State Monad in C ++, statemonadin

State Monad in C ++, statemonadin

A c ++ version of State Monad. C ++ 11 is required.

#include <functional>template<template<typename> class M> struct Monad {    template<typename A> M<A> ret(A);    template<typename A, typename B> M<B> bind(M<A>, std::function<M<B>(A)>);};template <typename A, typename B> struct Tuple{    A _1;    B _2;};template<typename S> struct State {    template <typename C> struct type : Monad<type> {        std::function<Tuple<S, C>(S)> runState;        template<typename A> type<A> ret(A a){            std::function<Tuple<S, A>(S)> f = [a](S s) -> Tuple<S, A> {                Tuple < S, A > r;                r._1 = s;                r._2 = a;                return r;            };            type<A> s;            s.runState = f;            return s;        }        template<typename A, typename B> type<B> bind(type<A> fa, std::function<type<B>(A)> f) {            std::function<Tuple<S, B>(S)> fr = [fa, f](S s) -> Tuple<S, B> {                std::function<Tuple<S, A>(S)> f0 = fa.runState;                Tuple<A, B> r = f0(s);                std::function<Tuple<S, A>(S)> f1 = f(r._2).runState;                return f1(r._1);            };            type<B> s;            s.runState = fr;            return s;        }    };};