Example for higher kinded types in C++ with std::list and std::optional (Maybe)

higher_kinded_types.hh

#include<list>
#include<optional>

template<template <typename> class F>
struct Functor {
  template<typename A, typename B>
  static F<B> fmap(B *(f)(A), F<A> val);
  
  // ($>)
  template<typename A, typename B>
  static F<A> replaceFunctor(A a, F<B> list);
};

template<>
struct Functor<std::list> {
  template<typename A, typename B>
  static std::list<B> fmap(B *(f)(A), std::list<A> list) {
    for (auto x : list) x = f(x);
  }

  template<typename A, typename B>
  static std::list<A> replaceFunctor(A a, std::list<B> list) {
      return new std::list<A>(a);
  }
};

template<>
struct Functor<std::optional> {
  template<typename A, typename B>
  static std::optional<B> fmap(B *(f)(A), std::optional<A> maybe) {
      return maybe.has_value() ? maybe.emplace(f(maybe.get())) : maybe;
  }

  template<typename A, typename B>
  static std::optional<A> replaceFunctor(A a, std::optional<B> maybe) {
      return new std::optional(a);
  }
};