Sure, but you don't necessarily need sum types for Either. Its more tedious, but totally doable without them, e.g. just with higher order functions:

  let left  = a => (l, r) => l(a)
  let right = b => (l, r) => r(b)

its now impossible to consume result without passing functions to handle both the left and the right value.

  let mapRight = f => val => val(identity, right => right(f(val)))