Either and Tuple (the canonical Sum/Product types, respectively) used to make a List

SumProductList.js

const {Left, Right}  = require('data.either')
const Tuple = require('fantasy-tuples').Tuple2

//List :: Either Null (Tuple a List)

const empty = () =>
  Left(null)

const cons = (x, l) =>
  Right(Tuple(x, l))

const map = (f, l) =>
  l.cata({
    Right: t => cons(f(t._1), map(f, t._2)),
    Left: e => empty()
  })

const reduce = (f, z, l) =>
  l.cata({
    Right: t => reduce(f, f(z, t._1), t._2),
    Left: e => z
  })

const toArray = l =>
  reduce((acc, x) => [x].concat(acc), [], l)
  
const fromArray = xs =>
  xs.reduce((acc, x) => cons(x, acc), empty())


const l = fromArray([1, 2, 3]) // || cons(3, cons(2, cons(1, empty())))

toArray(l)
// [ 1, 2, 3 ]
toArray(cons(4, l))
// [ 1, 2, 3, 4 ]
toArray(map(x => x + 1, l))
// [ 2, 3, 4 ]
reduce((acc, x) => acc + x, 0, l)
// 6