise_t::promise_t():
+        pm(new Promise()),
+        ref_cnt(new size_t(1)) {}
+
+    inline promise_t::~promise_t() {
+        if (pm)
+        {
+            if (--*ref_cnt) return;
+            delete pm;
+            delete ref_cnt;
+        }
+    }
+
+    template<typename T>
+    inline void promise_t::resolve(T result) const { (*this)->resolve(result); }
+
+    template<typename T>
+    inline void promise_t::reject(T reason) const { (*this)->reject(reason); }
+
+    inline void promise_t::resolve() const { (*this)->resolve(); }
+    inline void promise_t::reject() const { (*this)->reject(); }
+
+    template<typename T>
+    struct callback_types {
+        using arg_type = typename function_traits<T>::arg_type;
+        using ret_type = typename std::conditional<
+            std::is_same<typename function_traits<T>::ret_type, promise_t>::value,
+            promise_t, pm_any_t>::type;
+    };
+
+    template<typename Func,
+        typename disable_if_arg<Func, pm_any_t>::type * = nullptr,
+        typename enable_if_return<Func, void>::type * = nullptr,
+        typename function_traits<Func>::non_empty_arg * = nullptr>
+    constexpr auto gen_any_callback(Func f) {
+        using func_t = callback_types<Func>;
+        return [f = std::forward<Func>(f)](pm_any_t v) mutable {
+            try {
+                f(any_cast<typename func_t::arg_type>(v));
+            } catch (bad_any_cast &e) { PROMISE_ERR_MISMATCH_TYPE; }
+        };
+    }
+
+    template<typename Func,
+        typename enable_if_arg<Func, pm_any_t>::type * = nullptr,
+        typename enable_if_return<Func, void>::type * = nullptr,
+        typename function_traits<Func>::non_empty_arg * = nullptr>
+    constexpr auto gen_any_callback(Func f) {
+        return [f = std::forward<Func>(f)](pm_any_t v) mutable {f(v);};
+    }
+
+    template<typename Func,
+        typename enable_if_return<Func, void>::type * = nullptr,
+        typename function_traits<Func>::empty_arg * = nullptr>
+    constexpr auto gen_any_callback(Func f) { return std::forward<Func>(f); }
+
+    template<typename Func,
+        typename enable_if_arg<Func, pm_any_t>::type * = nullptr,
+        typename disable_if_return<Func, void>::type * = nullptr,
+        typename function_traits<Func>::non_empty_arg * = nullptr>
+    constexpr auto gen_any_callback(Func f) {
+        using func_t = callback_types<Func>;
+        return [f = std::forward<Func>(f)](pm_any_t v) mutable {
+            return typename func_t::ret_type(f(v));
+        };
+    }
+
+    template<typename Func,
+        typename disable_if_arg<Func, pm_any_t>::type * = nullptr,
+        typename disable_if_return<Func, void>::type * = nullptr,
+        typename function_traits<Func>::non_empty_arg * = nullptr>
+    constexpr auto gen_any_callback(Func f) {
+        using func_t = callback_types<Func>;
+        return [f = std::forward<Func>(f)](pm_any_t v) mutable {
+            try {
+                return typename func_t::ret_type(
+                    f(any_cast<typename func_t::arg_type>(v)));
+            } catch (bad_any_cast &e) { PROMISE_ERR_MISMATCH_TYPE; }
+        };
+    }
+
+    template<typename Func,
+        typename disable_if_return<Func, void>::type * = nullptr,
+        typename function_traits<Func>::empty_arg * = nullptr>
+    constexpr auto gen_any_callback(Func f) {
+        using func_t = callback_types<Func>;
+        return [f = std::forward<Func>(f)]() mutable {
+            return typename func_t::ret_type(f());
+        };
+    }
+
+    template<typename FuncFulfilled>
+    inline promise_t promise_t::then(FuncFulfilled on_fulfilled) const {
+        return (*this)->then(gen_any_callback(std::forward<FuncFulfilled>(on_fulfilled)));
+    }
+
+    template<typename FuncFulfilled, typename FuncRejected>
+    inline promise_t promise_t::then(FuncFulfilled on_fulfilled,
+                                    FuncRejected on_rejected) const {
+        return (*this)->then(gen_any_callback(std::forward<FuncFulfilled>(on_fulfilled)),
+                            gen_any_callback(std::forward<FuncRejected>(on_rejected)));
+    }
+
+    template<typename FuncRejected>
+    inline promise_t promise_t::fail(FuncRejected on_rejected) const {
+        return (*this)->fail(gen_any_callback(std::forward<FuncRejected>(on_rejected)));
+    }
+}
+
+#endif