composability-example04.js

const Right = x => ({
  map: f => Right(f(x)),
  chain: f => f(x),
  fold: (f, g) => g(x)
})

const Left = x => ({
  map: f => Left(x),
  chain: f => Left(x),
  fold: (f, g) => f(x)
})

const getPackages = user =>
      (user.premium ? Right(user) : Left('not premium'))
      .map(u => u.preferences)
      .fold(() => defaultsPackages, props => loadPackages(props))