The store class (simplified)

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template <class S, class A>
class reducpp::store {
public:
    typedef std::function<S(const S&, const A&)> reducer_t;
    typedef std::function<void()> callback_t;
    
    template <class F>
    store(const F& reducer) : m_reducer(reducer) { m_history.push_back(S()); }
    
    const S& getState() const { return m_history.back(); }

    void dispatch(const A& action) {
        m_history.push_back(m_reducer(m_history.back(), action));
        for (const callback_t& callback : m_subscriptions) {
            callback();
        }
    }

    template <class F>
    void subscribe(const F& callback) { m_subscriptions.push_back(callback); }
  
private:
    const reducer_t m_reducer;
    std::list<S> m_history;
    std::vector<callback_t> m_subscriptions;
};