This source code is from Bartosz Milewski https://github.com/BartoszMilewski/MoreMoney
Explains how to use the monad concept in C++
https://bartoszmilewski.com/2015/05/11/using-monads-in-c-to-solve-constraints-1-the-list-monad/ https://bartoszmilewski.com/2014/02/26/c17-i-see-a-monad-in-your-future/ https://bartoszmilewski.com/2014/10/17/c-ranges-are-pure-monadic-goodness/