An example of a monad written in ReasonML.

monad-example.re

type collection('a) = Collection('a) | None;
module CollectionMonad = {
  let lift = (value) => Collection(value);
  let map = (fn, x) =>  {
      switch (x) {
        | Collection(value) => Collection(fn(value)) 
        | None => None      
      }
  }
  let flatMap = (fn, collection) => switch(collection) {
   | Collection(value) => fn(value)
   | None => None 
  }
};