.js

 
 
 //https://medium.com/@dimpapadim3
 
  var node = (left, value, right) => ({
    left: left,
    value: value,
    right: right,
    map: f => node(left.map(f), f(value), right.map(f)),
    fold: (mempty, mappend) => 
    mappend(mappend(mappend(mempty, left.fold(mempty, mappend)), value), right.fold(mempty, mappend))
  });
  
  var leaf = value => ({
    value: value,
    map: f => leaf(f(value)),
    fold: (acc, mappend) => mappend(acc, value)
  });