contravariance-example1.js

const g  = z => z + 1;

g(0) // 1

const maybeValue = Maybe.of(0);

maybeValue.map(g) // Maybe(1)


Function.prototype.map = function (fn) {
  return compose(fn, this);
}

Function.prototype.contramap = function(fn) {
  return leftCompose(fn, this);
}

const greet = name => `Hello ${name}`;

greet('Jamie') // Hello Jamie.

const upperGreeter  = greet.map(toUpper);

upperGreeter('Jamie') // HELLO JAMIE.

const newUpperGreter = greet.contramap(toUpper);

newUpperGreter('Jamie') // Hello JAMIE