Back in 1971 (that is several decades ago), a man named Saunders Mac Lane wrote a book called "Categories for the Working Mathematician". In one of the pages, it is written:
All told, a monad in X is just a monoid in the category of endofunctors of X, with product × replaced by composition of endofunctors and unit set by the identity endofunctor. - Saunders Mac Lane
This simple file uses std::list and defines the two operations as free functions unit and prod in such a way as to respect this exact definition, verifies the three monadic laws allowing the infamous Kleisli triple (known as Monad) to emerge naturally. Compile it with -std=c++14 or whatever else is your (at least) C++14 compiler enabling mode. Comments help you navigate through the implementation as a concept more than anything else (it is really simple). The following is a comment-free snippet.
auto f = [](int64_t x) { return unit(x * x); };
auto g = [](int64_t x) { return unit(x + x); };
auto law1 = [=](auto x) {
return prod(f, unit(x)) == f(x);
};
auto law2 = [=](auto x) {
typedef std::list<int64_t>(*U)(int64_t const &);
return prod(static_cast<U>(&unit), unit(x)) == std::list<int64_t>{x};
};
auto law3 = [=](auto x) {
return prod(f, prod(g, unit(x)))
== prod([=](auto x) { return prod(f,g(x)); }, unit(x));
};The reason I wrote this is because C++ is a language that makes it difficult for such constructs (as monads) to emerge and be used naturally for a variety of reasons related to its design; yet, there are simple ways that these may come about, despite the mental acrobatics one must do to deploy them.